Show notes with a full set of references are available here: http://www.rosebook.club/episodes/2021/6/22/chapter-fourAlso, please fill out our listener survey: https://forms.gle/DVdcJikKZkzY56mXA

Chapter Three: How the proximal tubule is like Elizabeth Warren and other truths my friends from Boston taught me References for Chapter 3: Faisy C, Meziani F, PLanquette B et al. Effect of Acetazolamide vs. Placebo on Duration of Invasive Mechanical Ventilation among patients with chronic obstructive pulmonary disease: a randomized clinical trial. JAMA 2016 https://pubmed.ncbi.nlm.nih.gov/26836730/This randomized controlled double blinded multi-center study of acetazolamide to shorten the duration of mechanical ventilation (known as DIABLO) there was no statistically significant difference (though it may have been underpowered to do so).Salazar H, Swanson J, Mozo K, White AC, Cabda MM Acute Mountain sickness impact among travelers to Cusco, Peru J Travel Med 2012 https://pubmed.ncbi.nlm.nih.gov/22776382/ Investigators found that altitude sickness is common and alters travel plans for 1 in 5 travelers but was prescribed infrequently.Buzas GM and Supuran CT. Journal of enzyme inhibition and medicinal chemistry 2015 https://www.tandfonline.com/doi/full/10.3109/14756366.2015.1051042This review describes the use of acetazolamide to treat peptic ulcers and how it was later learned that H. pylori have carbonic anhydrase NORDIC idiopathic intracranial Hypertension Study Writing Committee. The effect of acetazolamide on visual function in patients with idiopathic intracranial hypertension and mild visual loss: the idiopathic intracranial hypertension treatment trial. JAMA 2014 https://pubmed.ncbi.nlm.nih.gov/24756514/In this multi-centered trial, acetazolamide and low sodium weight reduction diet improved mild visual loss more than diet alone. Mullens W et al. Rationale and design of the ADVOR (acetazolamide in decompensated heart failure with volume overload trial) Eur J Heart Failure 2018 https://pubmed.ncbi.nlm.nih.gov/30238574/This reference explains the rationale for this ongoing trial.Gordon CE, Vantzelfde S and Francis JM. Acetazolamide in Lithium-induced nephrogenic diabetes insipidus NEJM 2016 https://www.nejm.org/doi/full/10.1056/NEJMc1609483A case report of efficacy of acetazolamide in a patient with severe polyuria.Zehnder D et al. Expression of 25-hydroxyvitamin D-1alpha hydroxylase in the human kidney. JASN 1999 This report explores the activity in the enzyme in nephron segments and suggests that the distal nephron may play an important part in the formation of 1,25 vitamin D https://jasn.asnjournals.org/content/10/12/2465 Outline: Chapter 3    - This is chapter three, kind of the first real chapter of the book- Proximal Tubule- Reabsorbs 55-60% of the filtrate    - Active sodium resorption        - 65% of the sodium        - 55% of the chloride        - 90% of HCO3        - 100% glucose and amino acids    - Passive water resorption    - Water resorption is isosmotic    - Secretion of        - Hydrogen        - Organic anions        - Organic cations    - Anatomy        - S1, S2, S3 can be differentiated by peptidases         - S1 more sodium resorption and hydrogen secretion, high capacity        - S2 more organic ion secretion    - Cell model        - Basolateral membrane            - Na-K-ATPase powers all the resorption         - Luminal membrane        - 100 liters a day crosses the proximal tubule cells            - Microvilli to increase surface area            - Microvilli has brush border which has carrier proteins as well as carbonic anhydrase            - Water permeable, so sodium resorption leads to water resorption            - Aquaporin-1 (sounds like this transporter is unique to the proximal tubule and RBC)            - HCO3 is reabsorbed early, along with Na, resulting in increased chloride concentration which passively reabsorbed via paracellular route.            - Tight junction has only one strand (on freeze fracture) as opposed to 8 in distal nephron        - The Na-K-ATPase            - Lower activity than in the LOH and distal nephron            - Maintained intracellular Na at effective concentration of 30 mmol/L            - Interior of the cell is negative due to 3 sodium out and 2 K in, then K leaks back out.                - 3 Na out for 2 K in                - An ATP sensitive K outflow channel on the basolateral membrane                    - Increased ATP slows potassium eflux                    - The idea is if Na-K slows, ATP will accumulate and this will slow K leaving, because there is less potassium entering.            - K channel is ATP sensitive, ATP antagonizes K leak.            - Highly favorable ELECTROCHEMICAL gradient for sodium to flow into the cell through the luminal membrane            - Must be via a channel or carrier                - Cotransporters                    - Amino acids                    - Phosphate                    - Glucose                - Called secondary active transport                - Countertransporters                    - Only example is H excretion            - Basolateral membrane                - Na-3HCO3 transporter                    - Powered by the negative charge in the cell- Chloride resorption    - Formate chloride exchanger        - Formate combines with hydrogen in the lumen, becomes neutral formic acid, and is reabsorbed where the higher pH causes it to dissociate and recycle again.        - Dependent on continued H+ secretion        - Chloride moves across basolateral membrane thanks to Cl and KCl transporters, taking advantage of negative intracellular charge- Passive mechanisms of proximal tubule transport    - Accounts for one third of fluid resorption    - Mechanism        - Early proximal tubule resorts most of the bicarb and less of the chloride        - Tubular fluid gets a high chloride concentration        - Chloride flows through the tight junction down its concentration gradient            - Sodium and water follow passively behind            - Water moves osmotically into intercellular space from tubular fluid even though the osmolalities are equal since chloride is an ineffective osmole, so tonicity is not the same. ******    - Argues that bicarb is primarily important solute for passive resorbtion        - Acetazolamide blocks Na and chloride resorption        - Similar thing happens with metabolic acidosis where less bicarb is available to drive passive resorbtion of Na and Cl    - Summary        - Other than Na-K-ATPase Na-H antiporter main determinant of proximal Na and water resorption            - 1. Direct bicarb resorption            - Preferential bicarb resorbtion proximally drives passive chloride resorption            - Drives active the formate exchanger for chloride resorption- Neurohormonal influence    - AT2 drives a lot of Na resorption, primarily in S1 segment        - Does not have a net effect on H-CO3 movement    - Dopamine antagonizes sodium resorption        - Blocks both Na-K-ATPase and        - Na H antiporter- Capillary uptake    - Starlings. Again        - Low hydraulic pressure due to glomerular arteriole        - High plasma on oncotic pressure from loss of the filtrate        - The two together promote resorption        - There maybe movement from interstitial back into tubular fluid (back diffusion) conflicting data- Glomerular tubular balance    - The fractional tubular reabsorption remains constant despite changes in GFR (tubular load)    - It is essential the GFR is matched by resorption    - The rise in capillary osmotic pressure with increased GFR via increased filtration fraction is one mechanism of GT balance    - Glomerular tubular balance os one of three mechanisms that prevents fluid delivery from exceeding the resorptive capacity of the tubules         - GT balance        - TG feedback        - Autoregulation    - GT balance can be altered if patients are volume overloaded or depleted    - Closes this section with a story of a kid born without a brush border    - Primacy of sodium in proximal tubule activity        - Discusses bicarb resorbtion            - There is no Tm for Bicarb as long as volume overload is prevented, in rats can rise over 60!            - If you give NaHCO3 you get volume overload and the Tm I about 60        - Glucose            - S1 and S2 have high capacity, low affinity glucose resorption            - S3 has high affinity 2 Na fo every glucose             - Tm glucose is 375 mg/min                - For a GFR of 125t that comes out to 300mg/dL                - 125 ml/min * 3mg/ml (300 mg/dL) = 375 mg/min                - Functionally this is 200 mg/dL due to splay        - Urea            - Only 50-60 of filtered urea is excreted        - Calcium Loop and distal tubule        - Phosphate - 3Na-Phosphate high affinity transporters late in proximal tubule - three types of Na-Phos transporters, type 2 are the most important - regulated by PTH and plasma phosphate - PTH suppresses Phos resorption -Metabolic acidosis also reduces phosphate resorption (good to have phosphate in the tubule to soak up H+ - Decreased tubular pH converts HPO42- to H2PO4- which has lower affinity for phosphate binding site        - Mg Loop and distal tubule        - Uric Acid Why do I love acetazolamide?-   I love the proximal tubule-   Many uses-   Often forgottenMOA-   Inhibit carbonic anhydraseMain effects-   Renal: less bicarb reabsorption (ie less H secretion) à more distal Na/bicarb delivery à hypokalemic metabolic acidosis-   Brain: reduce CSF production, reduce ICP/IOP, aqueous humor-   Pulm: COPDNotes-   Tolerance develops in 2-3 days-   Sulfonamide derivative-   Highly protein bound, eliminated by kidneys Source: Buzas and upuran, JEIMC, 2016S Data:1968 - High altitudeHigh altitude usually results in respiratory alkalosisAcetazolamide – lessens symptoms of altitude sickness (insomnia, headache) which occur because of periodic breathing/apnea1979- NEJM study took 9 mountaineers asleep at 5360 meters à improvement in sleep, improved SaO2 from 72 to 78.7 mmHg, reduce periodic breathing, increased alveolar ventilation (pCO2 change from 37 mmHg to 30.8mm Hg)1950s - Seizures/migrainesCAI reduces pH (more H intracellularly), K movement extracellularly à hyperpolarization and increase in seizure thresholdWeak CAI (Topamax, zonisamide) but not though to be important mechanism of antiseizure effect (topamax enhances inhibitory effect of GABA, block voltage dependent Na and Ca channels)Pulmonary/COPDThought to help with the metabolic alkalosis and as a respiratory stimulant to increase RR, TV, reduce ventilator timeIn 2001 Cochrane review – no difference in clinical outcomes, but did reduce pH and bicarb minimallyDIABLO study (RCT) on ventilated COPD patients – no difference in median duration of mechanical ventilation despite correction of metabolic alkalosisHigh altitude erythropoiesis (Monge disease)First described in 1925 via Dr. Carlos Monge Medrano (Peruvian doctor), seen in people living > 2500-3000 meters (more common in South America than other high altitude areas)Usually chronic altitude sickness with HgB > 21 g/dL + chronic hypoxemia, pHTNAcetazolamide – reduces polycythemia because induces a met acidosis à increases ventilation and arterial PPO2 and SaO2 à blunts erythropoiesis and reduces HCT and improves pulmonary vascular resistanceGI ulcersWhen H2 and PPI available, less useHistory: 1932 – observed alkaline tide, presumed existence of gastric CA (demonstrated in 1939)Acetazolamide was used to inhibit acid secretion in 1960s, ulcer symptoms, with reversible metabolic acidosis, BUT lots of SE (electrolyte losses, used Na/K/Mg salts to help, renal colic, headache, fatigue, etc)Later found H. Pylori encodes for two different CasHelps to acclimatize to acidic environmentBasically, the Ca changes CO2 into H+ and HCO3They also have a urease which produces NH3The NH3 binds with H+, leaving an alkaline environment for them to live inInhibition of CA with acetazolamide is lethal for pathogen in vitro1940sFound there was CA in pancreasThought acetazolamide to reduce volume of secretions from NGT (output from exocrine pancreas) Source: Human Anatomy at Colby Blog Diuretic resistanceIf develop hyperchloremic metabolic alkalosis, short course of acetazolamide + spironolactone (b/c need distal Na blockage) à can helpMay help with urine alkalization (ie uric acid stone) but increases risk of calcium phosphate stonesADVOR trial acetazolamide in HF exacerbation in Belgiumuse may help to prevent new episode, lower total diuretic doseCSF reduction (pseudotumor cerebri)Reduces CSF by as much as 48% when > 99.5% of CA in choroid plexus is inhibitedNORDIC trial (acetazolamide v. placebo) – improvement in visual symptoms especially if advanced papilledema, and reduced opening pressure)Side note also used off label to help with increased ICP and CSF leaks, as alternative to VP shunts, repeat LPs, etc Source: Eftekari et al, Fluid Barriers CNS, 2019.

The exciting conclusion to Chapter Two: Renal Circulation and Glomerular Filtration Rate - Determinants of GFR    - First step in making urine is separation of an ultrafiltrate    - Governed by starling forces        - Balance of hydraulic and osmotic forces        - GFR = LpS (P gc – P us - Osmotic Pressure Cap p)            - Normal GFR 95 in women, 120 in men            - Cap Hydrolic pressure remains constant             - glom cap Oncotic progressively rises                 - Due to filtration of protein free fluid (protein concentration rises in the capillary)            - Filtration gradient begins at 13 mmHg and falls to zero after filtration of 20% or RPF!            - GFR is capped at 20% of RPF called filtration equilibrium            - So GFR is dependent on RPF, unless you can change glomerular hydraulic pressure        - Glomerular hydraulic pressure is controlled by balance of twin arteriole (afferent and efferent)            - Constriction of afferent arteriole reduces RPF, GFR, and glom pressure            - Dilation of afferent arteriole increases RPF, GFR, and glom pressure            - Constriction of the efferent arteriole increases Glom pressure, increasing GFR        - Besides glom hydrostatic pressure the other starlings forces are rarely relevant to changes in GFR Letty says: referred to this NEJM review article later JC thought she was referring to something else -see #2- and then Roger referred to this again)Normotensive Acute Renal Failure from Gary Abuelo in NEJM 2007. https://www.nejm.org/doi/10.1056/NEJMra064398 (note in this article, Dr. Abuelo acknowledges the newer terminology of the time, AKI rather than ARF but chooses not to embrace it). In figure 2, he highlights the classic examples of how autoregulation can be affected. In the table, additional examples are provided but all within the framework of alterations related to autoregulation and the interplay between the two resistance vessels. - Regulation of GFR    - Autoregulation        - The ability to keep glomerular pressure constant over wide range of systemic arterial pressure        - When pressure < 70 autoregulation fails and GFR will fall with decreases in systemic pressure        - When pressure falls below 40-50 GFR ceases        - At least some of this autoregulation is mediated with Ang2. Giving ACEi markedly disrupts autoregulation        - Nitric oxide, not important    - TGF         - Chloride in macula densa            - Blocked by furosemide            - Group affect of nephrons         - Ang 2 sensitizes        - Adenosine mediates        - Function of TGF            - 90% of filtrate is reabsobed in PT and LOH                - 10% is reabsobed dismally                - Need to control the amount of fluid delivered distally to prevent overwhelming the resorptive capacity of the distal nephron                - Talks about acute renal success without naming it (but did reference it)                - Mentions glucosuria blunts TGF. Hmmm...    - Neurohormonal influences         - Volume changes in ang2, sympathetic NS        - Role of PGE        - Interesting discussion of change of the nephrons perfumed with volume depletion, shifting of blood from outer coretex to inner medullary cortical gloms with their long loops        - Dopamine and ANP both increased with volume up            - Dopamine causes vasodilation of afferent and efferent arteriole            - ANP causes afferent vasodilation and efferent vasodilation constriction, increasing GFR without affecting RPF    - Glomerular hemodynamics and renal failure        - Decreased glomerular mass results in hyperfiltration of remaining gloms        - Mediated through afferent vasodilation JC talks about this classic study in critical care: High vs. Low blood pressure target in Septic Shock. https://www.nejm.org/doi/pdf/10.1056/NEJMoa1312173In this multi-center open label trial of 776 patients randomized to either a MAP of 65-70 or 80-85 with the primary endpoint of mortality. There was no difference in mortality at 28 days between the two groups (but a small difference in AKI in the patients who had chronic HTN- in the higher BP target, there was a decrease in need for RRT; there was also a higher incidence of afib in the high target group overall).          - Results in compensation and stable GFR in short term, long term maladaptive        - Reason for ACEi- Clinical Evaluation of Renal Circulation    - Concept of clearance and measurement of GFR        - GFR as an index of functioning renal mass            - Had a patient today s/p nephrotomy, 72 years old, Cr0.9! Melanie referred to this article in Circulation which demonstrates that SGLT2 inhibitors do decrease single nephron GFR (in mice) and that this is related to a decrease in the afferent arteriole diameter and then they show that this is related to a local increase in adenosine.  Kidokoro K, Cherney DZI et al. Evaluation of glomerular hemodynamic function by empagliflozin in diabetic mice using in vivo imaging  Circulation 140 (4) 2019https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.118.037418         - Fall in GFR earlier and only sign of renal disease        - Serial monitoring is used to assess severity and follow the course of disease        - GFR is useful for dosing drugs        - How to measure GFR             - Consider fructose polysaccharide inulin (love the parenthetical, not insulin)                 - Inulin filtered = inulin excreted                    - Filtered inulin = plasma inulin concentration x GFR                    - Inulin excreted = urine concentration x urine volume                    - Use Alber a to get GFR = [Urine]insulin x urine volume  / [plasma]inulin        - GFR = inulin clearance                    - There is not an available assay for inulin            - Creatinine clearance                - Freely filtered                - Not reanbsorbed                - Not metabolized                - Small amount excreted                    - CrCl exceeds GFR by 10-20% Roger says the SGLT2 inhibitor story is about the afferent arteriole and he thought it reminded him of the MDRD study and the concept that the lower protein intake would be protective and delay the progression of CKD. The concept was that low protein diets would decrease glomerular pressure by decreasing the intake of amino acids that lead to arteriolar vasodilation and increased GFR. Klaur S, Levey AS et al. The effects of Dietary Protein Restirciton and blood-pressure control on the progression of chronic renal disease. NEJM 1994 330:877-884. https://www.nejm.org/doi/full/10.1056/nejm199403313301301                     - Compensated for by noncreatinine chromogens (acetone proteins, as Orbi acid, pyruvate) that over estimate Cr by 10-20%                 - Cr Cl = [Urine]cr x urine volume / [Plasma]cr                - Two major limitations                    - Incomplete collections                        - 20-25 mg/kg in adult men                        - 15-20 mg/kg in adult women The term “Acute renal success” comes from Thurau K and Boylan JW. Acute renal success. The unexpected logic of oliguria in acute renal failure. Am J Med 1976  61(3): 3038-15.                          - Falls by 50% from age 50 to 90 to 10 mg/kg                    - Increased tubular secretion with decreased kidney function                         - GFR of 40-80 cr secretion may account for as much as 35% of creatinine excretion                        - In some cases CrCl can exceed GFR by a factor of 2                        - Give cimetidine 1200 mg!                - It is important to appreciate however that exact knowledge of GFR is not required. More important to know if GFR is changing                - Why is radio labeling the solution DTPA and iothalamate?                - Talks about the reality of progressive disease despite stable GFR and CrCl            - On to plasma Cr and GFR If you think placing dialysis lines is too easy, here is a wonderful review of micropuncture technique in the kidneys by Volker Vallon.Micropuncturing the Nephron. Pflugers Arch 2009 458(1): 189-201. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2954491/                 - Creatinine excretion = creatinine production (and this is constant)                    - Creatinine excretion = [Cr] x GFR = constant                    - If GFR falls in half, creatinine excretion will fall in half, while creatinine production remains the same, so creatinine will rise and rise until [Cr] x GFR = creatinine production and then it will level off.                - Changes in creatinine load                    - High protein diet can increase it                    - Vegetarian diet can decrease it JC brought up studies on fenoldopam, of which there are many. This is one such study in patients undergoing cardiac surgery. JAMA 2014 Bove T et al. Effect of fenoldopam on use of renal replacement therapy among patients with acute kidney injury after cardiac surgery: a randomized clinical trial https://pubmed.ncbi.nlm.nih.gov/25265449/                     - Cooked meat can increase Cr by 1 mg/dL                - Talks about need for steady state to assess GFR                - Talks about the curvilinear relationship                - Then he talks Cockcroft Gault The one, the only: The Cockcroft Gault: Prediction of creatinine clearance from serum creatinine. Nephron 16: 31–41, 1976 https://pubmed.ncbi.nlm.nih.gov/1244564/                 - Cirrhosis masks kidney insufficiency, low meat intake, low BUN production                - Can someone explain what we are supposed to take from figure 2-12                - Stable Cr does not mean stable kidney disease Roger describes the study design for the seminal paper on the use of ACE inhibitors to slow the decline in renal function in diabetic kidney disease (then called diabetic nephropathy) and the decision to use the doubling of the serum creatinine as an endpoint.  Lewis EJ The effect of Angiotensin-converting-enzyme inhibition on diabetic nephropathy NEJM 1993 https://www.nejm.org/doi/full/10.1056/NEJM199311113292004                     - Ketoacidosis can raise the Cr 0.5 to 2.0mg/dL    - On to BUN        - Destination of amino acids produces ammonia        - We detoxify ammonia by converting to urea            - Increased with increased protein load            - Increased catabolism Melanie mentioned an old study on ingestion of expired blood: Cohen TD. Induced azotemia in humans following massive protein and blood ingestion and the mechanism of azotemia in gastrointestinal hemorrhage. AM J Med Sci 1956 https://pubmed.ncbi.nlm.nih.gov/13302213/             - Tetracycline causes decreased anabolism            - Trauma            - Steroids        - Urea excretion is variable and tied to hydration and FF    - Renal plasma flow and PAH          

Back by popular demand…all two of you…the second chapter of The Clinical Physiology of Acid Base and Electrolyte Disorders. Chapter Outline- Renal Circulation and GFR    - RBF is 20% of cardiac output        - In terms of mL per 100 g organ weight it is 4x the liver and exercising muscle and 8x coronary blood flow!        - After the glomeruli the efferent arteriole have two fates            - Peritubular capillaries in the cortex                - Peritubular capillaries are not necessarily associated with their parent glomeruli. Weird.            - Vasa recta from juxtamedullary glomeruli in the medulla Joel Says: This seems wrong. Solute balance can be maintained down to a very low GFR. The R^2 here would be very low. Prove me wrong.     - States that GFR is an important determinant of solute and water excretion. - Glomerular anatomy and function    - Structure Four editions of the Bud Bible up top and a copy of Bud Light on the bottom.         - Glomerulus is a tuft of capillaries            - Enclosed in a capsule of epithelial cells, called Bowman’s capsule            - The epithelial cells of Bowman’s capsule are continuous with the epithelial cells of the proximal tubule Looking at scanning EMs of the glomerulus is one of life’s simple pleasures—Josh. Josh says: Look at the review in Nature Reviews Nephrology from Rachel Lennon’s groupComplexities of the glomerular basement membrane         - Filtration barrier             - Epithelial cell (podocyte)                - Epithelial cells adhere to the basement membrane via foot processes and the foot processes have slit diaphragms             - Basement membrane New Super-resolution structure of the GBM: https://elifesciences.org/articles/01149 Hi res microscopy is really hi-res. Technique is call ed STORM. Melanie talks about conduits through the glomeruli. Here is a cool review: Why until just now? Undiscovered uniqueness of the human glomerulus! by L. Gabriel Navar, Owen RichfieldAm J Physiol Renal Physiol. 2018 Nov 1; 315(5): F1345–F1346. Published online 2018 Aug 15. doi: 10.1152/ajprenal.00369.2018 PMCID: PMC6293291                 - Produced by both the endothelial cells and podocytes                - Formed from type IV collagen                    - Abnormalities of type 4 collagen cause Alport                        - The gene coding for the alpha 5 chain is the culprit                        - COL4A5                    - Abnormal Alpha 3 and 4 chains can also cause hereditary nephritis                - Has other substances                    - Laminin                    - Nidogen                    - Heparin sulfate proteoglycans                        - Provides the negative charge            - Enthothelial cell (fenestrated)        - Protein excretion            - Glomerular function: allow filtration of small solutes (Na and urea) while preventing filtration of larger molecules                - Insulin MW 5,200 is freely filtered (upper range of freely filtered)                - Preventing loss of protein prevents                    - Negative nitrogen balance                    - Development of hypoalbuminemia                    - Infection from loss of immunoglobulin                - Size and charge selectivity of the GBM                    - pores are between cords of type 4 collagen                    - The epithelial cells and slit diaphragms matter                        - Macromolecules that pass through GBM can accumulate underneath the epithelial layer                        - Isolated GBM in invitro studies is much more permeable to than intact glomerulus                        - There is increased protein filtration in areas where the epithelial cells have detached from the GBM Josh really likes this figure from another Nature Reviews Nephrology paper. This one by Moeller and Chia-Gil.                         - Mutations in nephrin, localized to the slit diaphragm causes congenital nephrotic syndrome                    - Charge selectivity is important                        - Neutral and cationic particle are more likely to be filtered                        - Albumin (negative charge) is filtered 5% as well as same size neutral dextrans                        - In glomerular disease, while there is increased filtration of proteins there is decreased filtration of small solutes due to loss of glomerular surface area JC says: Take a look at this research on the serving coefficient in glomerular disease. Some surprising results.Glomerular dysfunction in nephrotic humans with minimal changes or focal glomerulosclerosis                             - Why do people in remission have what appears to be spilling more high molecular radius particles than normal and why do patients with active MCD have lower clearance across all molecular diameters?        - Other glomerular functions Josh says: Take a look at this interesting paper by Butt et alA molecular mechanism explaining albuminuria in kidney disease             - Synthetic                - Epithelial cells produce GBM            - Phagocytic                - Remove circulating macromolecules that pass through GBM and get trapped in subepithelial space Josh says: The sFLT1 (soluble VEGF receptor) relationship to preeclampsia is just so cool. And here’s the paper:Excess placental soluble fms-like tyrosine kinase 1 (sFlt1) may contribute to endothelial dysfunction, hypertension, and proteinuria in preeclampsiaAnd in the NEJM: VEGF Inhibition and Renal Thrombotic Microangiopathy             - Endocrine                 - Enthothelial cells regulate vascular tone by releasing                    - Prostacyclin                    - Endothelin                    - Nitric oxide JC says: Do yourself a favor and spend some time learning about extraglomerular mesangial cells with Stuart Shankland Extraglomerular origin of the mesangial cell after injury. A new role of the juxtaglomerular apparatusJoel adds, if you ever get a chance to party with Dr. Shankland, don’t skip out.         - Mesangial cells, two types            - Intrinsic Mesangial cell                - Microfilaments similar to smooth muscle                - Responds to Ang2                - Regulates glomerular hemodynamics                - Can release cytokines                - Can respond to cytokines by proliferation            - Circulating macrophages and monocytes                - Phagocytic function                - Clear molecules that get through the endothelial wall but cant get through the GBM Josh says, “Topf, get it right. Its Ree-nin not renin. Classic letter to JAMA. - Renin-Angiotensin System    - Afferent arteriole contain specialized cells called juxtaglomerular cells        - Produce prorenin which cleaved into renin        - Stimuli for renin release            - Hypotension            - Volume depletion            - Increased sympathetic activity        - Renin catalyze the production of ang1 from angiotensinogen        - Ang1 is catalyze to Ang2 by ACE located in the            - Lung            - Endothelial cells            - Glomeruli itself pic.twitter.com/DaDfS7u8se— Roger Rodby (@NephRodby) February 22, 2021     - Discussion of local renin and Aniotensinogen        - Explains why ACEi are useful even with low systemic renin levels and Ang2    - Actions of Ang2        - Sodium and water retention            - By direct Na reabsorption in the early PT (and in the proximal tubule, water is permeable to the epithelium so every sodium reabsobed, brings a water molecule along for the osmotic ride.                - Stimulates the Na-H antiporter                - 40-50% of Na reabsorption in the S1 segment of the PT is due to Ang2            - By stimulation of aldosterone                - Ang2 that stimulates Aldo comes from the kidney and from the adrenal gland itself        - Vasoconstriction Josh talks angiotensin:Tenses the angios--love this Melanie!1961 paper from del Greco (who's endowed chair Dan Batlle has now) trying AT2 in "hopeless" patients and dialysis patients:https://jamanetwork.com/journals/jama/article-abstract/332265Great EM-crit/pulmcrit discussion here:https://emcrit.org/emcrit/deeper-vasopressors-athos-3/and caveats here:https://emcrit.org/pulmcrit/angiotensin-ii/             - Arteriolar vasoconstriction            - Ang2 important for raising BP in RAS            - Ang2 important in maintaining BP with volume depletion or in CHF, liver disease                - Giving ACEi to cirrhosis can cause BP to dump 25 points        - Regulation of GFR            - Affects constriction at afferent arteriole and efferent arteriole                - Mediated via thromboxane  JC talks about the ATHOS trial and how there is a signal for improved outcomes especially in patients requiring renal replacement therapy.Angiotensin II for the Treatment of Vasodilatory ShockOutcomes in Patients with Vasodilatory Shock and Renal Replacement Therapy Treated with Intravenous Angiotensin II                 - Afferent arteriole starts bigger so reductions have less of an effect than constriction does on the narrower efferent arteriole.                    - This results in a fall of RBF due to increased resistance but maintaining GFR by increasing inrtaglomerular pressure.                - Also stimulates prostaglandins which are vasodilator, modulating this affect Joel says: You haven’t heard of the Trolly Problem? Oh you need to take 5 minutes and read this.             - It can stimulate contraction of the mesangium reducing surface area of the glom reducing filtration.             - It sensitizes the afferent arteriole to TG feedback so it can reduce glomerular flow in response to increased chloride detection in the TLoH.    - Control of renin secretion Ever wanted to know about intrarenal renin concentrations? Yeah, me neither. But JC’s got you covered: Endogenous angiotensin concentrations in specific intrarenal fluid compartments of the rat.          - Primarily sodium intake, increased intake results in less renin        - Mediated by baroreceptors            - Baroreceptors in afferent vessel wall            - Cardiac and arterial baroreceptors which activate the sympathetic nervous system and catecholamines which then stimulates renin Roger says: Do your self a favor and read about Yanomamo IndiansBlood pressure and electrolyte excretion in the Yanomamo Indians, an isolated population             - Cells of the macula densa in the early distal tubule which detect decreased chloride delivery                 - This allows loop diuretics to be particularly effective at increasing renin as they block chloride resorption                - Suppression of renin in response to chloride is mediated by adenosine                - Stimulation of renin in response to decreased chloride is mediated by PGE                    - The PGE cause local vasodilation so the kidney maintained a rich blood flow while using renin and Ang2 to cause systemic vasoconstriction Anna’s notes for the deep dive in glomerular barrierOur understanding is based on technology available at the time. Even in 1920s, there was thought that tubular reuptake of protein may be important, but studies never demonstrated this til 2007 and even then are debated. 2007 Russo, et al (and BM at IU!)  showed that The normal kidney filters nephrotic levels of albumin and that failure of retrieval by proximal tubule cells is what separates proteinuria from nonproteinuria.  This was countered by a study in 2009 demonstrating much lower GSC and suggesting that the high GSC in the 2007 could be the result of nonphysiologic states.Check out this 2008 debate in JASN regarding the validity of the charge model and “normal” albumin in the glomerular filtrate.  Hotly debated with too many studies to cite.  2017: Lawrence et al publish their findings that the GBM and podocyte processes are sufficient and the slit diaphragm likely does not exist. They used labeled proteins and confocal microscopy to determine migration of particles through the enodthelium and GBM. They also injected NaSCN oligoclusters from the size of albumin (66kDa)up to the size of IgG dimers (300 kDa) into mice, then fixed. The size-sensitive permeation into the lamina densa of the GBM and the podocyte glycocalyx of albumin and uptake of any “escaping” albumin by the proximal tubule was also observed. This countered the common prior conception that the slit diaphragms pores are the site of albumin “capture.” For your reading pleasure the review of Clinical Physiology of Acid-Base and Electrolyte Disorders Fourth Edition in Annals of Internal Medicine

Please enjoy the first episode of our book club. Join us as we start our journey through The Clinical Physiology of Acid Base and Electrolyte Disorders. Chapter outline:Hello and welcome to chapter one: Introduction to Renal FunctionSummary of kidney functionsMaintenance of extracellular environmentHormone secretionReninAng2PGENOEndothelinBradykininEpo1,25 DIs soluble Klotho another hormone that should be added to this list?Misc: catabolism of protein and gluconeogenesisIs that all?Where’s BP in that list? Renal MorphologyIntroduces the nephronFunny. No simple definition of a nephronCortex and medulla From Brenner Rector I take it those white bars at the top mean these are at the same magnification?Reabsorption and secretionProximal tubuleLoopLong vs short loopsDCTCCTDuct vs TubuleReabsorption and SecretionParacellular vs across the cellIs there such a thing as paracellular secretion?Typo page 8 last paragraph, describing NaK2Cl as being in the CCT rather than LOHWhy do we filter so much only to reabsorption 98-99% of the water, sodium chloride and bicarbLove table 1-1 The role of the tight junction (see detailed dive by Josh below)Comparison of leaky TJ in PT andTight TJ in MCTThen there is the Molitores sectionMembrane recyclingComposition of the urineThe composition of the extracellular compartment is constant because of the variability of the content of the urine  Atomic weight and molarityEquivalenceOsmotic pressure and osmolalityJosh’s script on Tight Junctions:For all the love we give to ion channels and other transcellular processes, it’s paracellular transport of fluid through tight junctions does the most work of reabsorbing salt and water in the proximal tubule.Tight junctions are ways that two opposing cells can get close together  and stay there so that they create a barrier. This creates an inside the cells “lumen” and an outside the cells “interstitium”. It  also creates two domains of the cell membrane—an “apical” lumen-facing side and a “basolateral” interstitium facing side. These become important because certain transporters are only localized to one of these sides. I think we’ll spend a lot of time talking about these side-specific ion channels and transporters in later installments. What I want to spend a few minutes on is the tight junction itself. I hadn’t realized how mysterious these things were. One review I saw called them “the most enigmatic of all adhesion complexes.” There’s still a lot we don’t know about how they work. What we do know is that each of the two cells involved in forming a tight junction expresses a transmembrane protein called a Claudin. The extracellular domains of claudins fit together like the teeth of a zipper. A tight junction between two cells is made up of multiple Claudin zippers; the more zippers present, the tighter the junction.While these junctions are called “tight” they actually still let some things through. They do this through two mechanisms: called “pores” and “leaks"I want to talk about pores first. Just like individual particles of sand can get in between the teeth of a coarse toothed zipper, really small molecules, like single ions, can get through the pores formed by the Claudin teeth of a tight junction. Some claudin-claudin junctions allow for more pore-permeability, and some allow for  less. Some claudins seem to let only cations through, while others only let anions through. This ends up having clinical implications. Genetic deficiency of the magnesium-permeable Claudin, Claudin 16, results in tight junctions that don’t allow for normal magnesium reabsorption; this means that more magnesium remains in the tubule, and magnesium is wasted into the urine. The other mechanism by which tight junctions allow solutes to pass along the paracellular pathway is called “leakiness”. Leakiness is generated when tight junctions unzip, allowing cargos to move away from the lumen, and then re-zip, stopping the solutes from going back into the lumen. Like the opening and closing of locks in a canal, this process of opening and closing the individual Claudin zippers allows larger molecules to transit from the lumen to the interstitium. We still don’t understand how Claudin zippers decide to unzip and re-zip, but the number of zippers and the frequency of zipping probably play major roles in the transport of large solutes along the paracellular pathway. Different epithelial tight junctions have different levels of tightness. I was surprised to find out that really tight epithelia, like the ones in the bladder (where no fluid reabsorption occurs) have an electrical resistance that’s 50,000 times higher than the very leaky epithelia of the proximal tubule. The not-so-tight junctions of the proximal tubule are primed to reabsorb salt and water, and the tighter junctions of the downstream segments are critical to creating the barriers that allow for selective reabsorption of osmoles.However, it’s not clear that tight junctions are needed to create an apical and basolateral polarity—newer evidence shows that single cells in culture can still polarize (and because they don’t have neighbors, they don’t have tight junctions).