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).