Consider the following case: Blood volume decreases, so we retain more water (i.e., what we discussed during the previous class) But that would cause dilution of the body fluids … 4-1

4-2

Tubular reabsorption of Na Proximal Tubule 65% Distal Tubule 3-5% Loop of Henle 25-30% Collecting Tubule 1-3% Glomerulus Filter ~ 1 lb of Na daily Tubular reabsorption of Na 4-3

Proximal Tubule Sodium Reabsorption Proximal Tubule Epithelial Cell Capillary Lumen (blood) Tubular Lumen (urine) Na+ 2K+ ATP H+ 3 Na+ H+ Na+ Na+ H2O glucose amino acids PO42- 3 HCO3- OH- CO2 4-4

+ - Loop of Henle Sodium Reabsorption – thick ascending limb Na+ 2K+ Ascending Thick Limb of the Loop of Henle Epithelial Cell Capillary Lumen (blood) Tubular Lumen (urine) Na+ 2K+ ATP 2 Cl- 3 Na+ K+ K+ recycling K+ Cl- ROMK channel + - Na+ Ca+2 Mg+2 Paracellular Pathway 4-5

Ascending limb of the loop of Henle Point: if there are abnormalities in the various transport processes involved in Na+ reabsorption, there are serious consequences in terms of volume and osmotic regulation. Bartter’s syndrome: hypotension with salt wasting and hypokalemia as a result of loss of Na reabsorption in the thick ascending limb 4-6 Flatman (2008) Curr Opin Nephrol Hypertens 17: 186-192

Distal Tubule Sodium Reabsorption Capillary Lumen (blood) Distal Tubule Epithelial Cell Tubular Lumen (urine) Na+ 2K+ ATP Cl- 3 Na+ K+ Gitelman’s disease: a mutation in the Na, Cl cotransporter; results in sodium wasting and hypotension Cl- 4-7

- + Collecting Tubule 2K+ Na+ 3 Na+ K+ K+ Collecting Tubule Epithelial Cell Principal Cell Capillary Lumen (blood) Tubular Lumen (urine) 2K+ ATP Na+ 3 Na+ K+ K+ Here, ROMK and ENaC are the critical players. - + 4-8

Regulation of Tubular Na+ Transport Hormones regulate the extracellular fluid volume by altering renal Na+ excretion Hormones that enhance tubular Na+ reabsorption angiotensin II arginine vasopressin Aldosterone Hormones that inhibit tubular Na+ reabsorption atrial natriuretic peptide natriuretic factors Ouabain Ouabain analogs 4-9

Actions of Aldosterone (the simple version) Silverthorn Figure 19-12 4-10

The key point of the next few slides is that the actions of aldosterone and the regulation of Na+ transport are quite complex, and abnormalities in these regulatory processes can result in diseases. Model for the early transcriptional action of aldosterone on ENaC function. The ubiquitin ligase Nedd4-2 that tonically inhibits ENaC surface expression is highlighted in red, and red dashed arrows indicate pathways downregulating ENaC that are antagonized by aldosterone. Blue boxes represent the regulatory proteins implicated in the early aldosterone action that are rapidly induced via activated MR. The blue lines terminated by a dash indicate at what level these regulatory proteins interfere with ENaC inhibition. Verrey et al. Kidney International (2008) 73: 691-696 4-11

Figure 1. Regulation of ENaC activity in the distal nephron Left-hand panel: Segments of the distal nephron including the distal convoluted tubule (DCT; dark grey), connecting tubule (CNT; black), cortical collecting duct (CCD; light grey) and medullary collecting duct (MCD; white) are shown. The juxtamedullary nephrons have a long connecting tubule. Right-hand panel: Schematic of the principal cell of the connecting tubule or cortical collecting duct. Aldosterone (Aldo) binds mineralocorticoid receptor (MR) in the nucleus to stimulate expression of several genes: some important examples are shown. Aldosterone may also have non-genomic effects in the cortical collecting duct/connecting tubule (not shown). Nedd4 and Nedd4-2 promote endocytosis and ubiquitination of epithelial sodium channel (ENaC). Serum- and glucocorticoid-inducible kinase 1 (SGK1) regulates ENaC via inhibition of Nedd4-2 and possibly by a direct mechanism. Furin and channel-activating protease (CAP) proteins activate ENaC by proteolysis. Ub(n) is a polyubiquitin moiety and ER is the endoplasmic reticulum. From:   Thomas: Curr Opin Nephrol Hypertens, Volume 13(5).September 2004.541-548 4-12

4-13

Liddle’s mutations disrupt the interaction between Nedd4 and ENaC PY N C PY Internalization and Degradation Ubiquitination ENaC X X Ubiquitin N C PY X Nedd4 C2 WW Domains Ubiquitin Ligase Ubiquitin Ligase Liddle’s syndrome is an autosomal dominant form of salt sensitive hypertension Mutations or Truncations β or γ subunit 4-14

Expression of major sodium and potassium transport proteins in the distal nephron, including WNK kinases and associated regulatory proteins. Expression of major sodium and potassium transport proteins in the distal nephron, including WNK kinases and associated regulatory proteins. See text for abbreviations. Hoorn E J et al. JASN 2011;22:605-614 4-15

So, Aldosterone is important, but what controls aldosterone secretion? 4-16

AngII is the major stimulus of aldosterone secretion Silverthorn Figure 19-13 4-17

renin secreting cells of the juxtaglomerular apparatus Factors controlling renin secretion 4-18

Schematic depiction of the renin–angiotensin system components 2013 Only in tissue Decarboxylation of asp to ala in position 1 Schematic depiction of the renin–angiotensin system components and selected actions. The enzymes of the system are shown in red. Newly described enzymatic pathways are shown in yellow. Receptors are shown in the boxes. ACE indicates angiotensin-converting enzyme; Agt, angiotensinogen; Ang, angiotensin; APA, aminopeptidase A; AT1R, angiotensin type-1 receptor; AT2R, angiotensin type-2 receptor; MasR, Mas receptor; MrgD, Mas-related G-protein–coupled receptor; and PRR, (pro)renin receptor. Point here: there is a lot more going on with the renin-angiotensin system than just the classical stuff represented by the blue line in the figure. Carey R. Hypertension 2013;62:818-822 4-19

Sympathetic influences on renal function G.F. DiBona Fig. 1. Effects of increased renal sympathetic nerve activity (RSNA) on the 3 renal neuroeffectors: the juxtaglomerular granular cells (JGCC) with increased renin secretion rate (RSR) via stimulation of 1-adrenoceptors (AR), the renal tubular epithelial cells (T) with increased renal tubular sodium reabsorption and decreased urinary sodium excretion (UNaV) via stimulation of 1B-AR, and the renal vasculature (V) with decreased renal blood flow (RBF) via stimulation of 1A-AR. But these results are from studies in anesthetized animals, and there is evidence at in unanesthetized animals, renal blood flow varies with physiological changes in renal nerve activity 4-20

Atrial Natriuretic Hormone Germann figure 20.18 4-21 Silverthorn 19-15

A quick word about ‘salt appetite’ At least in some species, Na+ deficit elicits the motivation for salt seeking and increased salt intake 4-22

Issue of salt sensitivity of blood pressure, and dietary recommendations Percentage change in mean arterial pressure in normotensive subjects receiving incremental increases in sodium. Percentage change in mean arterial pressure in normotensive subjects receiving incremental increases in sodium. Blood pressure at the end of 7 days of low (10 mmol/d) salt intake was taken as baseline. All subjects demonstrated an increase in blood pressure with salt loading. Data adapted from Luft et al.13 Weinberger M H Hypertension 1996;27:481-490 (based on data from Luft et al., 1979) 4-23 Copyright © American Heart Association

Percentage change in mean arterial pressure in normotensive subjects receiving incremental increases in sodium. Current recommended Current US average Percentage change in mean arterial pressure in normotensive subjects receiving incremental increases in sodium. Blood pressure at the end of 7 days of low (10 mmol/d) salt intake was taken as baseline. All subjects demonstrated an increase in blood pressure with salt loading. Data adapted from Luft et al.13 Typical range for assessing salt sensitivity of BP Weinberger M H Hypertension 1996;27:481-490 (based on data from Luft et al., 1979) 4-24 Copyright © American Heart Association

Low salt: 20 mmol/day (460 mg) High salt: 240 mmol/day (5520 mg) 4-25

Flow chart of responses to severe dehydration: Silverthorn Figure 19-17 4-26

Now switching to K+ Figure 29-1 Normal potassium intake, distribution of potassium in the body fluids, and potassium output from the body. 4-27

Figure 29-2 Renal tubular sites of potassium reabsorption and secretion. Potassium is reabsorbed in the proximal tubule and in the ascending loop of Henle, so that only about 8 per cent of the filtered load is delivered to the distal tubule. Secretion of potassium into the late distal tubules and collecting ducts adds to the amount delivered, so that the daily excretion is about 12 per cent of the potassium filtered at the glomerular capillaries. The percentages indicate how much of the filtered load is reabsorbed or secreted into the different tubular segments. 4-28

4-29

ROMK Figure 29-3 Mechanisms of potassium secretion and sodium reabsorption by the principal cells of the late distal and collecting tubules. 4-30

Figure 29-4 Effect of plasma aldosterone concentration (red line) and extracellular potassium ion concentration (black line) on the rate of urinary potassium excretion. These factors stimulate potassium secretion by the principal cells of the cortical collecting tubules. (Drawn from data in Young DB, Paulsen AW: Interrelated effects of aldosterone and plasma potassium on potassium excretion. Am J Physiol 244:F28, 1983.) 4-31

Figure 29-5 Effect of extracellular fluid potassium ion concentration on plasma aldosterone concentration. Note that small changes in potassium concentration cause large changes in aldosterone concentration. 4-32

Figure 29-8 Effect of large changes in potassium intake on extracellular fluid potassium concentration under normal conditions (red line) and after the aldosterone feedback had been blocked (blue line). Note that after blockade of the aldosterone system, regulation of potassium concentration was greatly impaired. (Courtesy Dr. David B. Young.) 4-33

Figure 29-9 Effect of high sodium intake on renal excretion of potassium. Note that a high-sodium diet decreases plasma aldosterone, which tends to decrease potassium secretion by the cortical collecting tubules. However, the high-sodium diet simultaneously increases fluid delivery to the cortical collecting duct, which tends to increase potassium secretion. The opposing effects of a high-sodium diet counterbalance each other, so that there is little change in potassium excretion. 4-34

Model of the “aldosterone paradox Model of the “aldosterone paradox.” Two pathophysiological settings are depicted: hypovolemia (left) and hyperkalemia (right). Model of the “aldosterone paradox.” Two pathophysiological settings are depicted: hypovolemia (left) and hyperkalemia (right). Aldosterone acts as a sodium retaining hormone during hypovolemia, leading to a low urinary Na+ excretion (left). Conversely, aldosterone acts as a K+-secreting hormone during hyperkalemia, leading to a high urinary K+ excretion (right). Hypovolemia stimulates angiotensin II (Ang II), which in turn increases aldosterone (Aldo). Both contribute to renal Na+ retention. Ang II stimulates Na+ transport in the proximal tubule by activating the NHE3. Ang II also increases the activity of the NCC in the early DCT1. Aldo activates both NCC and the ENaC in the late DCT (DCT2), CNT, and CD. Note that because Na+ transport is stimulated at three locations, the distal delivery of Na+ decreases, contributing to the low Na+ excretion. The effects of Ang II and Aldo are primarily mediated via a WNK–SPAK pathway, whereas the effects of Aldo on ENaC primarily involves SGK1. Unknown factors increase the WNK1–KS–WNK1 ratio, leading to inhibition of the ROMK, helping to conserve potassium during hypovolemia. In the setting of hyperkalemia, the opposite occurs, because direct effects of a high serum K+ level decrease the WNK1–KS-WNK1 ratio (right). This leads to an activation of ROMK, stimulating potassium secretion. The lower WNK1–KS-WNK1 ratio also increases WNK4, preventing Aldo from activating NCC (dashed line). However, Aldo is still capable of activating ENaC, which stimulates Na+ exchange for K+ in the collecting duct. Hoorn E J et al. JASN 2011;22:605-614 The key point to take away from this is that the overall effect on Na+ reabsorption versus K+ secretion in the nephron is dependent upon whether the increase in aldosterone occurs with or without an increase in AngII. 4-35

A word about Ca++ homeostasis 4-36

Regulation of parathyroid hormone secretion by Ca++ Point: PTH secretion is directly sensitive to changes in blood Ca++ 4-37

Diuretic Drugs: (see table 31-1 in Guyton) Osmotic diuretics (e.g., mannitol) Loop diuretics (e.g., furosemide) Thiazide diuretics (e.g., hydrochlorothiazide) Aldosterone antagonists (e.g., spironolactone) Drugs that block Na channels in the collecting ducts (e.g., amiloride) Carbonic anhydrase inhibitors (e.g., acetazolamide) (notice that ADH antagonists are not on the list. Why?) 4-38

Is coffee a diuretic? 4-39

(Caffeine intake ~ 300 mg/day) TBW measured by dilution of D2O 40

In higher doses, caffeine is a diuretic; its action is mostly on the proximal tubule to reduce Na+ reabosorpton 41

Journal of Pharmacology and Experimental Therapeutics 313: 403-409, 2005 Caffeine dose: 45 mg/kg oral ~5-7 cups of coffee per day A1 knockout mice 42

And to put this back into physiology, the adenosine A1 receptors are required for the signaling of tubuloglomerular feedback! Macula densa J. Schnermann and J.P Briggs 43

Renal Failure Effects on plasma constituents of shutting down the kidneys (NPN = nonprotein nitrogen) 4-44

Renal Dialysis Table 31-7. Comparison of Dialyzing Fluid with Normal and Uremic Plasma Constituent Normal Plasma Dialyzing Fluid Uremic Plasma Electrolytes (mEq/L)   Na+ 142 133   K+ 5 1.0 7   Ca++ 3 3.0 2   Mg++ 1.5   Cl- 107 105   HCO3- 24 35.7 14   Lactate- 1.2   HPO4= 9   Urate- 0.3   Sulfate= 0.5 Nonelectrolytes   Glucose 100 125   Urea 26 200   Creatinine 1 6 4-45