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
Proximal Tubule Sodium Reabsorption Proximal Tubule Epithelial Cell ATP Tubular Lumen (urine) Capillary Lumen (blood) 2K + 3 Na + Na + H+H+ H+H+ H2OH2O 3 HCO 3 - OH - CO 2 Na + glucose amino acids PO 4 2-
Loop of Henle Sodium Reabsorption – thick ascending limb Ascending Thick Limb of the Loop of Henle Epithelial Cell ATP Tubular Lumen (urine) Capillary Lumen (blood) 2K + 3 Na + Na + K+K+ ROMK channel K+K+ K + recycling Na + Ca +2 Mg +2 Paracellular Pathway + - 2 Cl - Cl -
Flatman (2008) Curr Opin Nephrol Hypertens 17: 186-192 Bartter’s syndrome: hypotension with salt wasting and hypokalemia as a result of loss of Na reabsorption in the thick ascending limb 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. Ascending limb of the loop of Henle
ATP Tubular Lumen (urine) 2K + 3 Na + Na + Cl - Distal Tubule Sodium Reabsorption Capillary Lumen (blood) K+K+ Distal Tubule Epithelial Cell Gitelman’s disease: a mutation in the Na, Cl cotransporter; results in sodium wasting and hypotension
ATP 2K + 3 Na + Na + Capillary Lumen (blood) K+K+ Collecting Tubule Tubular Lumen (urine) K+K+ +- Collecting Tubule Epithelial Cell Principal Cell
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
Silverthorn Figure 19-12 Actions of Aldosterone (the simple version)
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 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.
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
ENaC Ubiquitin Ligase Ubiquitin Internalization and Degradation N C PY Ubiquitin Ligase WW DomainsC2 Nedd4 N C PY N C Ubiquitination X X X Liddle’s mutations disrupt the interaction between Nedd4 and ENaC Mutations or Truncations β or γ subunit Liddle’s syndrome is an autosomal dominant form of salt sensitive hypertension
Expression of major sodium and potassium transport proteins in the distal nephron, including WNK kinases and associated regulatory proteins. Hoorn E J et al. JASN 2011;22:605-614
So, Aldosterone is important, but what controls aldosterone secretion?
Silverthorn Figure 19-13 AngII is the major stimulus of aldosterone secretion
renin secreting cells of the juxtaglomerular apparatus Factors controlling renin secretion
Schematic depiction of the renin–angiotensin system components 2013 Carey R. Hypertension 2013;62:818-822 Decarboxylation of asp to ala in position 1 Only in tissue 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.
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. G.F. DiBona Sympathetic influences on renal function
A quick word about ‘salt appetite’ At least in some species, Na + deficit elicits the motivation for salt seeking and increased salt intake
Flow chart of responses to severe dehydration : Silverthorn Figure 19-17
Figure 29-1 Normal potassium intake, distribution of potassium in the body fluids, and potassium output from the body. Now switching to K +
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.
Figure 29-3 Mechanisms of potassium secretion and sodium reabsorption by the principal cells of the late distal and collecting tubules. ROMK
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.)
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.
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.)
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.
Model of the “aldosterone paradox.” Two pathophysiological settings are depicted: hypovolemia (left) and hyperkalemia (right). 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.
Regulation of parathyroid hormone secretion by Ca ++ Point: PTH secretion is directly sensitive to changes in blood Ca ++
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?)