Human Cortical Distal Nephron: Distribution of Electrolyte and Water Transport Pathways

ABSTRACT. The exact distributions of the different salt transport systems along the human cortical distal nephron are unknown. Immunohistochemistry was performed on serial cryostat sections of healthy parts of tumor nephrectomized human kidneys to study the distributions in the distal convolution of the thiazide-sensitive Na-Cl cotransporter (NCC), the β subunit of the amiloride-sensitive epithelial Na channel (ENaC), the vasopressin-sensitive water channel aquaporin 2 (AQP2), and aquaporin 3 (AQP3), the H+ ATPase, the Na-Ca exchanger (NCX), plasma membrane calcium-ATPase, and calbindin-D28k (CaBP). The entire human distal convolution and the cortical collecting duct (CCD) display calbindin-D28k, although in variable amounts. Approximately 30% of the distal convolution profiles reveal NCC, characterizing the distal convoluted tubule. NCC overlaps with ENaC in a short portion at the end of the distal convoluted tubule. ENaC is displayed all along the connecting tubule (70% of the distal convolution) and the CCD. The major part of the connecting tubule and the CCD coexpress aquaporin 2 with ENaC. Intercalated cells, undetected in the first 20% of the distal convolution, were interspersed among the segment-specific cells of the remainder of the distal convolution, and of the CCD. The basolateral calcium extruding proteins, Na-Ca exchanger (NCX), and the plasma membrane Ca2+-ATPase were found all along the distal convolution, and, in contrast to other species, along the CCD, although in varying amounts. The knowledge regarding the precise distribution patterns of transport proteins in the human distal nephron and the knowledge regarding the differences from that in laboratory animals may be helpful for diagnostic purposes and may also help refine the therapeutic management of electrolyte disorders.

Distribution of transcellular calcium and sodium transport pathways along mouse distal nephron

First published August 15, 2001; 10.1152/ajprenal. 00085.2001.—The organization of Na+ and Ca2+ transport pathways along the mouse distal nephron is incompletely known. We revealed by immunohistochemistry a set of Ca2+ and Na+transport proteins along the mouse distal convolution. The thiazide-sensitive Na+-Cl− cotransporter (NCC) characterized the distal convoluted tubule (DCT). The amiloride-sensitive epithelial Na+ channel (ENaC) colocalized with NCC in late DCT (DCT2) and extended to the downstream connecting tubule (CNT) and collecting duct (CD). In early DCT (DCT1), the basolateral Ca2+-extruding proteins [Na+/Ca2+ exchanger (NCX), plasma membrane Ca2+-ATPase (PCMA)] and the cytoplasmic Ca2+-binding protein calbindin D28K (CB) were found at very low levels, whereas the cytoplasmic Ca2+/Mg2+-binding protein parvalbumin was highly abundant. NCX, PMCA, and CB prevailed in DCT2 and CNT, where we located the apical epithelial Ca2+ channel (ECaC1). Its subcellular localization changed from apical in DCT2 to exclusively cytoplasmic at the end of CNT. NCX and PMCA decreased in parallel with the fading of ECaC1 in the apical membrane. All three of them were undetectable in CD. These findings disclose DCT2 and CNT as major sites for transcellular Ca2+ transport in the mouse distal nephron. Cellular colocalization of Ca2+ and Na+ transport pathways suggests their mutual interactions in transport regulation.

Superficial distal and deep nephrons in correction of metabolic alkalosis

Chloride is necessary and sufficient to correct alkalosis induced by dialysis vs. 0.15 M NaHCO3. To determine the contribution of the cortical (SC) distal convolution (DCT) and juxtamedullary (JM) nephrons to correction, we examined Cl and total CO2 (tCO2) transport in alkalotic Sprague-Dawley rats infused with 5% dextrose (group DM) or with 5% dextrose and 80 mM Cl (group CC); in papillary studies in alkalotic Munich-Wistar rats, 6% albumin was added to the infusate. In cortical studies, changes in plasma Cl and tCO2 were 4.9 +/- 0.7 vs. 0.7 +/- 0.9 and -6.0 +/- 0.8 vs. 0.4 +/- 0.9 meq/l and in tCO2 excretion (133 +/- 28 vs. -8 +/- 10 mueq/min) in groups CC and DM, respectively; results in papillary studies were similar. Delivery of tCO2 out of late SC DCT (CC 146 +/- 20 and DM 146 +/- 23 pmol/min) and Henle's loop (CC 145 +/- 18 and DM 202 +/- 56 pmol/min) and reabsorption within DCT (CC 15 +/- 24 and DM 45 +/- 19 pmol/min) did not differ. During correction of chloride-depletion alkalosis, the increment in bicarbonate excretion does not emanate from DCT of SC nephrons or JM nephrons but rather from the collecting duct.

Chloride-depletion alkalosis with a normal extracellular fluid volume

Current concepts hold that volume expansion is essential to the correction of chloride-depletion alkalosis (CDA) with chloride repletion in a permissive role. In this scheme, intranephronal fluid reabsorption would be redistributed with increased delivery to the distal nephron where the provided chloride is readily reabsorbed and the limited capacity for bicarbonate reabsorption would promote bicarbonate excretion and correction of CDA. In a model of CDA produced by peritoneal dialysis against 0.15 M NaHCO3, we have shown complete correction of CDA within 24 h without volume expansion by either oral isotonic sodium or chloride salts with 70 mM chloride and despite an obligatory bicarbonate load and negative sodium and potassium balance. During correction of CDA without volume expansion in rats by intravenous isotonic fluids containing 80 mM chloride, fractional fluid and chloride reabsorptions in the proximal convoluted tubule and in the loop segment of superficial nephrons were not different from controls but chloride reabsorption was enhanced in the collecting duct segment and probably within the distal convolution. Despite no differences in serial hematocrits, blood pressure, and measured plasma volume, kidney and nephron glomerular filtration rate (GFR) were reduced in CDA and returned to normal upon recovery 24 h later.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of phosphate deprivation on phosphate reabsorption in rat nephron: role of PTH

The sites of enhanced phosphate (PO4) reabsorption after PO4 deprivation were investigated before and after infusion of parathyroid hormone (PTH) in acutely thyroparathyroidectomized rats. Animals were fed either a control PO4 diet (1.6% P) or a low PO4 diet (0.025% P) for 2 days or 7-10 days. In control rats, PTH decreased PO4 reabsorption in the proximal tubule, loop of Henle, and distal convolution. PO4 reabsorption in the proximal tubule was enhanced after 2 days of PO4 deprivation. In this group, proximal PO4 reabsorption was decreased by PTH but remained greater than in control rats (70 +/- 6 vs. 45 +/- 6 pmol/min; P less than 0.025). After PTH, PO4 reabsorption increased in the loop of Henle from 3 +/- 0.5 to 13 +/- 2 pmol/min (P less than 0.005), whereas it was unaltered in the distal convolution in PO4-deprived rats. PTH markedly increased fractional excretion of PO4 in control rats but not in PO4-deprived rats. After prolonged PO4 deprivation, PO4 reabsorption along the nephron was unaltered by PTH. These results demonstrate that acute PO4 deprivation enhances PO4 reabsorption in the proximal tubule, although the phosphaturic effect of PTH in this segment is not abolished. Resistance to the inhibitory effect of PTH on PO4 reabsorption in some portion of the loop of Henle and possibly also in the distal convolution accounts for the absence of a significant phosphaturic effect of the hormone in acutely PO4-deprived rats. Prolongation of PO4 deprivation results in unresponsiveness to PTH extending to the proximal tubule.

Net transtubular movement of water and urea in saline diuresis

Anesthetized rats were given urea-C14 intravenously and infused with 5% NaCl solution containing inulin-methoxy-H3. Fluid was collected by micropuncture from surface tubules for simultaneous determination of H3 and C14 activities and osmolality. Inulin clearances, plasma and urine sodium, urine volume, and urea excretion were all greatly increased, but fractional water and urea reabsorption in the proximal convolution was similar to that in nondiuretic rats. Inulin fluid/plasma ratios in hypotonic early distal samples were slightly lower than those observed in nondiuretic rats. Urea/inulin ratios in distal samples and in ureteral urine were similar to the late proximal convolution, indicating little or no addition of urea in the loop of Henle or loss from distal convolution or collecting ducts, in contrast to nondiuretic rats. These results suggest that in the proximal convolution sodium (and therefore water) reabsorption is proportional to the rate of delivery of sodium to the tubule. Furthermore, the absence of demonstrable net transtubular movement of urea beyond the proximal convolution under conditions in which concentration gradients are reduced is consistent with the hypothesis that urea movement is largely passive.