Characteristics of Volume Reabsorption in Rabbit Superficial and Juxtamedullary Proximal Convoluted Tubules

Abstract

A B S T R A C T Segments of superficial and juxtamedullary proximal convoluted tubules of the rabbit were perfused in vitro to examine the mechanisms responsible for net volume reabsorption. The very early postglomerular segments were not studied. Fluid reabsorptive rates and transepithelial potential differences were compared under various conditions: (a) with perfusate that simulated glomerular filtrate; (b) with perfusate that lacked glucose, amino acids, and acetate and that had HCO3 and Cl concentrations of 5 and 140 mM, respectively; (c) with perfusate that lacked glucose, amino acids, and acetate but with 20 meq ofNaHCO3 replaced with 20 meq ofNa cyclamate; (d) with the same perfusate as in b but in the presence of ouabain in the bath; (e) with ultrafiltrate of rabbit serum titrated with HC1 to final HCO3 and Cl concentrations of 2 and 134 mM, respectively. Tubules were perfused with this titrated ultrafiltrate at 37°C, 21°C, and in the presence of 0.1 mM ouabain in the bath. Bath fluid in all experiments was regular rabbit serum. Under conditions a and b superficial proximal convoluted tubule (SFPCT) and juxtamedullary proximal convoluted tubule (JMPCT) behaved similarly with the exception that SFPCT exhibited a lumen-positive and JMPCT a lumen-negative electrical potential under condition b. However, undeT condition c SFPCT failed to exhibit net volume reabsorption, whereas reabsorption in JMPCT continued unchanged. Ouabain did not affect volume reabsorption in SFPCT under condition

Calcium transport in the rabbit superficial proximal convoluted tubule.

“…For example, in clearance and micropuncture experiments in rabbits by Chonko et al (36), the J, rate in the proximal convoluted tubule was 1.9 nl/mm * min. However, most studies of J, in the proximal tubule in vitro find 50% of this rate, i.e., 0.8-1.0 nl/mm * min (15,18,33,37 The finding of passive calcium transport in the S2 segment of the superficial proximal convoluted tubule of the rabbit is consonant with the published literature that suggests that there is an interdependence of sodium and calcium (38,39). Those maneuvers, which depress proximal tubular reabsorption of sodium and water, such as volume expansion (38), PTH, (40), insulin (41), renal vasodilators (42), and acetazolamide (43), also similarly depress proximal tubular reabsorption of calcium, which results in parallel excretion of sodium and calcium in the final urine.…”

Neonatal rabbit juxtamedullary proximal convoluted tubule acidification.

“…Transporter activity was assayed by continuously monitoring fluorescence at 500 nm, the pH-sensitive wavelength, as either the luminal or bathing solution was changed. The initial rate of change (dF~w/dt) was used to calculate the initial rate ofchange in cell pH, dpH1/dt2: dpHi dF500/dt dt F45o X S' (1) where F430 is the pH-insensitive wavelength interpolated from readings before and after the fluid exchange and Sis the slope ofthe pH calibration curve relating F5w/F45o to pH. Once a steady state was reached, three measurements at 500 and 450 nm were made.…”

“…Clearance studies examine the composite of several nephron segments which may each be at different stages of maturation. These studies do not differentiate differences in transport between superficial and juxtamedullary nephrons (1)(2)(3). This problem of nephron heterogeneity encountered in adult animals is accentuated in the developing kidney where there is a centrifugal pattern of nephron maturation (4)(5)(6)(7)(8).…”

Apical membrane chloride/base exchange in the rat proximal convoluted tubule.

“…Sodium chloride absorption across the proximal convoluted tubule (PCT)' occurs by paracellular and transcellular pathways (1)(2)(3)(4). Transcellular NaCl absorption is electroneutral (3,5,6), but the exact apical and basolateral membrane mechanisms remain obscure.…”