Control of fluid absorption in the renal proximal tubule

Abstract

A B S T R A C T Glomerulotubular balance was investigated in isolated, perfused rabbit proximal tubules in vitro in order to evaluate some of the mechanisms proposed to account for the proportionate relationship between glomerular filtration rate and fluid absorption generally observed in vivo. The rate of fluid transport from lumen to bath in proximal convoluted tubules in vitro was approximately equal to the estimated normal rate in vivo. The absorption rate in proximal straight tubules however was approximately one-half as great. If the mechanism responsible for maintenance of glomerulotubular balance is intrinsic to the proximal tubule, as has been proposed on the basis of micropuncture studies, the rate of fluid absorption in vitro should be directly related to the perfusion rate and/or tubule volume. In the present studies absorption rate was only minimally affected when perfusion rate was increased or the tubule distended. Thus, glomerulotubular balance is not mediated by changes in velocity of flow of the tubular fluid or tubular diameter and therefore is not an intrinsic property of the proximal tubule. It has also been proposed that glomerulotubular balance results from a humoral feedback mechanism in which angiotensin directly inhibits fluid absorption by the proximal convoluted tubule. In the present experiments, angiotensin was found to have no significant effect on absorption rate.

Mechanosensory function of microvilli of the kidney proximal tubule

“…Burg and Orloff (20) had observed that in rabbit proximal tubule a 3-fold increase in perfusion rate produced a 37% increase in volume reabsorption, which was not statistically significant (Table 2 in ref. 20). However, this finding occurred in association with a 41% increase in tubule diameter.…”

“…Accordingly, 3 H-inulin was added to the luminal perfusate as a volume marker, and fluid reabsorption was calculated by 3 H-inulin-concentration changes between the original and collected fluid (20). First, the flow-dependent proximal tubule transport was examined under the control condition in individual tubules.…”

An inhibitory effect of furosemide on sodium reabsorption by the proximal tubule of the rat nephron

“…to a large extent, inferential evidence, it has been widely held until recently that the cross-sectional area of the lumen of the proximal tubule is the principal determinant of the rate of sodium reabsorption by this segment (19)(20)(21)(22). More recent studies in the rat (11,23) and rabbit (24) have provided information that strongly opposes this interpretation. In these studies despite marked increases in calculated and measured values for tubule radius produced by elevation of ureteral pressure, renal venous occlusion and dilatation of isolated perfused proximal tubules, respectively, far less than proportionate increases, and often reductions in absolute reabsorptive rate were noted.…”

Physical properties of isolated perfused renal tubules and tubular basement membranes

“…Isolated intact tubules absorb salt and water by active transport at the 280C temperature employed in these experiments. We assumed that the rate of active transport was unaffected by changes in tubular geometry (9). Hydraulic conductivity was calculated from the change in transmembrane flow occurring in response to a change in applied hydrostatic pressure.…”