Variation of proximal tubular reabsorptive capacity by volume expansion and aortic constriction during constancy of peritubular capillary protein concentration in rat kidney

Abstract

The present study was undertaken in anesthetized rats to examine the effect of aortic constriction and volume expansion on proximal tubular sodium reabsorption during constancy of flow and fluid composition in the peritubular circulation of the kidney. Efferent arterioles and branch capillaries were perfused at 625 nl/min with an artificial perfusate containing 9 g per oiter of albumin both before and during either aortic constriction or saline infusion. Results of recollection micropuncture studies in those tubules surrounded by artificially perfused capillaries were compared with results in control tubules in which the peritubular capillary flow and fluid composition were allowed to change during aortic constriction or volume expansion following saline infusion. Changes in single nephron filtration rate, fractional and absolute reabsorption induced by both aortic constriction and saline infusion were found to be qualitatively and quantitatively comparable in tubules with constant peritubular capillary microperfusion and in the control tubules with changing peritubular capillary environment due to the experimental maneuvers. Taken together, therefore, the present results indicate that with the use of the peritubular microperfusion technique no evidence is found to support a role of alteration in the peritubular environment in modulating the effect of aortic constriction or saline infusion on tubular sodium reabsorption in the rat nephron. Rather, these results provide indirect evidence in support of intraluminal factors as mediating these responses in tubular reabsorption to volume expansion and aortic constriction.

A Study in Vivo of Peritubular Oncotic Pressure and Proximal Tubular Reabsorption in the Rat

“…These findings of no effect of peritubular capillary protein concentration upon proximal tubular reabsorption are at variance with certain previous reports (BreMer et al, 1971 ;Spitzer & Windhager, 1970), but are in agreement with some others (Bank, Aynedjian & Wada, 1972;Holzgreve & Schrier, 1975a;Lowitz et ul., 1969;Rumrich & Ullrich, 1968). Such discrepancies are disturbing and the reasons for the differences are not apparent.…”

“…Rat plasma made either protein-free or hyperoncotic was used as the capillary perfusion solution. From a physiological point of view this was considered preferable to the use of artificial perfusates with various concentrations of either bovine albumin or dextran that have been used in similar experiments by others Holzgreve & Schrier, 1975a;Lowitz et al, 1969;Rumrich & Ullrich, 1968;Spitzer & Windhager, 1970). It is possible that plasma contains metabolic factors which act at the peritubular border to maintain normal levels of proximal tubular reabsorption.…”

“…Such studies by Spitzer & Windhager (1970), and provided evidence for a direct relation between the rate of proximal tubular reabsorption and theoncotic pressure of fluid perfusing the peritubular capillary circulation. However, similar studies employing capillary perfusion in viuo, by Rumrich & Ullrich (1968), Lowitz, Stumpe & Ochwadt (1969) and Holzgreve & Schrier (1975a), failed to demonstrate an effect of oncotic pressure on proximal tubular reabsorption. The reasons for these conflicting results are not clear.…”

Renal Tubular Transport of Calcium

“…Extracellular volume expansion inhibits salt and water reabsorption, and extracellular volume contraction increases reabsorption (22, 100,112,131,171). Because calcium reabsorption in the proximal tubule is coupled in large part to the transport of sodium and water, calcium excretion is affected in a parallel fashion.…”

The effect of intraluminal flow rate on glomerulotubular balance in the proximal tubule of the rat kidney

“…The latter explanation of dependence on intratubular flow rate is supported by experiments with direct manipulation of the intratubular flow rate in the absence of alterations in renal haemodynamics and peritubular factors (Bartoli, Conger & Earley, 1973). The true reason for glomerulotubular balance is still elusive and studies that oppose both the peritubular (Grantham, Qualizza & Welling, 1972;Holzgreve & Schrier, 1975;Conger, Bartoli & Earley, 1976) and intratubular (Burg & Orloff, 1968;Imai, Seldin & Kokko, 1977) theories have been reported. Nevertheless, the issue seems important, since this adaptation underlines the existence of a mechanism that can reset an active transport pump rate (Green & Giebisch, 1984) in the absence of a measurable change in the transtubular gradient.…”