Effects of luminal fluid anions on calcium transport by proximal tubule

Abstract

To determine whether the anion composition of tubule fluid affects calcium absorption by the renal proximal tubule, in vivo microperfusion techniques were employed in anesthetized rats. Experiments were designed so that total calcium and sodium concentrations were kept constant in fluids entering the tubule. A control solution, in which the main anion was chloride, was modified either by addition of ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetra-acetic acid or by replacing most of the chloride with nitrate, thiocyanate, sulfate, or citrate. Sufficient mannitol was added to the perfusion fluids to reduce net fluid flux to near zero. Net fluxes of calcium and sodium were calculated from measurements of total concentrations in perfused and collected fluids. Electrochemical driving forces across the tubule wall were calculated from measurements of transepithelial voltage and of Ca2+ activity in perfused and collected fluids. Results showed that calcium absorption by the proximal tubule depends on both the luminal Ca2+ activity and the transepithelial voltage. With zero transepithelial electrochemical driving force calcium absorption was significantly different from zero. Calcium and sodium transport rates were seen to vary independently. We conclude that the calcium absorptive mechanism involves active transport and can be dissociated from the sodium transport pathway. Calcium transport is also affected by changes in transepithelial electrochemical driving forces with an apparent permeability similar to values reported for sodium and potassium.

Ultracytochemical Study of the Pathway of Ca2+ Transportation across the Epithelial Cell Layer of a Nephron.

“…Micropuncture studies have found that about 60% of filtered Ca2+ is reabsorbed in segment 1 and 2 of the proximal convoluted tubule, and 10% in the proximal straight tubule [13 ,17, 31]. It has been proposed that nearly all Ca2+ absorption in these tubules is passive and occurs via the paracellular route [1,5,21]. The present findings in segment 2 and 3 of the proximal tubules support the paracellular pathway of Ca2+ absorption.…”

Calcium transport in canine renal basolateral membrane vesicles. Effects of parathyroid hormone.

“…(----in), BLMV The demonstration of active Ca2`reabsorption in the proximal nephron (7) indicates an important role of transcellular calcium movements in the reabsorptive fluxes of this segment. This implies both an apical influx and basolateral efflux step for calcium in the proximal cell.…”

“…These findings have been criticized on technical grounds (6). However, recent findings of Bomsztyk and Wright (7,8) confirm the presence of net Ca2+ absorption in the absence of transepithelial electrochemical driving forces which indicate active Ca2+ transport. In these studies, there was also clear dissociation between Na+ and Ca2+ transport rates in certain experimental settings.…”

Bicarbonate absorption stimulates active calcium absorption in the rat proximal tubule.

“…Male Sprague-Dawley rats (Bantin & Kingman, Freemont, CA) weighing 250-300 g were anesthetized by intraperitoneal injection of 5-sec-butyl-2-ethyl thiobarbituric acid (Inactin, Byk-Gulden, Konstanz, Federal Republic of Germany) (100 mg/kg body weight) and were prepared for in vivo micropuncture and microperfusion as previously described (6,7). A continuous intravenous infusion ofsolution containing 140 mM NaCl and 4.0 mM KCI at a rate of0.1 ml/min per kg body weight was administered and arterial blood pressure was monitored throughout the experiment.…”

“…With the recent introduction of Ca2" ion selective microelectrodes direct in situ measurements are now possible (6).…”