Effects of luminal fluid anions on calcium transport by proximal tubule

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.