CaBP(r) facilitates intracellular diffusion for Ca pumping in distal convoluted tubule

The system of renal Ca transport in the rat is modeled in terms of two classes of processes: a nonsaturable flux that predominates in the proximal tubule, and an active, vitamin D-dependent flux with major expression in the distal convoluted tubule. There transport is against an electrochemical gradient, with much of the efflux probably mediated by the Ca/Mg-ATPase. Calculations of the rate of free Ca diffusion in tubular cells indicate that an unaided flux would be only one-seventy-seventh of that found experimentally. It is suggested that the vitamin D-induced renal calcium binding protein, CaBP(r), M(r) ~ 28,000, in raising total cellular calcium by three orders of magnitude, increases the transcellular Ca flux and thus the free intracellular Ca ion concentration at the basolateral pole, allowing the Ca/Mg-ATPase to function near its maximum. Analysis of the rate of nonsaturable Ca flux throughout the kidney tubule suggests a paracellular pathway via bulk flow, following water that is driven osmotically. Evaluation of whole animal data in terms of these two classes of calcium fluxes indicates that our model is consistent with experimental observations and assigns a functional role to active calcium transport.