Calcium transport by the proximal tubule.

Taken together the results of these in vivo microperfusion experiments indicate that calcium absorption by the proximal tubule depends on more than one transport mechanism. We have observed that net calcium flux is affected by changes in calcium ion activity (even with constant total calcium concentration) and in transepithelial voltage. This sensitivity of calcium flux to changes in electrochemical driving force points to a diffusional component of calcium transport. Ng et al. (1984) have recently concluded that simple diffusion accounts for the majority of calcium absorption by superficial proximal convoluted tubules of the rabbit. The pathway for this diffusional component may involve paracellular channels. The permeability of this pathway appears to be as high for calcium as it is for sodium, potassium and chloride. Calcium flux is also affected by changes in osmotic water flow. The effect of changes in volume flow on calcium transport occurs even in the absence of concentration changes in bulk solutions. Thus, it does not appear to be the result of changes in passive driving forces secondary to dilution or concentration of tubule fluid. At present we are not able to distinguish between two other possible mechanisms: solute polarization in a microscopic unstirred fluid layer adjacent to the cell membrane, or true entrainment of calcium in the stream of osmotically driven water flow (solvent drag). Either mechanism could provide an additional component of total calcium transport independent of changes in bulk phase ion concentrations and electrical driving forces. A third component of total calcium absorption appears to involve active transport.(ABSTRACT TRUNCATED AT 250 WORDS)