Ca2+ transport in diluting segment of frog kidney.

In the isolated-perfused frog (Rana pipiens) kidney the question of whether transepithelial transport of Ca2+ is a passive voltage driven process or involves active mechanisms was investigated. With conventional and ion-sensitive microelectrodes transepithelial electrical and electrochemical potential differences were measured. Luminal activities and transepithelial net fluxes of Ca2+ and Cl- were evaluated. Different transepithelial electrical voltages in a wide range (+20 to -4 mV) were generated by "chemical voltage clamping" and the dependence of Ca2+ net fluxes on these voltages investigated. The hormonal control of both Cl- and Ca2+ transport was studied by evaluating the effect of the cell-permeable cAMP analogue, db-cAMP and of the adenylate cyclase stimulator, forskolin. The experiments reveal that: (a) Ca2+ is reabsorbed along the diluting segment of frog kidney. (b) Ca2+ reabsorption is inhibited by furosemide because of the elimination of the transepithelial voltage. (c) There is a direct relationship between transepithelial voltage and Ca2+ reabsorption. (d) Neither Cl- nor Ca2+ reabsorption are affected by db-cAMP or forskolin. We conclude that Ca2+ reabsorption is passive, driven by the lumen-positive transepithelial voltage. It most likely occurs via the paracellular shunt pathway.