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.