Intracellular calcium activity in split frog skin epithelium: Effect of cAMP

Summary Measurement of intracellular calcium activity (a _Ca ^c ) by ion-selective microelectrodes has previously been technically limited to relatively large cells (20 m). We now report results obtained with this technique in the small epithelial cells (10 m) of split frog skin using microelectrodes having an outer tip diameter of <0.2 m. The basolateral membrane potential was measured with Ca²⁺-selective microelectrodes (E _Ca ^sc ) and with reference micropipettes ( ^sc ) either sequentially or simultaneously in 15 successful experiments. Under baseline conditions,a _Ca ^c was measured to be 215±39nm (mean±se), in close agreement with the mean values estimated from published data obtained withNecturus proximal tubule. Stimulation of Na⁺ transport across six skins with 1mm serosal 8p-chlorophenylthio-3,5 cyclic AMP (CPTcAMP) increaseda _Ca ^c by a factor of 2.6±0.6. The increase ina _Ca ^c preceded the CPTcAMP-induced increase inI _sc. The results of the present study indicate that electrometric determination of intracellular calcium activity is now feasible in a much wider range of cell systems than heretofore possible. CPT cAMP elevates intracellular Ca²⁺ activity; this phenomenon is an early event, preceding the natriferic effect of CPTcAMP.