cAMP stimulates proximal convoluted tubule Na(+)-K(+)-ATPase activity

The effect of adenosine 3',5'-cyclic monophosphate (cAMP) was examined on the electrophysiological properties of nonperfused proximal convoluted tubule in vitro. In 5 mM bath K+, the basolateral membrane potential (Vbl) was -66 +/- 1 mV (n = 26). Low bath K+ (0.1 mM) led to a transient hyperpolarization of Vbl followed by a sustained decrease to reach -48.6 +/- 5.0 mV. Return to 5 mM bath K+ produced a rapid and transient Vbl hyperpolarization of 24.6 +/- 1.4 mV (n = 5). This hyperpolarization was completely blocked by 100 microM strophanthidin (n = 4), demonstrating that the hyperpolarization was caused by reactivation of the Na(+)-K(+)-adenosinetriphosphatase (ATPase). Addition of 1 microM forskolin (forsk) + 100 microM 8-(4-chlorophenylthio)-cAMP (cp-cAMP) significantly increased this hyperpolarization to 30.8 +/- 10 mV (P < 0.005, n = 5). In a separate series of experiments, addition of 1 microM forsk + 100 microM 3-isobutyl-1-methylxanthine increased this hyperpolarization from 21.7 +/- 2.8 to 27.1 +/- 1.6 mV (P < 0.05, n = 5), which excludes any nonspecific effect of cp-cAMP. Forsk + cp-cAMP decreased the apparent partial conductance to Cl- (tCl) from 0.049 +/- 0.003 to 0.031 +/- 0.007 (P < 0.06, n = 6), decreased that to K+ (tK) from 0.56 +/- 0.05 to 0.43 +/- 0.03 (P < 0.05, n = 6), slightly decreased that mediated by the Na-HCO3 cotransporter (tNaHCO3) from 0.26 +/- 0.03 to 0.21 +/- 0.05, and had no effect on the absolute conductance mediated by the Na-HCO3 cotransporter. Forsk + cp-cAMP had no effect on tK when determined using bath K+ steps from 15 to 45 mM (tK = 0.84 +/- 0.02, n = 5) instead of K+ steps from 5 to 15 mM as previously done, and did not affect the value of tK measured in the presence of strophanthidin (tK = 0.41 +/- 0.03, n = 5). These results demonstrate that the decrease of tK by forsk + cp-cAMP observed using K+ steps from 5 to 15 mM is due to modulation by these agents of the stimulated hyperpolarizing Na(+)-K(+)-ATPase current produced by the bath K+ steps. Consequently, the increased Vbl initial recovery from low bath potassium observed when intracellular cAMP is increased could not be the result of modulation of passive basolateral membrane properties and represents a stimulation of the pump current. The present work thus demonstrates that the Na(+)-K(+)-ATPase is stimulated by cAMP.