Mechanism of activation by cGMP-dependent protein kinase of large Ca²⁺-activated K⁺ channels in mesangial cells

The patch-clamp method was employed to establish the mechanism of regulation by guanosine 3',5'-cyclic monophosphate (cGMP)dependent protein kinase (PKG) of large Ca²⁺-activated K⁺ channels (BK(Ca)) in human mesangial cells. Dibutyryl cGMP (DBcGMP) significantly increased open probability (P(o)) of BK(Ca) in the absence but not in the presence of staurosporine in cell-attached patches. In excised patches, BK(Ca) was activated by simultaneous addition of MgATP plus cGMP but not cAMP plus MgATP. Activation by cGMP plus MgATP was blocked by KT-5823, an inhibitor of PKG, but not by KT-5720, an inhibitor of cAMP-dependent protein kinase (PKA). Thus a cGMP-specific endogenous kinase is associated with mesangial BK(Ca). In excised patches, exogenous PKG but not PKA or protein kinase C activated BK(Ca). The half-activation potential (V( 1/4 )), defined as the potential at which the P(o) = 0.5 with 1 μM Ca²⁺, was -34 and 42 mV for activated and inactivated BK(Ca), respectively; however, the gating charge (z(g)), a measure of voltage sensitivity, was not affected by PKG. Similarly, the Ca( 1/4 ) (free Ca²⁺ concentration required to activate to P(o) = 0.5 at 40 mV) decreased from 1.74 to 0.1 μM on addition of PKG, but the Hill coefficient, a measure of Ca²⁺ sensitivity, was not affected. Activation of BK(Ca) by PKG was heterogeneous with two populations: the majority (67%) activated by PKG and the minority unaffected. It is concluded that an endogenous PKG activates BK(Ca) by decreasing the Ca²⁺ and voltage activation thresholds independently of sensitivities.