Regulation of large calcium-activated potassium channels by protein phosphatase 2A

Abstract

Vasodilating agents induce relaxation of mesangial cells, in part through cGMP-mediated activation of large calcium-activated potassium channels (BK(Ca)). Normally quiescent in cell-attached patches, the response of BK(Ca) to nitric oxide, atrial natriuretic peptide, and dibutyryl cGMP (Bt2cGMP) is characterized by a biphasic increase and then decrease ('rundown') in open probability. Using the patch-clamp method in conjunction with phosphatase inhibitors, we investigated whether the run-down phase was the result of dephosphorylation by an endogenous protein phosphatase. In cell-attached patches, cantharidic acid (500 nM), okadaic acid (100 nM), and calyculin A (100 nM), nondiscriminant inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A) at these concentrations, caused a significantly greater and sustained response of BK(Ca) to Bt2cGMP. Within 2 min, the response of BK(Ca) to the combination of cantharidic acid and Bt2cGMP was greater than the response to these agents added separately. Incubation of mesangial cells with okadaic acid for 20 min at a concentration (5 nM) specific for PP2A increased the basal open probability of BK(Ca) and completely inhibited rundown after activation by Bt2cGMP. Incubation with calyculin A (10 nM), a more potent inhibitor of PP1, did not affect BK(Ca) activity. In inside-out patches, Bt2cGMP plus MgATP caused a sustained activation of BK(Ca) that was inhibited by exogenous PP2A but not PP1. It is concluded that either BK(Ca) or a tightly associated regulator of BK(Ca) is a common substrate for endogenous cGMP-activated protein kinase, which activates BK(Ca), and PP2A, which inactivates BK(Ca), in human mesangial cells.