Regulation of mesangial large Ca-activated K channels by protein phosphatase 2A

Large, Ca-activated K channels (BK) play a major role in the relaxation of mesangial cells. Normally quiescent in cell attached patches, BK respond to nitric oxide, atrial natriuretic peptide and DB-cGMP with a biphasic increase and then decrease ("rundown") in open probability. Using the patch clamp method, we determined if the rundown phase was the result of the activity of an endogenous protein phosphatase. In cell attached patches, cantharidic acid (500 nM), okadaic acid (CA; 100 nM), and calyculin A (CA; 100 nM), nondis-criminant inhibitors of protein phosphatases 1 and 2A at these concentrations, caused a significantly greater and sustained response of BK to DB-cGMP (0.01 mM). Incubation of MC with OA for 20 to 50 min. at a concentration (5 nM) specific for PP2A increased the basal open probability of BK and completely inhibited rundown after activation by DB-cGMP. Incubation for 20 to 50 min. with 10 nM CA, a more potent inhibitor than OA of PP1, did not affect BK activity. In inside out patches, DB-cGMP (0.01 mM) plus Mg-ATP (0,1 mM) caused a sustained activation of BK that was inhibited by exogenous PP2A (0.1 U/ml). It is concluded that BK (or a regulator of BK) is a common substrate for endogenous PKG, which activates BK, and PP2A, which inactivates BK, in human mesangial cells.