ROLE OF LARGE CA2-ACTIVATED K+ CHANNELS IN REGULATION OF MESANGIAL CONTRACTION BY NITROPRUSSIDE AND ANP()

The patch-clamp method, in conjunction with measurements of cell contr action, was employed to investigate activation by guanosine 3',5'-cycl ic monophosphate (cGMP) and guanylyl cyclase-stimulating vasodilators of large Ca2+-activated K+ channels (BKCa) in human glomerular mesangi al cells (MC). In cell-attached patches, with physiological NaCl bathi ng solutions, BKCa was activated transiently by nitroprusside [NP; a n itric oxide (NO) donor], atrial natriuretic peptide (ANP), and dibutyr yl cGMP (DBcGMP), reaching peak responses between 10 and 60 s and decr easing to near baseline activity within the next 120 s. In the presenc e of LY-83583, a specific inhibitor of guanylyl cyclase, BKCa was acti vated on cell by DBcGMP but not by NP or ANP. In all cases, the increa se in channel activity coincided with a decrease in channel amplitude, indicating that the membrane potential was approaching equilibrium po tential as BKCa was activated. If membrane potential was maintained de polarized with 140 mM KCl in the bathing solution, DBcGMP induced a su stained activation of BKCa. In the continued presence of DBcGMP, BKCa was further activated when 100 nM angiotensin II (ANG II) was added to the bathing solution. Experiments were performed to determine the rol e of BKCa in the regulation by vasorelaxants of mesangial contraction measured as percent maximal and defined by reduction in length induced by replacing 135 mM bath NaCl with KCl. Contraction by ANG II (100 nM = 60.5%) was attenuated by NP (100 mu M), ANP (1.0 mu M), and DBcGMP (10 mu M) in the absence, but not the presence, of iberiotoxin, a spec ific inhibitor of BKCa. These results indicate that guanylyl cyclase-s timulating vasorelaxants counteract ANG II-induced contraction of MC, in part, by repolarizing the membrane through activation of BKCa chann els.