LARGE CA2+-ACTIVATED K+ CHANNELS RESPONSIVE TO ANGIOTENSIN-II IN CULTURED HUMAN MESANGIAL CELLS

The patch-clamp method was used to determine the properties and response to angiotensin II (ANG II) of K+ channels in subpassages of human mesangial cell cultures. In cell-attached patches, with 140 mM KCl in the bath and cell potential equal to 40 mV, the open probability (P-o) of large K+ channels (MK(Ca)) was 0.8 with 0.5 mM Ca2+ in the bath and < 0.05 if the bath Ca2+ concentration was reduced to 1.0 mu M. Open and closed dwell-time histograms of MK(Ca) displayed both fast and slow time constants. Addition of ANG II (100 nM) to the bath solution (Ca2+ = 1.0 mu M) increased the P-o of MK(Ca) in cyclic bursts by decreasing the time constant of the slow closed state. In excised inside-out patches, the mean single-channel conductance of MK(Ca) was 206 pS in symmetrical 140 mM KCl. The selectivity sequence, established in asymmetrical cationic solutions, was K+ (1.0) > Rb+ (0.54) > NH4+ (0.11) > > Cs+ = Na+ (< 0.05). The P-o of MK(Ca) was increased by depolarizing potentials and high bath Ca2+. The Boltzmann distribution was consistent with an effective valence of 1.0, and the Hill coefficient for Ca2+ activation was 0.52. We conclude that MK(Ca) has properties similar to large Ca2+-activated K+ channels and may act to repolarize the membrane of mesangial cells in response to an agonist-induced mobilization of intracellular Ca2+.