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 μM. 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 μM) 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⁺.