Evidence for a Ca-activated inwardly rectifying K channel in human macrophages

Cell-attached patch studies of cultured human macrophages demonstrate that exposure to ionomycin induces inward-rectifying single-channel currents that differ from the voltage-dependent 28 pS inward-rectifying K currents previously described in these cells (J. Membr. Biol. 103: 55-66, 1988). With 150 mM KCl in the electrode and NaCl Hanks' solution in the bath, the ionomycin-induced single-channel conductance for inward currents was 37 pS, and the reversal potential was 57 mV. Channel activity was often associated with a shift in the base-line current level indicating that the cell membrane potential hyperpolarized. The ability of ionomycin to induce channel activity depended on extracellular [Ca] supporting the view that the channels were gated by calcium. Ionomycin-induced channels were permeable to K, relatively impermeable to Cl or Na, exhibited bursting kinetics, and had no apparent voltage dependence. Barium (3 mM in the patch electrode) did not significantly block the ionomycin-induced channel at rest but blocked channel activity when the patch was hyperpolarized beyond the resting membrane potential. Exposure of macrophages to platelet-activating factor, which is known to increase intracellular [Ca] [( Ca]i) (J. Cell Biol. 103: 439-450, 1986), also transiently induced channel activity. In excised patches with 3 microM [Ca]i bursting inward-rectifying channels with a 41 pS conductance were noted that probably correspond to the ionomycin-induced channels present in cell-attached patches. Increasing [Ca]i from 10(-8) to 3 x 10(-6) M induced inward-rectifying channel activity in previously quiescent excised patches.(ABSTRACT TRUNCATED AT 250 WORDS)