Highly co-operative Ca2+ activation of intermediate-conductance K+ channels in granulocytes from a human cell line

1. To study Ca(2+)-activated K+ currents in dimethyl sulphoxide (DMSO)-differentiated HL-60 cells (HL-60 granulocytes), we have combined the patch clamp technique with microfluorimetric measurements of the cytosolic free Ca2+ concentration ([Ca2+]i). 2. Elevations of [Ca2+]i induced by the receptor agonist N-formyl-L-methionyl-L-phenylalanine (f-MLP), by cellular spreading or by the Ca2+ ionophore ionomycin, activated whole-cell currents. The kinetics of the current elevations closely paralleled the kinetics of the elevations in [Ca2+]i. Cellular spreading induced oscillations in [Ca2+]i and parallel oscillatory changes in the amplitude of the recorded currents. 3. The reversal potential of the Ca(2+)-activated current was a function of the extracellular K+ concentration (56.1 mV per log [K+]), demonstrating that the underlying conductance was selective for K+. 4. The current was blocked by charybdotoxin, but insensitive to apamin. 5. The whole-cell current was inwardly rectifying. No time-dependent activation or inactivation of the current could be observed within the range of voltages tested (-100 to +100 mV). 6. The dependence of the current amplitude on the measured [Ca2+]i revealed a half-maximal activation at approximately 350 nM [Ca2+]i, and a highly co-operative activation by [Ca2+]i with an apparent Hill coefficient of approximately 8. Neither the half-maximal activation by [Ca2+]i nor the apparent Hill coefficient depended on the voltage, and they were identical for Ca2+ elevations caused by the ionophore and the receptor agonist. 7. Analysis of Ca(2+)-activated single-channel events in cell-attached recordings revealed an inwardly rectifying K+ channel with a slope conductance of 35 pS. Fluctuation analysis of the Ca(2+)-activated whole-cell current suggested an underlying single-channel conductance of a similar size (28 pS). 8. In summary, we describe a charybdotoxin-sensitive, intermediate-conductance Ca(2+)-activated K+ channel in HL-60 granulocytes. The characteristics of the Ca2+ activation of this current (i.e. sensitivity to submicromolar [Ca2+]i, high co-operativity and voltage independence) are similar to the Ca2+ activation of the apamin-sensitive small-conductance K+ channel. Our results also suggest that [Ca2+]i elevations are the predominant, if not the only, activators of this channel during physiological stimulation of HL-60 granulocytes.