Association of granular exocytosis with Ca(2+)-activated K+ channels in human eosinophils

We studied the mechanism of degranulation caused by Ca(2+)-activated K+ channels (KCa channels) in eosinophils isolated from mildly atopic donors using negative immunoselection. Stimulation of eosinophils with 0.1 microM platelet-activating factor (PAF) caused activation of single channels as recorded by the cell-attached patch-clamp technique. These channels were selectively permeable to K+ because the reversal potential was close to the equilibrium potential for K+. However, the channels were not permeable to Na+ or Cl- as demonstrated by ion substitution experiments. The calcium ionophore A-23187, at 1 microM, increased the K+ channel activity in the presence of Ca2+ in the external perfusate but did not induce channel activity in the absence of Ca2+. Similar results were obtained with another calcium ionophore, ionomycin (1 microM), and the Ca(2+)-releasing agent thapsigargin (10 microM). K+ channels activated by PAF and A-23187 had similar characteristics: two levels of single-channel conductances were observed, 10 +/- 1.5 and 22 +/- 1.7 pS as induced by PAF and 11 +/- 1.3 and 24 +/- 1.9 pS by A-23187; the mean open times of the large-conductance channels were 1.45 +/- 0.3 ms as induced by PAF and 1.26 +/- 0.5 ms by A-23187. These results indicate that PAF activates KCa channels. Both KCa currents and major basic protein release caused by A-23187 were blocked by quinidine. It is suggested that KCa channels are associated with granule secretion in human eosinophils.