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

M. Saito, R. Sato, N. M. Munoz, A. Herrnreiter, M. Oyaizu, H. Kasugai, T. Narahashi and A. R. Leff Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago 60611-3008, USA. 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.