ASSOCIATION OF GRANULAR EXOCYTOSIS WITH CA2-ACTIVATED K+ CHANNELS IN HUMAN EOSINOPHILS()

We studied the mechanism of degranulation caused by Ca2+-activated Kchannels (K-Ca channels) in eosinophils isolated from mildly atopic do nors using negative immunoselection. Stimulation of eosinophils with 0 .1 mu M platelet-activating factor (PAF) caused activation of single c hannels as recorded by the cell-attached patch-clamp technique. These channels were selectively permeable to K+ because the reversal potenti al was close to the equilibrium potential for K+. However, the channel s were not permeable to Na+ or Cl- as demonstrated by ion substitution experiments. The calcium ionophore A-23187, at 1 mu M, 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 re sults were obtained with another calcium ionophore, ionomycin (1 mu M) , and the Ca2+-releasing agent thapsigargin (10 mu M). K+ channels act ivated by PAF and A-23187 had similar characteristics: two levels of s ingle-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 me an open times of the large-conductance channels were 1.45 +/- 0.3 ms a s induced by PAF and 1.26 +/- 0.5 ms by A-23187. These results indicat e that PAF activates K-Ca channels. Both K-Ca currents and major basic protein release caused by A-23187 were blocked by quinidine. It is su ggested that K-Ca channels are associated with granule secretion in hu man eosinophils.