Evidence for contribution of Ca2+ storage sites on unitary K+ channel currents in inside-out membrane of rabbit portal vein.

While making use of the inside-out membrane patch, we examined the effects of caffeine and heparin on unitary currents of the large conductance Ca(2+)-dependent K+ (maxi-K+) channel in the rabbit portal vein. About half of the inside-out membranes we used contained a functional Ca(2+)-store site which facilitated modification of the maxi-K+ channel. When high-K+ solution containing 0.05mM EGTA was superfused in the bath, simultaneous openings of more than 20 maxi-K+ channels were observed in 39 of 83 patch membranes, and multi-channel opening appeared periodically or continuously at the holding potential of -10mV. Most channel activities of these patch membranes were inhibited by caffeine or heparin, and some heparin-insensitive channel activities were inhibited by caffeine. The remaining patch membranes (44 out of 83) showed low activity of the maxi-K+ channel, and neither caffeine nor heparin modified channel activity. Therefore, in our experimental set-up, half the number of excised patch membranes contained a Ca2+ store site. Most Ca2+ store sites have inositol 1,4,5-trisphosphate (InsP3)-activated Ca2+ release (IACR) and caffeine-activated Ca2+ release (CACR) channels and few lack the IACR channel. The mechanisms of activation of the maxi-K+ channel in relation to release of Ca2+ from the store sites can be examined in detail using the approaches we have described.