The contribution of Ca2+-activated and delayed rectifying K+ channels to the voltage-dependent outward current involved in spike repolarization in mouse pancreatic β-cells (Rorsman, P., and G. Trube. 1986. J. Physiol. 374:531-550) was assessed using patch-clamp techniques. A Ca2+-dependent component could be identified by its rapid inactivation and sensitivity to the Ca2+ channel blocker Cd2+. This current showed the same voltage dependence as the voltage-activated (Cd2+-sensitive) Ca2+ current and contributed 10-20% to the total β-cell delayed outward current. The single-channel events underlying the Ca2+-activated component were investigated in cell-attached patches. Increase of [Ca2+]i invariably induced a dramatic increase in the open state probability of a Ca2+-activated K+ channel. This channel had a single-channel conductance of 70 pS ([K+]o = 5.6 mM). The Ca2+-independent outward current (constituting >80% of the total) reflected the activation of an 8 pS ([K+]o = 5.6 mM; [K+]i = 155 mM) K+ channel. This channel was the only type observed to be associated with action potentials in cell-attached patches. It is suggested that in mouse β-cells spike repolarization results mainly from the opening of the 8-pS delayed rectifying K+ channel.
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