High-conductance K+ channel in pancreatic islet cells can be activated and inactivated by internal calcium.

The Ca2+-activated K+ channel in rat pancreatic islet cells has been studied using patch-clamp single-channel current recording in excised inside-out and outside-out membrane patches. In membrane patches exposed to quasi-physiological cation gradients (Na+ outside, K+ inside) large outward current steps were observed when the membrane was depolarized. The single-channel current voltage (I/V) relationship showed outward rectification and the null potential was more negative than -40 mV. In symmetrical K+-rich solutions the single-channel I/V relationship was linear, the null potential was 0 mV and the single-channel conductance was about 250 pS. Membrane depolarization evoked channel opening also when the inside of the membrane was exposed to a Ca2+-free solution containing 2mM EGTA, but large positive membrane potentials (70 to 80 mV) were required in order to obtain open-state probabilities (P) above 0.1. Raising the free Ca2+ concentration in contact with the membrane inside ( [Ca2+]i) to 1.5 X 10(-7) M had little effect on the relationship between membrane potential and P. When [Ca2+]i was increased to 3 X 10(-7) M and 6 X 10(-7) M smaller potential changes were required to open the channels. Increasing [Ca2+]i further to 8 X 10(-7) M again activated the channels, but the relationship between membrane potential and P was complex. Changing the membrane potential from -50 mV to +20 mV increased P from near 0 to 0.6 but further polarization to +50 mV decreased P to about 0.2. The pattern of voltage activation and inactivation was even more pronounced at [Ca2+]i = 1 and 2 microM.(ABSTRACT TRUNCATED AT 250 WORDS)