Calcium-activated potassium channels in single smooth muscle cells of rabbit jejunum and guinea-pig mesenteric artery.

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Single-channel studies were made using the patch-clamp technique of K channels in dispersed single smooth muscle cells from rabbit longitudinal jejunal muscle and guinea-pig small (less than 0.2 mm o.d.) mesenteric arteries. In isolated inside-out patches from these two types of smooth muscle cell there was a population of K channels which had single-channel conductances of about 100 pS in near physiological K gradients and about 200 pS with symmetrical 126 mM-K solutions. Their conductance and other properties distinguish them from a K channel of smaller conductance which we have previously described in these cells. The relative permeability of the channel with respect to K was 1.4 Tl:1.0 K:0.7 Rb: less than 0.05 Na: less than 0.05 Cs. Cs (1 mM applied to the outside of the membrane) interfered with inward K movement when the membrane was hyperpolarized. Rb conductance of the channel when both sides of the membrane were exposed to 126 mM-Rb was 30 pS. When the Ca concentration on the inside of the membrane ([Ca]i) was about 10(-9) M, K channel opening was rarely observed and then only at strongly positive potentials. At [Ca]i between 10(-9) M and 10(-7) M mean channel open time increased and the probability of channel opening increased steeply; both were further increased by increasing membrane positivity. At [Ca]i between 10(-6) M and 2.5 mM the channel was mainly in the open state and the probability of channel conducting state often declined with increasing membrane positivity. The effects of varying [Ca]i from 10(-7) M to 2.5 mM on the kinetic activity of a single channel was studied largely in mesenteric artery patches containing one active channel. The distribution of open times could be fitted by a single exponential at low (less than 10(-6) M) [Ca]i but a component of fast openings (to less than 1.0 ms) was observed at all potentials at [Ca]i 2.5 mM. Closed time distribution required the sum of three exponentials to fit it all [Ca]i greater than 10(-7) M; at [Ca]i 10(-6) M or greater evidence of a fourth component, probably caused by Ca block of open channels, was obtained. Raising [Ca]i increased the mean duration of the (long) open state and decreased or had no effect on the duration of short, intermediate, and long mean closed states.

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