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

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Abstract
1. 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 (< 0.2 mm o.d.) mesenteric arteries. 2. In isolated inside-out patches for 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. 3. The relative permeability of the channel with respect to K was 1.4 Tl1:0 K:0.7 Rb: < 0.05 Na: < 0.05 Cs. Cs (1 mM applied to the outside of the membrane); it 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. 4. 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 steeeply; 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. 5. 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 (< 10-6 M) [Ca]i but a component of fast openings (to < 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 at all [Ca]i > 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.