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

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 (Cai) was about 10(-9) M, K channel opening was rarely observed and then only at strongly positive potentials. At Cai 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 Cai 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 Cai 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) Cai but a component of fast openings (to less than 1.0 ms) was observed at all potentials at Cai 2.5 mM. Closed time distribution required the sum of three exponentials to fit it all Cai greater than 10(-7) M; at Cai 10(-6) M or greater evidence of a fourth component, probably caused by Ca block of open channels, was obtained. Raising Cai 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.