Some observations on the potassium-induced contracture in guinea-pig ureteric smooth muscle

Summary  The potassium (K) induced contracture of the guinea-pig ureter was examined to study the source of calcium for confraction and the mechanism of relaxation in this smooth muscle. The K-contracture showed 2 components; a rapidphasic response followed by a slowertonic component. The tonic component relaxed, almost to the baseline, over a 20 min period. The experimental findings suggest that the calcium for the phasic component is released from some loosely bound intracellular store of the activator, while the tonic component is dependent upon the utilization of extracellular calcium. The mechanism of relaxation of the K-contracture was investigated and the evidence suggests that relaxation is effected by active calcium pumping. No evidence was found for the operation of a Na/Ca exchange mechanism under normal physiological conditions, although Na/Ca exchange in the muscle cells could be demonstrated subsequent to contraction in potassium Krebs.