Calcium-movement in smooth muscle

Abstract

Calcium movements in smooth muscle preparations of guinea pig were examined. In ileum and vas deferens, the rate of Ca-efflux into Ca-free medium was increased by acetylcholine, but the rate of Ca-efflux from aorta was not affected by acetylcholine and increased by norepinephrine. The time course of efflux of preloaded radioactive calcium from single smooth muscle cells of taenia coli into modified Tyrode solution consisted of two phases,and the half-times of Ca-exchange were estimated to be 1.87 + 0.27 and 6.78 ± 0.43 min. Acetylcholine increased the Ca-exchange reaction, but repetitive application of acetylcholine was ineffective. In muscle strip preparation of taenia coli, the time course of decrease of contraction in Ca-free medium was 15 min,and repetitive application of acetylcholine was reported to induce the repetitive stimulation of calcium-exchange reaction. The rate of decrease in acetylcholine-contraction of muscle strip preparation in Ca-free medium was portal vein > vas deferens > trachea, while the sensitivity of acetylcholine-contraction to manganese was vas deferens > portal vein > trachea. This suggests that relative contribution of loosely bound calcium on acetylcholine-contraction is higher in portal vein than in vas deferens, and the sensitivity of contraction to the concentration of extracellular calcium is not the indicator of the contribution of extracellular calcium to the contractile response.