Differences between Ca Contractures in the Depolarized Longitudinal and Circular Muscles of Guinea Pig Stomach

Abstract

Ca-induced contracture and the effect of Na on it were investigated in the K-depolarized longitudinal and circular muscles of guinea pig stomach. In the absence of Na, Ca produced rapid contracture in longitudinal muscle, but slow and S-shaped contracture in circular muscle. The sensitivity to Ca for the contracture was about tenfold higher in longitudinal muscle than in circular muscle. The presence of external Na (7.5-60 mM) inhibited the contracture induced by Ca (0.1 mM) in longitudinal muscle in a dose-dependent manner. In circular muscle, however, Na potentiated the initial speed of contracture induced by Ca (1 mM), and inhibited the height of contracture at 30 min showing maximum inhibition at 15 mM Na. Li instead of Na produced inhibition of Ca contractures in both muscle layers. Na-induced potentiation in circular muscle was markedly depressed by lowering the temperature, and was reduced by a high concentration (1 mM) of ouabain.These observations indicate that the Ca contracture in longitudinal and circular muscles differs and the effect of Na on this contracture may modulate Ca ion movements through the muscle membrane by inhibiting Ca influx in both muscle layers, and initially potentiating it in circular muscle in an ouabain-sensitive process.