Mechanism of mechanical inhibition of smooth muscle by ouabain.

1 Ouabain (10 mug/ml) caused an initial increase followed by a decrease in tension in guinea-pig taenia coli bathed in normal sodium and potassium containing medium. 2 The excitatory effect of ouabain was prevented by the elevation of extracellular potassium while the inhibitory effect was abolished in the absence of sodium in the bathing medium. 3 In the presence of sodium, the inhibitory effect of ouabain disappeared when the drug was washed out, but in the presence of lithium (which is not extruded by the Na-K pump) the inhibition was not abolished. 4 The inhibitory effect of ouabain was accompanied by an increase in tissue Na and a decrease in tissue K concentration. The increase in intracellular [45Ca] which occurred when extracellular [k] was varied, was also reduced by ouabain. 5 It is concluded that a high intracellular level of sodium inhibits contractions in visceral smooth muscle probably by a mechanism involving decreased Ca influx.