EFFECTS OF SODIUM REMOVAL ON THE ELECTRICAL AND MECHANICAL ACTIVITY OF THE GUINEA-PIG STOMACH MUSCLE
The effects of Na-free solution on the electrical and mechanical activity of smooth muscle cells of the guinea~pig stomach were investigated. With application of Nafree solution (replaced by choline), the membrane was transiently hyperpolarized, the ionic conductance increased, and then the membrane was depolarized to less than the resting membrane potential level. The amplitude of slow potential changes was reduced and the frequency of spikes increased. Simultaneously, the spontaneous
... pontaneous contractions increased in frequency and amplitude and then ceased within several minutes (early response). The transient contraction required the presence of [K], and [Ca]", but the transient hyperpolarization did not. With prolonged treatment (20-30 min), spikes without slow potential changes and a sustained contraction reappeared (delayed response). These results indicate that the early response is mainly due to Na efliux and Ca influx. This Ca influx, however, may not be related to activation of the electrogenic Na-Ca exchange diflusion. The delayed response is due to depolarization of the membrane, reappearance of the spike, and suppression of Ca efl'lux, which result from a decrease in [Na],.