In the circular muscle of guinea-pig gastric antrum, the effects of removal and reapplication of K+ and Cl- were studied on the slow wave, which consists of the lower, first and upper, second components. The first component appeared to be triggered by the driving potential generated in the interstitial cells. K+ removal slightly depolarized the membrane, increased frequency, and shortened the first component and driving potential, and K+ reapplication hyperpolarized and prolonged these potentials transiently. Ouabain abolished the K+-induced hyperpolarization but had no inhibitory effect on the K+-induced potentiation. The K+-induced prolongation was much reduced in Ca2+-deficient and increased in Ca2+-excess solution. BAPTA- AM, thapsigargin, and cyclopiazonic acid shortened the slow wave and inhibited the K+-induced prolongation but did not block the slow wave. Effects of Cl- removal were stronger than K+ removal in shortening and increasing the frequency. In Cl--deficient solution, no prolongation was observed on K+ reapplication. Although no conclusive evidence was obtained as to the ionic mechanism involved in the effects of K+ or Cl- removal and reapplication, a possibility is considered that the sarcoplasmic reticulum is involved in determining the duration of the driving potential and the first component of the slow wave.
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