Electrical activity was recorded intracellularly from the muscle cell of guinea-pig taenia caeci in Locke solution. Membrane potential was —46.4mV, and spike potentials were discharged spontaneously. Isoprenaline (3 μm) hyperpolarized the membrane and suppressed the spike discharge. The hyperpolarization by isoprenaline was increased at low K (2 mM), while decreased at high K (11.8, 29.5, 59 mm). The hyperpolarization by isoprenaline was potentiated in the presence of external Mg ions, depending on the concentration of Mg (0–9.6 mM). Forskolin (3 μm) and papaverine (30 μm) hyperpolarized the membrane; the effects were augmented by 1.2 mm Mg. The hyperpolarization in response to 3 μm isoprenaline or 100 μm papaverine was inhibited by Mn, Co, and low Ca (1 mm) whereas it was not affected by high Ca (7.5 mm). Verapamil (0.5, 2 μm) had no influence of the hyperpolarization caused by isoprenaline. It was discussed that extracellular and/or intracellular Mg and Ca ions played important roles in the β-adrenoceptor-mediated action on the smooth muscle membrane of taenia caeci.
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