Comparison of Mg, Mn, and Co ions affecting the beta-adrenoceptor-mediated membrane response in the guinea-pig taenia caeci.

Electrical activity was recorded intracellularly from the muscle cell of guinea-pig taenia caeci in Locke solution. Membrane potential was -46.4 mV, and spike potentials were discharged spontaneously. Isoprenaline (3 microM) 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 microM) and papaverine (30 microM) hyperpolarized the membrane; the effects were augmented by 1.2 mM Mg. The hyperpolarization in response to 3 microM isoprenaline or 100 microM 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 microM) 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 beta-adrenoceptor-mediated action on the smooth muscle membrane of taenia caeci.