Effects of Low Temperature on the Membrane Currents and Tension Components of Bullfrog Atrial Muscle

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  • 後藤 昌義
    Department of Physiology, Faculty of Medicine, Kyushu University
  • 津田 泰夫
    Department of Physiology, Faculty of Medicine, Kyushu University
  • 八谷 アツ子
    Department of Physiology, Faculty of Medicine, Kyushu University
  • 斎藤 雅彦
    Department of Physiology, Faculty of Medicine, Kyushu University

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  • Effects of Low Temperature on the Membr

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In order to clarify the nature of inotropic action of low tempera-ture, the effects of cooling on the membrane currents and tension com-ponents were studied on the bullfrog atrial muscle under voltage clamped and unclamped conditions with double gap method. Cooling (in between 35°-7°C) produced an increase of overshoot and a prolongation of the action potential accompanied by a slight depolarization of the membrane, a decrease of basal tension and an increase of twitch contraction. Under voltage clamp, a marked augmentation of contraction also occurred despite a decrease of basal tension, suggesting that the inotropic effect of cooling is not merely dependent on the prolongation of action potential. When the components of membrane current and tension were isolated in modified Ringer solutions, it became clear that ICa and ICa-dependent tension markedly increased at low temperature, while all other currents (INaf, INas, IKl, Ix) and ICa-independent tension decreased. Leaky mem-brane current (Il) for hyperpolarizing pulses also diminished. Tempera-ture coefficient (Q10) of the ICa-independent tension component was 1.2-1.5 between 7°C and 17°C, while that of ICa-dependent tension varied depending on depolarization voltages. These data were discussed in relation to possible alteration of Ca concentration at outer and inner layers of the membrane which may depend on temperature.

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