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In order to clarify the nature of inotropic action of low temperature, the effects of cooling on the membrane currents and tension components were studied on the bullfrog atrial muscle under voltage clamped and unclamped conditions with double gap method. Cooling (in between 35 degrees-7 degrees 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, IK1 Ix) and ICa-independent tension decreased. Leaky membrane current (I1) for hyperpolarizing pulses also diminished. Temperature coefficient (Q10) of the ICa-independent tension component was 1.2-1.5 between 7 degrees C and 17 degrees 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.

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