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ABSTRACT A study of the mechanical properties of longitudinal muscle in relation to the membrane properties was carried out under isometric conditions. When the stimulus duration exceeded 50 msec., active tension development was followed by sustained contraction. The sustained contractions were not related to spike generation. The critical potassium concentration to produce the contracture depolarized the membrane from –36 to –20 mV. Reduced sodium concentration and increased calcium and increased potassium (up to 27 mM) concentrations enhanced the amplitude of the active tension. Reduced sodium concentration enhanced the amplitude and duration of the sustained tension, but increased potassium and calcium concentrations reduced them. Caffeine (12 mM) induced contractures of the muscle, and reduced the membrane resistance and capacitance. Spikes were not elicited by intracellular stimulation. 5-Hydroxytryptamine (10−6 g./ml.) blocked the generation of the sustained contraction but no effect was observed on the phasic tension.
Hidaka et al. (Tue,) studied this question.