TETANIC CONTRACTION AND TENSION-LENGTH RELATION OF FROG VENTRICULAR MUSCLE

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Abstract
Complete tetanic contraction was generated in frog ventricular muscle by repetitive electrical stimulation. The maximum stimulus was a transverse alternating current at 10 Hz and 17-20 V/cm in peak to peak voltage in an external solution containing 9 mM Ca2+. The maximum isometric tension obtained was twice or more than that of the twitch tension at 20.degree. C. The tetanic tension and its rate of rise declined with decreasing external Ca2+ concentration and less than half of the maximum tension was generated at 1.8 mM Ca2+. Various tetanic tensions less than the maximum were obtained in the partially depolarized muscle in excess K+ solution by reducing the stimulus intensity. Adrenaline [epinephrine] (5 .times. 10-6 g/ml) potentiated the submaximal tetanic tension and the twitch tension, although no effect was observed for the maximum tetanic tension. The tension-length relation for the tetanic contraction of ventricular muscle was similar to that of the skeletal muscle, but the tension fell almost linearly at shorter lengths than 0.9 Lm, where Lm is the optimum length at which the maximum tension, Fm, was generated. Fm was 4.6 g/mm2; the sarcomere length at Lm was 2.0-2.2 .mu.m.