Contractile properties of isolated frog skeletal muscles under the influence of Na-octanoate

Na-octanoate (2-10 mM) altered the characteristics of the isometric twitch of isolated frog skeletal muscles. The peak tension, the maximum rate of tension rose, the time to peak tension, and the half relaxation time decreased. The tetanus tension was reduced by the same amount in percent as a single isometric twitch so that no changes occurred in the twitch/tetanus relationship. The fusion frequency of tetanic contractions increased. The isotonic contractions had a reduced shortening period. The shortening velocity was not affected. The K+ induced contractures were diminished. The S-shaped curve which relates peak tension to potassium concentration was shifted to more positive potentials. The maximum contractile strength was not attained even by complete depolarization. 1-2 mM caffeine removed the octanoate effects described above. The contracture induced by 7.5 mM caffeine produced the same maximum tension as that obtained in Ringer's solution, but the contracture began later and the rise of tension was retarded. The tension development of glycerol-extracted muscles was not affected by octanoate. The results are consistent with the hypothesis that the octanoate-induced changes of muscle contraction are due to a reduced Ca++ release from cellular stores.