An analysis of post-contracture potentiation in frog twitch skeletal muscle

Post-tetanic potentiation (PTP) was mimicked by post-contracture potentiation (PCP) in the twitch muscle of frog. A marked PCP was observed in small bundles of semitendinosus muscle in tetrodotoxin (TTX)-containing solution under current clamp condition. Contractures were induced by a constant depolarizing current pulse of 1 s every 5 min. Each contracture (C) was preceded and followed by a twitch-corresponding short contracture, T1 and T2, with an interval of 2 s. The ratio of twitch height, T2/T1, was measured and compared under various conditions. In TTX-containing Ringer solution, T2/T1 depended on magnitude of both T and C. When T2/T1 was plotted against C/T1 a quasi-linear relationship was found. The slope of the curve was 0.076 and was found to decrease when the duration of T was increased to 1 s. In TTX-containing solution, addition of 2 mM Ba2+ or 1 mM caffeine potentiated T as well as C but decreased T2/T1 markedly. Similar effects were observed when Na+ was replaced with TEA+ or when bath temperature was lowered from 20 to 8 degrees C. In all cases T2/T1 vs. C/T1 curve was shifted to the left and downwards. It was considered that these results could be attributable to an elevation of the resting Ca2+ concentration in the sarcoplasm. Thus the phenomenon of PCP probably reflects the time course of Ca2+ transients in the conditioning contracture C.