Isometric contractile properties and instantaneous stiffness of amphibian skeletal muscle in the temperature range from 0 to 20 °C

Abstract

The isometric contractile properties of frog (Rana pipiens) and toad (Bufo bufo) sartorii have been studied over the temperature range from 0 to 20 °C. The isometric twitch tension was found to vary considerably between these two species and between muscles in the same species. Between 0 and 4 °C ther was very little change in maximum isometric twitch tension. Between 4 and 12 °C several muscles from frog or toad showed a potentiation of twitch tension whereas others showed a decline. Over this temperature range the toad sartorii consistently demonstrated a greater potentiation. By 12 °C a steady decline in twitch tension in both muscles was seen as the temperature approached 20 °C. The maximum isometric tetanic tension recorded between 18 and 20 °C increased fractionally to an average of 1.504 ± 0.029 (n = 4) for frog sartorii and to 1.377 ± 0.008 (n = 5) for toad sartorii. The time to peak twitch tension and the half-relaxation time decreased markedly with an increase in temperature. Moreover, the half-relaxation time was reduced by a greater proportion than the time to peak twitch tension. Measurements of instantaneous stiffness by controlled velocity releases from the plateau of isometric tetani revealed that the large increase in isometric tetanus tension as the muscle was warmed was not accompanied by a corresponding increase in the total number of active cross-bridges. The possibility that a decreased availability of intracellular Ca²⁺ ions at the contractile sites contributing to the fall of isometric twitch tension at elevated temperatures is discussed. The possibility exists that at elevated temperatures a change in the intrinsic-contractile ability of the muscle occurs which produces an increased tension per cross-bridge.