The interactive effects of temperature and pH on the isometric contraction of toad sartorius muscle

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The main purpose of this study was to examine the interactive effects of pH and temperature on the performance of isolated muscle in order to estimate the effect of temperature in vivo. The development of tetanic tension by the sartorius muscle isolated from toads was tested at either 5 or 25°C. The pH of the physiological solution bathing the muscle was changed from 9.0 to 6.0 stepwise by 0.5 pH units. In order to see if the temperature at which the toads were maintained modified the muscle activity, two groups of toads were used: those acclimated to low temperature (5°C) and those acclimated to high temperature (25°C). Acclimation temperature had no significant effect (Fig. 1, Table 1). Both the amount and rate of tension development increased when test temperature increased (Fig. 1A, B). Maximum tension and rate of tension development were significantly reduced at low pH. The latent period, time to half-maximum tension, and half-relaxation time were longer at low pH (Fig. 1C, D, E). The interactive effects of pH and test temperature were significant for all variables, except the latent period (Table 1). That is, the effects of pH were greater at 5 than at 25°C and the effects of test temperature depend on extracellular pH. We calculated that the in vivo decrease in tetanic tension when the body temperature of a toad decreases from 25 to 5°C is smaller than previously supposed. This is because in previous studies on the effect of test temperature on muscle function, unrealistic pH regimes generally have been employed.