The effects of temperature and pH on the contractile properties of skinned muscle fibres from the terrapin, Pseudemys scripta elegans

Abstract

Fibre types in the iliofibularis muscle of the freshwater terrapin Pseudemys scripta elegans have been characterized on the basis of their histochemical characteristics, nerve endings and contractile properties. Three types of focally innervated fibres are present, corresponding to the fast glycolytic (Fg), fast oxidative glycolytic (FOG) and slow oxidative (SO) fibre types of other vertebrates. Single fibres or small bundles of fibres representing each histochemical type were identified on the basis of their light scattering properties under dark-field illumination. Fibres were detergent-skinned using Brij 58, and their maximum isometric tension (P0) and unloaded contraction velocity (V0) were determined by the slack test method. At 15 degrees C, fast glycolytic fibres generated maximum isometric tensions of 184 +/- 5 kNm-2 and V0 values of 5.5 +/- 0.3 muscle lengths per second (L0s-1). Slow oxidative fibres produced tensions of 70.6 +/- 3 kNm-2 and had V0 values of 1.3 L0s-1. Tensions and V0 values of fast oxidative glycolytic fibres were between those of Fg and SO fibres. The force-velocity (P-V) characteristics of slow oxidative fibres were studied at 5 degrees and 15 degrees C. Points below 0.6 P0 on the curves could be fitted by a linear form of Hill's equation. Maximum contraction velocities (Vmax) extrapolated from the P-V relationship were 0.62 L0s-1 at 5 degrees C and 0.91 L0s-1 at 15 degrees C. The curvature of the P-V relationship was relatively independent of temperature over the range 5 to 15 degrees C. Values for Hill's constant a/P0 were 0.29 and 0.33 at 5 degrees C and 15 degrees C, respectively. The temperature dependence of P0 and contraction velocity at near zero load (Vi) were studied at constant pH, and under conditions designed to simulate the changes in intracellular pH which occur with temperature in vivo (delta pH/delta T = -0.0186). Changes in pH in the range 6.6 to 7.8 had no effect on either tension or Vi at temperatures between 0 degrees and 20 degrees C. However, below and above this pH range, both tension and Vi were depressed. It is concluded that pH changes within the normal physiological range (6.7-7.8) have no effect on the temperature dependence of P0 and Vi.