Ventricular Isomyosins and the Tonic Regulation of Cardiac Contractility

Abstract

The molecular basis for cardiac muscular contraction is generally regarded as being the same as that for skeletal muscle: energy is transduced from ATP into mechanical work by cyclic interactions of cross-bridges of myosin with actin, this cycling being coupled to the hydrolysis of ATP (1,2). The concept that myosin controls the speed and power of muscle contraction is now reasonably well established for skeletal muscles. Close (3) showed that fast and slow skeletal muscles within the same species differ essentially in their force-velocity properties. Whereas the tetanic stress (force per unit cross-sectional area) of these muscles is about the same, the intrinsic speed of shortening of sarcomeres for the fast muscle is about twice as fast as that for the slow muscle. These findings imply that the rate of mechanical energy output, and hence the rate of chemomechanical energy transduction from ATP is about twice as high for the fast muscle as that of the slow muscle. Since the force-velocity properties of skeletal muscles were obtained from fully activated muscles during tetanic contractions, the results further imply that the difference in energy transduction rate is not related to any possible difference in the time course of excitation contraction coupling, but is necessarily due to the difference in the contractile machinery itself, namely, the contractile proteins myosin or actin.

This work is supported by a grant-in-aid from the National Heart Foundation of Australia.