Synthesis by Fast Muscle of Myosin Light Chains characteristic of Slow Muscle in Response to Long-term Stimulation

Abstract

MYOSIN isolated from slow (red) skeletal muscles of the rabbit differs in several respects from myosin isolated from fast (white) skeletal muscle. In particular the ATPase of slow muscle myosin shows lower specific activity than that of fast muscle myosin and, unlike the latter, is labile at alkaline pH. White muscle myosin contains three kinds of small subunits, the so called light chains, in the 20,000 dalton range, while myosin from red muscle contains two classes of light chains which differ in molecular weight from any of the light chains in white muscle myosin^1,2. Salmons and Vrbová³ have found that the contractile speed of a fast muscle can be changed by continuous stimulation of the motor nerve over a period of weeks, which produces a marked slowing of the time course of contraction and relaxation. As Bárány⁴ has shown that the myosin ATPase activity of a muscle correlates closely with its contractile speed, it seemed appropriate to examine what changes, if any, might be evident in the ATPase activity and light chain complement of myosin extracted from muscles whose contractile speed had been altered experimentally.