Thick-filament-linked calcium regulation in vertebrate striated muscle

Abstract

CALCIUM is the trigger initiating muscular contraction, and relaxation occurs on its removal. As contraction is caused by the cyclic attachment and detachment of myosin-crossbridges of the thick filament with actin of the thin filament, calcium, in principle, could modify muscular activity by affecting regulatory systems on either the thick or the thin filaments. It is generally accepted that a thin-filament-linked Ca²⁺-regulatory system, involving troponin and tropomyosin, is responsible, at least in part, for the on-off switch governing contraction in vertebrate striated muscle^1,2. Several studies also suggest the possibility of a simultaneously operating Ca²⁺-regulatory system on the thick filaments, presumably involving the myosin molecules^3–6. However, the actin-activated Mg-ATPase of purified vertebrate striated muscle myosin has not been shown to be Ca²⁺ dependent, and hence direct evidence for a thick-filament-linked regulatory system is lacking⁶. Nonetheless, it is debatable whether the latter evidence precludes the existence of a thick-filament-linked system. The effect may simply result from an essential component of the thick filament being lost or denatured during standard myosin purification procedures. Alternatively, actomyosin which is routinely formed from repolymerised pure proteins may not assume the proper quaternary structure allowing for expression of myosin's Ca²⁺ sensitivity. In view of the ambiguities inherent in these studies, we adopted another approach and report here our attempts to test for thick-filament-linked Ca²⁺ regulation in actomyosin preparations in which we had removed troponin–tropomyosin from the thin filaments. The Ca²⁺ sensitivity of intact thick filaments could then be measured with both the residual and added pure actin. Our results do suggest the presence of a thick-filament-linked Ca²⁺-regulatory system.