Calcium and regulation of contraction: a short review.

The purpose of this paper is to review very briefly how our understanding of the control of muscle contraction by calcium has developed, and to give an overview of the different regulatory systems that have been shown so far. It should provide a background for some of the more specific presentations that appear later in the symposium. Three different molecular mechanisms for calcium regulation of actin-activated myosin Mg-ATPase activity have been identified. Control of contraction and relaxation in different types of muscle is dominated by one or the other of these regulatory mechanisms. The troponin-tropomyosin system associated with the actin filaments is the best known of the calcium control systems. It operates, for instance, in vertebrate skeletal and cardiac muscles. Direct binding of Ca2+ to myosin controls contraction in muscles of certain invertebrates such as the scallop. This calcium binding is dependent on the presence of the regulatory light chains on the myosin molecules. In vertebrate smooth muscle, calcium in conjunction with calmodulin activates an enzyme, myosin light chain kinase, which phosphorylates the regulatory light chains of the myosin, and this phosphorylation triggers contraction. Relaxation is brought about by dephosphorylation of the light chains through the action of phosphatase(s). Additional regulatory mechanisms, not yet fully identified, appear to operate in smooth muscles.