Regulation of contraction in cardiac and smooth muscles.

Newer aspects of the contractile and regulatory proteins of cardiac and smooth muscles are described. Contraction in both tissues depends on Ca2+. In heart muscle, a fast, direct Ca2+-trigger system involving the troponin complex is operative on the actin filaments. Both the Ca2+-binding troponin-C and the phosphorylatable troponin-I display evolutionary diversification of properties characteristic of the heart muscle. Further modulation of heart-muscle contraction is governed by phosphorylation reactions, which are dependent on either Ca-calmodulin (CM) or cAMP. Both these regulatory pathways are operative in parallel. In smooth muscle, no fast Ca2+-trigger system is present. Smooth-muscle contraction is regulated by two systems, which operate indirectly and slowly via covalent protein modification. One involves phosphorylation of the myosin regulatory light-chain, the other phosphorylation of caldesmon on the actin filaments. Both are dependent on the Ca2+-binding CM. The CM-dependent myosin light-chain kinase (MLCK) itself is, in smooth muscle only, subject to control by a cAMP-dependent phosphorylation system. Thus, drugs that cause an increase in cAMP in smooth muscle, unlike in heart muscle, may lead to relaxation without lowering cytosolic free Ca2+ concentration.