Phosphorylation of contractile proteins in heart and skeletal muscle

Contractile performance of cardiac and skeletal muscles may be regulated by cyclic AMP or Ca²⁺, two second messengers that stimulate the phosphorylation of specific myofibrillar proteins. Cyclic AMP-dependent protein kinase catalyzed the rapid phosphorylation of a single site in the inhibitory subunit of cardiac troponin in vitro and in perfused hearts. Skeletal muscle troponin was not phosphorylated by this enzyme in vivo. Although there was a correlation between cardiac troponin phosphorylation and the positive inotropic response to catecholamines, a biochemical mechanism that could account for a functional relationship between the two processes has not been discoverd. Phosphorylation of skeletal muscle myosin was catalyzed by myosin light chain kinase in the presence of Ca²⁺ and the ubiquitous, multifunctional Ca²⁺-dependent regulator protein (CDR). The activation of kinase activity appeared to proceed via a trimolecular reaction process in which Ca²⁺ bound to CDR and the Ca²⁺. CDR complex then interacted with the enzyme. In rat extensor digitorum longus muscle, a 1 sec tetanic contraction resulted in phosphorylation of myosin light chain with the maximal phosphate incorporated 20 sec after the contraction. The light chain phosphate content declined slowly and correlated to post-tetanic potentiation of isometric twitch tension. Phosphorylation of skeletal muscle myosin may be important in modulating contraction.