The turnover of phosphate bound to myosin light chain-2 in perfused rat heart.

The rate of exchange of phosphate bound to ventricular myosin light chain-2 (LC2-P) was measured in rat hearts perfused with [32P]Pi at various levels of perfusate Ca2+. Computer simulations of the light-chain labelling suggested the presence of two isotopically distinct pools of LC2-P, one large pool comprising 90% of the total and a small pool consisting of the remaining 10%. At control levels of perfusate Ca2+ the phosphate of the large pool turned over very slowly (t 1/2 congruent to 250 min), whereas that of the small pool turned over much more rapidly (t 1/2 congruent to 1 min). At high levels of perfusate free Ca2+ (5mM) the turnover of the phosphate of the small pool decreased markedly, whereas that of the large pool remained little changed. Conversely, at low perfusate free Ca2+ (0.2 mM), the turnover of the large pool decreased, whereas that of the small pool remained unchanged. The possible identity of these two pools is discussed. The total myosin-light-chain kinase activity of rat ventricle was found to be only 2-3-fold higher than the kinase activity expressed in the heart under control conditions. This, coupled with the very low turnover of most of the LC2-bound phosphate, implies that, in heart, there is insufficient myosin-light-chain kinase activity to cause a rapid rise in the overall level of light-chain phosphorylation, even under conditions of increased cytoplasmic Ca2+.