Fine-tuning of cross-bridge kinetics in cardiac muscle of rat and mouse by myosin light chain isoforms.

Cross-bridge kinetics underlying stretch-induced force transients was studied in cardiac muscle strips with different myosin heavy chain (MHC) and myosin light chain (MLC) isoforms. The force transients were induced by stepwise stretches of maximally Ca(2+)-activated skinned muscle strips. The MHC and MLC isoforms were analyzed by electrophoreses after the mechanical experiments. Muscle strips of euthyroid rats and mice exclusively containing alpha-MHC were used. In addition, muscle strips of hyper- and hypothyroid rats containing different combinations of MHC and MLC isoforms were used. The thyroid hormone is known to alter the expression of MHC but not of MLC isoforms. In muscle strips containing exclusively alpha-MHC, atrial MLC isoforms (all atria of rats and mice) were associated with about 30% faster kinetics than ventricular MLC isoforms (ventricles of hyperthyroid rats and some muscle strips of ventricles of euthyroid rats and mice). On the other hand, in muscle strips containing exclusively ventricular MLC isoforms, alpha-MHC (ventricles of hyperthyroid rats) was associated with about 2.6 times faster kinetics than beta-MHC (ventricles of hypothyroid rats). We conclude that the MLC isoforms fine-tune cross-bridge kinetics, which underlies stretch-induced force transients, whereas the MHC isoforms mainly determine this kinetics. The effect of MLC isoforms on the cross-bridge kinetics may partially contribute to the faster twitch contraction in atria than in ventricles. Furthermore, it may play a role in various cardiomyopathies where atrial MLC isoforms are partially expressed in ventricles or ventricular MLC isoforms are partially expressed in atria.