Phosphorylation of rodent cardiac myosin light chain 2: effects of exercise

The purpose of this study was to ascertain whether the degree of cardiac myosin light chain 2 (P-light chain) phosphorylation occurs as a function of changes in cardiovascular functional state as induced by 1) treadmill exercise (20-27 m/min, 0% grade for 20, 30, 45 min) (phase I) and 2) pharmacological intervention (phase II) in adult female Sprague-Dawley rats. It was hypothesized that cardiac myosin phosphorylation is regulated in accordance with time-dependent sustained elevations in myocardial work demands requiring alterations in either heart rate or left ventricular pressure development. Exercise heart rates (HR) and double products (HR x DP) were equivalent among the three exercise groups and were significantly elevated in comparison with the normal-rest (NR) group (P less than 0.05). In phase II, isoproterenol elicited higher HR, although the atenolol group exhibited a marked reduction in HR, mean arterial pressure, and double product relative to NR (P less than 0.05). Percent myosin P-light chain phosphorylation exhibited both a HR- and a work load-dependent modulation in P-light chain levels (-9% to +23% change) in the two phases of the study (P less than 0.05). These data are consistent with the view that the above responses are associated with modulations in intracellular calcium concentrations commensurate with the alterations in HR and left ventricular pressure. Also, elevations in P-light chain phosphorylation could serve to augment left ventricular pressure development under these functional states.