Phosphorylation of myosin in perfused rabbit and rat hearts

Working perfused rabbit and rat hearts were used to determine whether changes in myosin P-light chain phosphate content could affect cardiac contractility. Control perfused rabbit and rat hearts contained 0.48 ± 0.02 and 0.61 ± 0.02 mol phosphate/mol P-light chain, respectively. Perfusion of (1) rabbit hearts with 0.1 μM isoproterenol for 30 s, (2) rabbit or rat hearts with a five-fold increase in perfusate [Ca²⁺] for 30 s, or (3) rabbit hearts with a three-fold increase in [Ca²⁺] for 5 and 15 min caused significant increases in LV dP/dtmax, but had no effect on P-light chain phosphate content. Perfusion of rabbit hearts with Ca²⁺-free buffer or buffer containing 22 mM K⁺ caused complete cessation of contractile activity within 30 s and reduced P-light chain phosphate content to 50 and 70%, respectively, of control values after 30 min. Reperfusion of hearts exposed to 22 mM K⁺ with control buffer restored LV dP/dtmax to control values within 10 min, whereas P-light chain phosphate content remained at 70% of control value. Thus, changes in cardiac contractile state were not accompanied by changes in the extent of phosphorylation of P-light chain.