Effects of extracellular calcium concentrations on myosin P light chain phosphorylation in hearts from running-trained rats

Certain biochemical responses to clevated perfusate calcium concentrations were studied in the isolated hearts of sedentary and running-trained rats from which natural actomyosin was prepared. Both the V_max and Ca²⁺ sensitivity of the Ca²⁺-stimulated, Mg²⁺-dependent actomyosin adenosine triphosphatase (ATPase) were similar in the hearts of trained and sedentary animals, and were not altered when the concentration of extracellular Ca²⁺ was increased. Enhanced myosin Ca²⁺-ATPase activities and phosphate contents of myosin P light chains were found in the hearts of trained animals compared with controls, and these differences were still maintained when perfusate Ca²⁺ concentrations were increased. Although treatment of the perfused hearts with isoproterenol also increased both parameters in the trained and sedentary series, the V_max for myosin Ca²⁺-ATPase and the alkali-labile phosphate contents of myosin P light chains remained, throughout, significantly greater in the hearts of trained rats than in their sedentary counterparts. These differences were not eliminated by the combined use of isoproterenol and high perfusate Ca²⁺. The results suggest that an enhanced capacity for trans-sarcolemmal Ca²⁺ flux in the hearts of trained animals may be responsible for enhanced Ca²⁺-dependent phosphorylation of myosin P light chains, and thus improving cardiac function.