Phosphorylation of myosin in permeabilized mammalian cardiac and skeletal muscle cells.

The effect of myosin phosphorylation on tension production at less than 50% maximal activation by Ca2+ was examined in rabbit psoas and ventricular muscle. For psoas fibers, tension was determined at pCa 6.0, 5.8, 5.6, 5.5, and 5.4. Myosin light chain kinase (0.15 microM) and calmodulin (2 microM) were added, and the fibers were incubated at pCa 5.4, which resulted in an increase in light chain phosphorylation (P-light chain) from 5-10 to 60-75%. After 5 min, the sequence of pCa activations was repeated. An identical protocol was followed for cardiac muscle, except the activation solutions were pCa 6.2, 6.0, 5.9, 5.8, and 5.6. Phosphorylation of P-light chain increased tension in both permeabilized cardiac and skeletal muscle fibers. The effect manifested itself as a leftward shift in the pCa-tension relationship at levels below 50% maximal activation, with a decrease in the slope of the pCa-tension relationship. These results indicate that P-light chain phosphorylation affects actin-myosin interactions in cardiac and skeletal muscles at submaximal levels of Ca2+ activation.