Frequency-dependent myosin light chain phosphorylation in isolated myocardium.
A specific light chain subunit (P-light chain) of myosin from striated and smooth muscles is phosphorylated by Ca2+ calmodulin-dependent myosin light chain kinase. Phosphorylation of cardiac P-light chain was examined in isolated perfused rabbit ventricular septae to determine the effect of contraction frequency on this Ca2+-dependent reaction. Muscles stimulated at 42 beats/min had 0.23 mol phosphate/mol P-light chain which decreased to 0.12 mol phosphate/mol P-light chain when the muscles were made quiescent (0 beats/min in the presence of 22 mM K+ for 30 min). Rephosphorylation of P-light chain to 0.24 mol phosphate/mol P-light chain occurred in muscles stimulated at 84 beats/min for 90 min but not in muscles stimulated at 42 beats/min for 30 min (0.15 mol phosphate/mol P-light chain). Stimulation at frequencies ranging from 0 to 126 beats/min for 30 min produced a frequency-dependent increase in P-light chain phosphorylation from 0.1 to 0.4 mol phosphate/mol P-light chain. Increased inotropy for 30 s with isoproterenol was not associated with significant increases in P-light chain phosphorylation in muscles stimulated at 42 beats/min. The rates of myosin P-light chain phosphorylation and dephosphorylation in ventricular muscle are much slower than the reported rates of phosphorylation in either fast-twitch skeletal or smooth muscles. The extent of cardiac P-light chain phosphorylation appears dependent upon the steady-state frequency of contraction.
D000818:Animals; D006339:Heart Rate; D006346:Heart Septum / Q000502:physiology; D066298:In Vitro Techniques; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism*; D009219:Myosin-Light-Chain Kinase; D009218:Myosins / Q000378:metabolism*; D010477:Perfusion; D011189:Potassium Chloride / Q000494:pharmacology; D011494:Protein Kinases / Q000378:metabolism*; D011817:Rabbits; D013997:Time Factors
D011189:Potassium Chloride; D011494:Protein Kinases; D009219:Myosin-Light-Chain Kinase; D009218:Myosins