Phosphorylation of C-protein in intact amphibian cardiac muscle: Correlation Between 32P Incorporation and Twitch Relaxation

Abstract

The molecular mechanisms by which neurotransmitters modulate the force of contraction of cardiac muscle are incompletely understood. Hartzell and Titus (1982. J. Biol. Chem. 257:2111-2120) have recently reported that C-protein, an integral component of the thick filament, is reversibly phosphorylated in response to ionotropic agents. In this communication, Cprotein phosphorylation (as measured by isotopic labeling with 32P) is correlated with changes in the rate of relaxation of twitch tension. On the average, isoproterenol simultaneously increases peak systolic tension twofold, decreases twitch relaxation time from a control value of ~450 to ~300 ms, and increases C-protein phosphorylation two- to threefold, with a maximum effect occurring <60 s after addition of 1 µM isoproterenol. Carbamylcholine, in contrast, decreases peak systolic tension more rapidly than it affects relaxation or Cprotein phosphorylation. The maximum decrease in peak tension (60%) occurs within 1 min of addition of 0.5 µM carbamylcholine, but relaxation time increases slowly to 800 ms over ~6 min. The increase in relaxation time correlates well with the decrease in 32P incorporation into C-protein (τ = 0.94). Changing beat frequency between 0.2 and I/s has no effect on C-protein phosphorylation but does alter relaxation time (relaxation time decreases ~100 ms when beat frequency is changed from 0.5 to 1/s) and thus alters the quantitative relationship between C-protein phosphorylation and relaxation rate. These results suggest that two separate processes affect relaxation. It is proposed that the level of C-protein phosphorylation sets the boundaries over which relaxation is regulated by a second process that is dependent upon beat frequency and probably involves changes in intracellular Ca. © 1984, Rockefeller University Press., All rights reserved.