Multiple structures of thick filaments in resting cardiac muscle and theirinfluence on cross-bridge interactions

Based on two criteria, the tightness of packing of myosin rods within the b ackbone of the filament and the degree of order of the myosin heads, thick filaments isolated from a control group of rat hearts had three different s tructures. Two of the structures of thick filaments had ordered myosin head s and were distinguishable from each other by the difference in tightness o f packing of the myosin rods. Depending on the packing, their structure has been called loose or tight. The third structure had narrow shafts and diso rdered myosin heads extending at different angles from the backbone. This s tructure has been called disordered. After phosphorylation of myosin-bindin g protein C (MyBP-C) with protein kinase A (PKA), almost all thick filament s exhibited the loose structure. Transitions from one structure to another in quiescent muscles were produced by changing the concentration of extrace llular Ca. The probability of interaction between isolated thick and thin f ilaments in control, PKA-treated preparations, and preparations exposed to different Ca concentrations was estimated by electron microscopy. Interacti ons were more frequent with phosphorylated thick filaments having the loose structure than with either the tight or disordered structure. In view of t he presence of MgATP and the absence of Ca, the interaction between the myo sin heads and the thin filaments was most likely the weak attachment that p recedes the force-generating steps in the cross-bridge cycle. These results suggest that phosphorylation of MyBP-C in cardiac thick filaments increase s the probability of cross-bridges forming weak attachments to thin filamen ts in the absence of activation. This mechanism may modulate the number of cross-bridges generating force during activation.