Force transient time course in heart muscle with high and low V1 to V3 myosin isoenzyme ratio.

Values of mechanical and biochemical indicators of muscle performance measured in heart muscle having mostly the V1 isoenzyme of myosin are different than measurements of the same indicators made in muscle having mostly the V3 isoenzyme of myosin. It has been suggested that these differences in performance indicators might be attributable to subtle differences in myosin-actin crossbridge cycling kinetics between the V1 and the V3 isoforms of myosin. To investigate this, we derived information about myosin-actin cycling kinetics from the time course of force transients following rapid small amplitude length releases applied to chemically "skinned", isometrically contracting trabeculae from the hearts of normal (greater than = 90% V1) and propylthiouracil treated (greater than = 90% V3) rats. The rate constant for rapid force recovery measured in trabeculae from normal rats was twice that measured in trabeculae from treated rats (88.8 +/- 18.8 (n = 12) vs. 43.7 +/- 6.5 (n = 10)/s, mean +/- S.D.). We interpret this difference in rate constants as evidence that the kinetics of at least one step in the interaction of myosin with actin depends on the isoenzyme of myosin present in the heart.