INFLUENCE OF MYOSIN ISOFORMS ON TENSION COST AND CROSSBRIDGE KINETICSIN SKINNED RAT CARDIAC-MUSCLE

1. In attempting to consolidate the role of ventricular isomyosins in regulating the contractility of the myocardium, actomyosin ATPase and crossbridge kinetics were obtained at 24 degrees C in chemically skinn ed isometrically contracting cardiac muscles containing V-1 and V-3 is omyosins. 2. The ATPase activity was measured at various levels of Ca2 + activation by the enzymatic coupling of ATP hydrolysis with the conv ersion of NADH to NAD(+). The crossbridge kinetics were inferred from small-amplitude perturbations of muscle length and muscle tension, and characterized by the frequency-domain parameter f(min). 3. The ATPase rates of V-1 and V-3 muscles obtained at various levels of Ca2+ activ ation were plotted against the corresponding proportional tensions. Th e ATPase vs tension plots were linear with slopes of 4.92 nmol/min(-1) per mm per mN and 1.98 nmol/min(-1) per mm per mN, respectively for, V-1 and V-3 muscles. Individual calculations of ATPase-to-tension rati os (nmol/min(-1) per mm per mN) gave corresponding averages of 4.98+/- 0.12 (s.e.m., n = 12) and 2.16+/-0.12 (s.e.m., n = 10). The myosin iso form induced proportional change in tension cost was accompanied by a similar change in f(min) (4.1+/-0.1 Hz and 1.95+/-0.03 Hz, means +/- s .e.m., for V-1 and V-3 muscles, respectively). 4. The observations and other published kinetic data are discussed in the context of models o f crossbridge cycling. It is suggested that the tension economy of V-3 muscle arises principally from an increase in the fraction of time, d uring the crossbridge cycle, when the crossbridge is exerting force.