Influence of myosin isoforms on tension cost and crossbridge kinetics in 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°C in chemically skinned isometrically contracting cardiac muscles containing V₁ and V₃ isomyosins. 2. The ATPase activity was measured at various levels of Ca²⁺ activation by the enzymatic coupling of ATP hydrolysis with the conversion 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 V₁ and V₃ muscles obtained at various levels of Ca²⁺ activation were plotted against the corresponding proportional tensions. The 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, V₁ and V₃ muscles. Individual calculations of ATPase‐to‐tension ratios (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 isoform 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 V₁ and V₃ muscles, respectively). 4. The observations and other published kinetic data are discussed in the context of models of crossbridge cycling. It is suggested that the tension economy of V₃ muscle arises principally from an increase in the fraction of time, during the crossbridge cycle, when the crossbridge is exerting force.