The Energy Expenditure of Actomyosin-ATPase, Ca(2+)-ATPase and Na+,K(+)-ATPase in Guinea-pig Cardiac Ventricular Muscle

1. The rate of heat production (heat rate) and isometric twitch tension of ventricular trabeculae isolated from guinea-pig heart were measured at 37 degrees C in order to determine the relative contributions of actomyosin-ATPase, Ca(2+)-ATPase and Na+,K(+)-ATPase to myocardial energy metabolism. 2. The increase in heat rate recorded during isometric contractions at optimal length (contraction-related heat production) was 19.1 +/- 1.2 mW cm-3 at a stimulation rate of 2 Hz. The tension-time integral of individual contractions measured under the same conditions was 147 +/- 15 mM s cm-2. 3. The heat production of the actomyosin-ATPase was determined by inhibiting the contractile proteins with 2,3-butanedione monoxime (BDM). Contraction-related heat production was reduced by 0.219 +/- 0.010 and the isometric tension-time integral was reduced by 0.288 +/- 0.016 in the presence of 1 mM BDM. From these data an estimate of 0.76 for the relative contribution of the actomyosin-ATPase to contraction-related heat production was derived. 4. The heat production related to actomyosin-ATPase plus Ca(2+)-ATPase was studied by blocking Ca2+ influx into the myocardial cells with a solution containing 100 microM Ca2+ and 400 microM Ni2+. In this solution contraction-related heat production was reduced by 0.907 +/- 0.012. Comparison of this value with the component attributable to the actomyosin-ATPase yields an estimate of 0.15 for the relative contribution of the Ca(2+)-ATPase to contraction related heat production. 5. The heat production related to the Na+,K(+)-ATPase in resting preparations was studied by blocking the sodium pump with 400 microM dihydro-ouabain (DHO). DHO produced a transient decrease in heat rate lasting 1-2 min, which was followed by a secondary increase. From the heat transient produced by DHO the heat rate related to the Na+,K(+)-ATPase in the steady state was extrapolated. The relative contribution of the sodium pump to resting heat production was estimated to be 0.17. 6. The heat production related to the Na+,K(+)-ATPase in contracting preparations was studied by first blocking Ca2+ influx with 100 microM Ca2+ and 400 microM Ni2+, and then inhibiting the sodium pump with 400 microM dihydro-ouabain (DHO). The relative contribution of the sodium pump to contraction-related heat production extrapolated from these data was 0.10, which agreed well with the fraction of contraction-related heat production persisting after blockage of actomyosin-ATPase and Ca(2+)-ATPase (0.09).(ABSTRACT TRUNCATED AT 400 WORDS)