The energy expenditure of actomyosin‐ATPase, Ca(2+)‐ATPase and Na+,K(+)‐ATPase in guinea‐pig cardiac ventricular muscle.

Abstract

1. The rate of heat production (heat rate) and isometric twitch tension of ventricular trabeculae isolated from guinea-pig heart were measured at 37°C in order to determine the relative contributions of actomyosin-ATPase, Ca2+-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 + 1P2 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 mN s cm2. 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 Ca2+-ATPase was studied by blocking Ca2+ influx into the myocardial cells with a solution containing 100 /M Ca2+ and 400 #M 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 Ca2+-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 uM 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 uM Ca2+ and 400,uM Ni2+, and then inhibiting the sodium pump with 400 uM 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 Ca2+-ATPase (0 09).7. It is concluded that contraction-related heat production can be completely accounted for by the three major ATPases, and that under our experimental conditions actomyosinATPase, Ca2+-ATPase and Na+,K+-ATPase contribute about 76, 15 and 9%, respectively, to myocardial energy expenditure. 8. In cardiac muscle contracting under aerobic conditions the heat rate measured in the steady state is proportional to the rate of ATP hydrolysis, 1 1uW corresponding to about 11 pmol ATP s-5. From our heat and tension measurements the mechanical impulse per ATP molecule was calculated to be 0-027 pN s.

Cardiac Basal Metabolism.

“…They thus attribute 14% of the basal metabolic rate to the Na ϩ -K ϩ ATPase. This relative value is in good agreement with that of Schramm et al [113], despite the difference of experimental conditions: heat production of intact guinea pig trabeculae at 37°C versus ATPase activity of skinned rat trabeculae at 20°C.…”

“…If the Q 10 of the pump is taken to be about 1.5 (see section VII [ii]), this value would be inflated to about 2 mW g Ϫ1 at 37°C. Thus both their absolute and relative values are in good agreement with those of Schramm et al [113]. Nevertheless, caution is advised since the net effect of saponin-skinning is difficult to estimate.…”

“…Calorimetric studies on very thin guinea pig preparations (50-350 m diameter) give astonishingly high values of basal metabolism (see Table 2). Values of 20 mW g Ϫ1 with glucose as substrate and values in excess of 30 mW g Ϫ1 with pyruvate as substrate have been reported [67,68,113]. Readers will find a thoughtful discussion of the problem by Daut and Elzinga [68].…”

“…A novel approach to estimating the contribution of the Na ϩ -K ϩ ATPase has been reported by Schramm et al [113]. These authors also employed the microcalorimetric method of measuring the rate of basal heat production of isolated trabeculae, but chose to use trabeculae from the guinea pig instead of from the rat.…”

“…This provides a brief "window-of-opportunity" during which the metabolic consequence of inhibiting the Na ϩ -K ϩ ATPase is revealed-as a brief diminution in the rate of resting heat production. Exploiting this opportunity (see their figure 7), Schramm et al [113] estimated that some 17% of the basal rate of heat production of guinea pig trabeculae (about 2 mW g Ϫ1 ) could be attributed to the Na ϩ -pump.…”