ISOPROTERENOL-DEPENDENT DECREASE IN OXYGEN-UPTAKE AND RESPIRATORY ENZYME-ACTIVITIES IN RAT MYOCARDIAL TISSUE AND MITOCHONDRIA

Objective: Myocardial damage induced by isoproterenol is believed to b e secondary to increased oxygen demands on the heart, Our objective wa s to test an additional primary action of isoproterenol on tissue and mitochondrial oxidative metabolism and to compare these effects with t he effects of other adrenergic agents in the presence of adrenergic in hibitors. Design: Prospective, dose-response study. Setting: Research laboratory at a university hospital. Subjects: Fifty Sprague-Dawley fe male rats (200 to 350 g), slightly anesthetized with ether and divided into several groups. Interventions: In 26 rats, the heart was removed , cut into fine slices (0.5-mm thickness), and placed in an ice-cold b uffer. In 22 animals, the hearts were perfused in the Langendorff mann er and chopped and processed for mitochoncarial studies. Measurements and Main Results: We determined the following: a) the direct ''in vitr o'' effects of isoproterenol and related catecholamines on normal oxyg en uptake using myo cardial slices; b) rat heart oxygen consumption an d mitochondrial oxygen uptake from isolated organs, perfused with isop roterenol; c) measurements of enzyme activities in submitochondrial pa rticles from the same perfused hearts; and d) the direct effects of is oproterenol on normal mitochondria isolated from normal nonperfused he arts. The oxygen uptake was determined polarographically with a Clark- type electrode and enzymatic activities were assayed by spectrophotome tric reduction of cytochrome c at 550 nm with different mitochondrial substrates. Isoproterenol (0.01 to 100 nM) decreased the oxygen uptake by the heart slices in a dose-dependent manner. In comparison, epinep hrine or norepinephrine per se did not change the parameter. However, with the addition of alpha-adrenergic receptor inhibitors, oxygen upta ke decreased to values similar to those values obtained with isoproter enol. Also, mitochondria isolated from hearts perfused with isoprotere nol had decreased state 3 respiratory rates (by 50%) and decreased res piratory control ratios (by 30%), without changes in adenosine 5'-diph osphate/oxygen ratios. The respiratory chain enzyme activities were al so lowered. Conclusions: The data suggest that while isoproterenol inc reases ''in vivo'' oxygen uptake by the working rat heart, isoproteren ol can simultaneously decrease maximal adenosine 5'-diphosphate-induce d mitochondrial oxygen uptake and in vitro myocardial tissue oxygen up take, probably by modifying the mitochondrial respiratory enzymes, Thi s action could be counteracted by alpha-adrenergic agonist effects.