Production in Heart Homogenates

H The metabolic requirements of a continu-ously functioning organ such as the heart differ in important respects from those of other structures. Tt is, therefore, not surpris-ing that many differences in cellular compo-nents and metabolism have been observed between the myocardium and other tissues. Thus, in contrast to white skeletal muscle, myocardial cells contain a very high concen-tration of mitochondria per unit weight.1 Be-cause of this, the heart is able to oxidize a great variety of substances at a rapid rate2 and to maintain a high level of adenosinetri-phosphate (ATP) by oxidative phosphoryla-tion.8 It has been demonstrated that the myocardium is capable of keeping the intra-cellular concentration of ATP constant under conditions during which the heart is markedly stimulated or depressed. In previous publications,5"7 we have de-scribed the properties of what may be called a "glucose oxidation systein " of rat heart. Homogenates, free of cellular debris, were found to oxidize glucose for many hours at a rate approximating that of the intact heart in vivo. Under optimum conditions, the glu-cose utilized was completely oxidized to CO2 and H2O, and there was little or no accumu-lation of intermediary metabolites. When intermediates of the glycolytie and tricar-boxylic acid pathways were added to the sys-tem, they were found to be oxidized rapidly.5 It is evident that the heart homogenate i