The results reported in this paper indicate that dinitrophenol acts directly on the isolated heart, increasing its metabolic rate. It also produces heart failure associated with a low phosphocreatine content of the muscle but with no change in adenosine triphosphate, which may or may not be due to a relative hypoxia of the cardiac tissue. Experimental arterial hypoxaemia, if severe, produces a similar picture of heart failure with a decrease in phosphocreatine and no change in adenosine triphosphate. Ligation of the coronary arteries results in disappearance of the major part of the phosphocreatine within a few minutes regardless of whether or not ventricular fibrillation ensues; the adenosine triphosphate remains unchanged.2: 4-Dinitrophenol increases oxidative metabolism in animals several-fold by direct cellular action (Tainter and Cutting, 1933). It is also known to uncouple phosphorylation and oxidation in tissue homogenates or particulate systems derived therefrom (Loomis and Lipmann, 1948). Hence, if dinitrophenol were added to a welloxygenated heart-lung preparation, one would expect to observe a failure of the heart. This failure would be due to lack of production of high-energy phosphate bonds, instead of lack of utilization of such bonds as is the case in heart failure due to barbiturates (Wollenberger, 1947;Fawaz and Hawa, 1953). This study was undertaken to determine to what extent observations made on homogenates and simpler systems can be applied to organs in activity. Since dinitrophenol may produce relative hypoxia by increasing metabolism, the effects of arterial hypoxaemia and ischaemia on the phosphorus compounds of the heart were also studied. METHODSPentobarbitone anaesthesia was used in all animals. Heart-lung preparations were made by the conventional procedure except that the blood donors were anaesthetized with chloroform.Detailed information for preparing samples from the left ventricle and for estimating the phosphorus compounds has been described previously (Fawaz and Hawa, 1953). In the present work, the labile nucleotide phosphorus was determined by hydrolysing the trichloracetic acid filtrate in N-HCl for 7 instead of 10 min. Experiments with pure crystalline adenosine triphosphate (ATP) showed that this was a sufficient period for 2/3 of the ATP phosphorus to be liberated. Our values are therefore slightly lower than before. In several control experiments and in a dinitrophenol experiment we compared the results obtained by this method with the paper chromatographic method for ATP determination. Nucleotides were precipitated from the trichloracetic acid filtrates with mercuric acetate (Kerr, 1940). After removal of the mercury by H2S and evaporation to a small volume, the nucleotides were chromatographed on filter paper by the method of Magasanik, Vischer, Doniger, Elson, and Chargaff (1950). It was found that at least 90% of the 7 min. value, as determined on the trichloracetic acid filtrate, was accounted for by true ATP.The phosphorus values in this paper are expressed not as...

The values reported in the literature for phosphocreatine in mammalian cardiac muscle vary widely. For the intact rat heart the published figures, expressed in terms of phosphocreatine-P, vary from 5.7 to 12 mg % (1-4). For the intact guinea. pig heart and cat heart, the figures are 5.2 and 6.0 mg %, respectively( 5,6). For the intact dog heart, the figures vary from 4.2 to 9.0 mg %(6-8). For the isolated dog heart (heart-lung preparation) the published values range from 6.2 t o 2 1 .S mg %, depending on the experimental conditions (8-10) . Wollenberger ( 8) and Pollack et d ( 9 ) state that the phosphocreatine content of the heart in the intact animal is considerably below that of the normal heartlung preparation. Mommaerts( 11) states, without citing references, that cardiac muscle may contain one-tenth {as much phosphocreatine as striated muscle, and he gives the value 2 x l W mole per g for cardiac muscle, viz.6.2 mg % phosphocreatine-P.In connection with our studies on the effect of drugs on the metabolism and performance of the isolated dog heart (heart-lung preparation), we found it necessary to redetermine the "resting" values of phosphocreatine in mammalian heart muscle. At the same time we have made determinations of the corresponding values for the labile "nucleotide" phosphorus.Methods. Pentobarbital anesthesia was used in all animals including those designated for heart-lung preparations. Artificial respiration was applied before opening the chest. Heart-lung preparations were performed by the conventional procedure except that the blood donors were anesthetized with chloroform. A sample of heart muscle was cut from the left ventricle and quickly immersed in liquid air or in a mixture of COZ-snow and ether (temp. -60°C).When liquid air was *This study has been supported by a grant from the American Heart Assn. used the frozen tissue was ground into a fine powder in a mortar chilled by adding successive portions of liquid air to make sure that the tissue did not thaw. The powdered materid was then transferred to a previously weighed Erlenmeyer flask containing 5 % trichloracetic acid using a spoon and a widenecked funnel that had been dipped in liquid air. The flask was shaken to mix the contents and then reweighed. The volume of 5% trichloracetic acid employed was adjusted to give a dilution of 1 : 10, reckoning the water content of the tissue as 801%. Not more than 10 minutes was allowed to elapse between the fixing of the powder with acid and the time when the protein-free filtrate was neutralized. During this period of time all operations were carried out iat a temperature which did not exceed 2 "C. Inorganic phosphate and phosphocreatine were determined by the method of Fiske and Subbarow( 1 2 ) and the labile "nucleotide" phosphorus was calculated by subtracting the sum of the phosphocreatine and inorganic phosphorus from the value for the total inorganic phosphorus obtained after hydrolysis for 110 minutes in 1 N HC1 at li0O"C. In a subsequent publication we are discussing the extent to which ...