The Use of Glucose, Lactate, Pyruvate, and Palmitic Acid by Muskrat and Guinea Pig Hearts

Hearts were removed from anesthetized guinea pigs and muskrats and were perfused with a buffer containing glucose, pyruvate, lactate, and palmitic acid under both working and nonworking conditions. The uptake of glucose and the release of lactate was greater in muskrat hearts than in guinea pig hearts under all conditions. The uptake of pyruvate was small in the hearts of both species but was significantly greater in guinea pig hearts. The oxidation of exogenous palmitic acid was similar under nonworking and low-level working conditions, but the guinea pig hearts oxidized more palmitic acid under the higher-level working conditions than did muskrat hearts. Under all working conditions the muskrat hearts failed to produce a cardiac output in excess of that required to supply their own coronary flow. Guinea pig hearts produce a cardiac output and a rise in left ventricular pressure. Mitochondrial function in the hearts of the two species was similar except that state 4 respiration rate in the muskrat heart was almost twice as great as the rate in mitochondria from guinea pig hearts. Gross morphology of the hearts of the two species was different. Muskrat hearts were narrower and had thinner walls than did guinea pig hearts. Measurement of myocardial concentrations of glycolytic intermediates under normoxic and hypoxic conditions suggests that, in guinea pig hearts, glycolysis may be limited at the phosphofructokinase and glyceraldehyde 3-phosphate dehydrogenase steps. The rate-limiting step could not be determined in muskrat hearts. Lactate dehydrogenase activity was much higher in muskrat hearts than in guinea pig hearts and contained more of the M-type subunit. Compared to guinea pig hearts, muskrat hearts rely more on O₂-conserving carbohydrate metabolism and do not pump well under working conditions.