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Response of Isolated Muskrat and Guinea Pig Hearts to Hypoxia

The hypoxic resistance of hearts of a diving mammal (Ondatra zibethica) and of a nondiver, guinea pig, was investigated in a Langendorff perfused preparation. Hearts were perfused for 20-30 min under hypoxic conditions followed by 20-30 min of reoxygenation. Hearts were paced at 250 beats per minute and perfused at a pressure of 70 cm H₂O. In the absence of insulin, left ventricular pressure decreased less in the muskrat heart during hypoxia and recovered more during reoxygenation compared with guinea pig hearts. Lactate production was similar in the two species. Glucose uptake could not be demonstrated by chemical means. When insulin was added to the perfusion medium, glucose uptake was about 60% greater in muskrat hearts compared with the guinea pig hearts. Lactate production was also significantly greater. In spite of the greater anaerobic metabolism in the muskrat, left ventricular pressure was similar during hypoxia in the two species. However, during reoxygenation mechanical performance recovered more completely in the muskrat. When hearts were perfused with a buffer containing both glucose and palmitic acid under normoxic conditions, the preferred fuel was glucose for the muskrat and fatty acid for the guinea pig. It is concluded that increased energy supply and reduced energy demand are mechanisms that confer hypoxic resistance to isolated muskrat hearts.