The metabolism of semen in the presence and in the absence of carbon dioxide was examined using isotopically labelled substrates. When carbon dioxide was present, both oxygen consumption and carbon dioxide formation were determined in the one manometric flask. Respired carbon dioxide had no effect on oxygen uptake, substrate oxidation or aerobic fructolysis of ram spermatozoa. The addition of low levels of potassium and magnesium ions to the diluent stimulated the metabolism of washed ram spermatozoa, but there was still no effect of carbon dioxide on aerobic metabolism. Incubation of ram spermatozoa in an atmosphere containing 3·8% carbon dioxide increased the amount of lactate accumulated but had no effect on oxygen uptake or substrate oxidation. The necessity to distinguish between the effects of pH and carbon dioxide has been shown. In the presence of added fructose, the respiratory quotient of ram spermatozoa was approximately unity but was 0·78 for washed semen incubated in phosphate-buffered saline without added substrate.
The presence of respired carbon dioxide during incubation of bull spermatozoa had no effect on oxygen uptake or substrate oxidation but increased lactate accumulation. With fructose as substrate, incubation of bull spermatozoa in phosphate-buffered saline and saline gave respiratory quotients of 0·97 and 0·83 respectively.
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