Strategy of oxidative metabolism in bull spermatozoa

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Abstract
Hypotonically treated bull epididymal spermatozoa (HTBES) were prepared by the same technique used for hypotonically treated rabbit epididymal spermatozoa (HTRES) in order to compare the mitochondrial activities of the bull gamete to those of the rabbit gamete. Bull sperm mitochondria oxidize pyruvate plus malate, lactate plus malate, and acetyl‐L‐carnitine plus malate with energy coupling. The rate of O2 uptake by HTBES with pyruvate plus malate is more rapid than lactate plus malate. Bull sperm mitochondria also oxidize glutamate plus malate, L‐3‐glycerolphosphate, and palmitoyl CoA plus carnitine as well as palmitoyl‐L‐carnitine. The first mitochondrial activity enables bull sperm to utilize the malate/aspartate shuttle as well as the lactate/pyruvate shuttle to transfer reducing equivalents from NADH produced by glycolysis in the cytosol to the mitochondria for complete oxidation by O2. The latter two activities enable bovine sperm to utilize endogenous lipids and phospholipids for energy metabolism. Rabbit sperm mitochondria lack these three mitochondrial oxidative activities but have maximal oxidative activity with lactate as substrate (Storey and Kayne ('77). Bull spermatozoa are equipped metabolically for survival in the bovine oviduct lumen, which has a low content of oxidizable substrate, while rabbit spermatozoa are equipped for survival in the rabbit oviduct lumen, which has a high lactate content. The hypothesis was proposed that mitochondrial activities of sperm of a given species are adapted to the substrate content of the oviduct lumen of that species and so have predictive value concerning oviduct lumen content.