Metabolism and capacitation

Having investigated the structure, development and maturation of spermatozoa, it is now necessary to consider their metabolism, function and movement. Detailed understanding of these areas should allow the development of appropriate conditions for the storage and transfer of spermatozoa with minimal interruption to their normal function and fertilizing capacity.

Metabolism

Testicular spermatozoa differ in several ways from spermatozoa found within the cauda epididymis (that is, post maturation). Many of these differences are related to changes in metabolism and/or biochemical reactions within the spermatozoa. Testicular spermatozoa are largely immotile, though they may show limited motility after incubation in vitro. Testicular spermatozoa are also incapable of fertilization (Setchell et al., 1969; Voglmayr, 1975; Voglmayr et al., 1978).

Upon examination it can be seen that the composition of testicular spermatozoa differs from that of caudal epididymal and ejaculated spermatozoa, especially in relation to their lipid content. Testicular spermatozoa have higher concentrations of phospholipids (Setchell, 1991). These phospho-lipids have been identified in bull and ram spermatozoa as phosphatidylcholine (63%), phosphatidylethanolamine (15%) and lysophosphatidylcholine (10%) (Parks et al., 1987). In the case of testicular spermatozoa, the predominant fatty acid within the phospholipids is palmitic acid (16C saturated fatty acid), whereas phospholipids in caudal epididymal spermatozoa contain predominantly docosahexanoic acid (22C unsaturated fatty acid with six double bonds) (Parks and Hammerstedt, 1985). A significant decline in the cholesterol content of the membranes of cauda epididymal spermatozoa and testicular spermatozoa has also been reported. It has been suggested that this change may be necessary for spermatozoon transit and storage within the epididymus (Lopez and Souza, 1991).

If testicular spermatozoa are incubated in vitro in a phosphate-buffered saline resembling serum, they metabolize a high percentage of available glucose to carbon dioxide, amino and carboxylic acids plus inositol. On the other hand, if cauda epididymal spermatozoa are similarly treated they convert most of the glucose to lactate (Setchell et al., 1969; Voglmayr, 1975). It is apparent that some significant changes occur in the metabolic and biochemical reactions within the spermatozoa between their release from testicular tissue and their presence in the caudal epididymis; that is, during maturation. Spermatozoa emerging from the cauda epididymis have, therefore, already undergone and completed the vast majority of their metabolic changes. The components required for spermatozoan function and metabolism are in place, having been developed during spermatogenesis. The metabolic processes within mature spermatozoa are, however, limited to those providing energy for movement, initiation of glycolysis and other catabolic processes, the maintenance of ionic balance and cell function. Their ability to divide and repair cell damage has been lost (Hiipakka and Hammerstedt, 1978; Amann and Graham, 1993).

For subsequent activity the prime energy source available to spermatozoa are carbohydrates from the extracellular substrate. Stallion spermatozoa readily metabolize monosaccharide glucose but have a limited ability to utilize fructose. They also have a limited and variable ability to utilize other sugars or more complex carbohydrates as energy sources (Mann, 1964a). In contrast to stallion spermatozoa, bull and ram spermatozoa are able to utilize fructose and sorbitol as energy sources (Mann, 1964a), and will rapidly use all the monosaccharide energy sources available in seminal plasma within 15-20 min at 37°C. This does not occur so rapidly with stallion spermatozoa (Amann and Graham, 1993).

As can be seen in Fig. 4.31, the use of extracellular glucose results in the production of two ATP molecules via anaerobic metabolism and 36 by aerobic metabolism, making 38 ATP molecules available for the maintenance and movement of the spermatozoon. In order for the energy sources to be utilized, they have to be transported into the spermatozoon across the plasma

Glucose-6-phosphate -

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