Capacitation- and fertilization-related alterations in mouse sperm oxygen consumption

Summary. On the basis of experiments performed some years ago, it has been generally accepted that mammalian sperm metabolic activity, as determined by manometric measurement of O2 uptake, increases during capacitation. However, when mouse sperm suspensions were incubated for a total of 120 min in glucose-containing medium which promotes both capacitation and fertilization, O2 consumption, measured with an O2 electrode at 30 and 120 min, consistently declined with time. This same pattern was also observed when pyruvate was the sole exogenous energy substrate, despite the fact that these cells, although capacitated, cannot fertilize eggs until supplied with glucose. Since mouse spermatozoa require a suitable glycolysable substrate to express full functional ability, these results suggest that a shift in metabolic activity from oxidative phosphorylation to glycolysis normally accompanies the onset of fertilizing ability in this species. It was further demonstrated that this pattern, in the presence of glucose, can be markedly modified by alterations in the medium composition which have been shown to modulate fertilizing ability. All conditions chosen inhibit fertilization while retarding (Ca2+-free), accelerating (hyperosmotic high Na+) or nor affecting (iso-osmotic high K+) the rate of capacitation. When suspensions were assessed in these media, O2 consumption increased rather than decreased over the 120 min, despite the continuous presence of glucose. When these same suspensions were assessed at 30 and 120 min in media altered to permit expression of fertilizing ability, the pattern observed was one of declining O2 consumption. For the condition which accelerated capacitation, the decline relative to the inhibitory medium was seen at both 30 and 120 min, while with the other two, the shift was only observed at 120 min when suspensions would have obtained some degree of competence. The ionic composition of the extracellular environment therefore markedly affects sperm metabolic activity which in turn may modulate sperm functional ability both in vitro and in vivo.