ATP levels in hamster spermatozoa during capacitation in vitro.

Abstract

Capacitated sperm have been reported to exhibit elevated rates of motility, oxygen uptake, and glucose consumption. In order to investigate the biochemical basis for this elevation of energy metabolism, we have developed a serum-based test-tube system that capacitates sufficient quantities of hamster sperm to permit biochemical analysis. The rate of motility of sperm incubated in this system is greatly elevated over that of controls. Measurement of sperm ATP content revealed that the capacitating sperm maintained an ATP level one-third to one-fifth that of the control sperm whose content was about 50 nmoles ATP/108 sperm. This reduction in ATP content was found to be caused by the low molecular weight fraction of human serum. It was accompanied by large reductions in sperm ATP/ADP ratios. Data are presented that show the reduction in ATP content of capacitating sperm was not due to external ATPase, differential cell lysis, or nucleotide extraction artifacts. The average concentration of ATP within uncapacitated sperm was approximated to be 5 mM and that of capacitated sperm about 1 mM. This is 100 times the amount known to be required for hamster sperm flagellation and indicates that under these conditions ATP level does not control motility. Evidence from the literature is presented indicating that sperm motility is instead controlled via cAMP and possibly cAMP-dependent protein kinase. In capacitating hamster sperm an increase in cAMP synthesis has been reported. This observation is consistent with the elevated motility described here and would account for the reduction of sperm ATP levels and ATP/ADP ratios to new values. These lower values are known to activate sperm oxidative phosphorylation and glycolysis, thus accounting for the reported elevation in energy metabolism of capacitated sperm. Stimulation of sperm motility, with the consequent increase in energy metabolism, may be required for the penetration of the zona in fertilization.