The Effect of Moderate Epididymal Aging on the Kinetics of the Acrosome Reaction and Fertilizing Ability of Hamster Spermatozoa1

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Abstract
Spermatozoa within one sample do not appear to complete the capacitation process at the same rate. This could reflect a population composed of cells of different absolute age and thus of different maturational state, or the existence of inherent programmed differences among subpopulations of spermatozoa in regard to their capacitation potential. To explore this question, hamster spermatozoa aged within the range of their viable life in the cauda epididymidis were assessed, in comparison with controls, for onset of the acrosome reaction (AR) and hyperactivation as a function of capacitation time in vitro. Aging spermatozoa in the cauda epididymidis beyond the normal period did not synchronize onset of the motile population to react and, coincidentally, reduced their fertilization performance in vivo. Experiments using the calcium ionophore A 23187 indicate that these subtle epididymal aging effects reflected in the function-but not motility-of spermatozoa probably are exerted on the regulation of ion channels in the plasmalemma. We conclude that the asynchrony of onset of the AR and hyperactivation during capacitation probably refelcts inherent or programmed differences among spermatozoa. The fact that moderate aging tends to suppress the ability of many motile spermatozoa to react and to fertilize in vivo suggests that an optimal functional state can persist in the cauda for not much longer than spermatozoa remain there before normal replacement by younger cells.