Facilitative effect of pulsed addition of dibutyryl cAMP on the acrosome reaction of noncapacitated human spermatozoa

Abstract

The in vitro acrosome reaction of noncapacitated human spermatozoa was induced by both calcium ionophore (A23187) and dibutyryl adenosine cyclic monophosphate (Bu2cAMP), a membrane permeant cyclic nucleotide analog, in a dose-dependent manner. Maximal frequencies of acrosome-reacted spermatozoa above baseline values (12%; 90% confidence limits = 10.6 to 14.2%) were similar for Bu2cAMP and A23187 (24.5% and 25.1%, respectively). The concentration of Bu2cAMP required for a half-maximal response was 14.3 mumol/L, while that for A23187 was 24.5 pmol/L. The ability of A23187 to induce the acrosome reaction depended on the presence of calcium ion in the incubation medium. The A23187-induced reaction was prevented by the inclusion of human serum albumin in the medium; the inhibitory effect of albumin was partially reversed after preincubation of spermatozoa for 3 hours under capacitating conditions. In contrast, the Bu2cAMP-induced acrosome reaction was unaffected by either Ca2+ or albumin. Pulsed addition of Bu2cAMP enhanced the frequency of acrosome-reacted spermatozoa. This effect appeared to be influenced by pulse frequency: additions made every 5 minutes produced a greater maximal response than additions made every 2 minutes or every 15 minutes. The maximum theoretical acrosome reaction above baseline values (12%) was 88% of the total number of cells, accounting for almost the entire sperm population. Pulsed addition of A23187 did not increase the frequency of acrosome-reacted spermatozoa above values obtained from single equimolar additions of this agent. These data indicate that: (1) intracellular mechanisms for the human acrosome reaction are functional in noncapacitated spermatozoa; (2) the acrosome reaction can be separated from the process of capacitation; and (3) the acrosome reaction is affected by the pattern, as well as the type, of activation.