ARE CAPACITATION OR CALCIUM-ION INFLUX REQUIRED FOR THE HUMAN SPERM ACROSOME REACTION

Objective: To determine if the acrosome reaction of human spermatozoa can occur without prior incubation to induce capacitation or in calciu m-free medium. Design: Noncapacitated (washed ejaculated) or capacitat ed (incubated for 3 hours in the presence of albumin) human spermatozo a were treated with either solubilized zonae pellucidae (ZP); a calciu m ionophore (A23187); or activators of protein kinases A, G, and C, an d the acrosomal status was monitored by the double stain technique. Du ring agonist treatment, the capacitated spermatozoa were in medium eit her with or without calcium ions (Ca2+). The noncapacitated spermatozo a were always in Ca2+-containing medium. Sperm motility was monitored throughout the experiments. Results: Solubilized ZP and kinase activat ors induced acrosomal exocytosis of capacitated spermatozoa (both in C a2+-containing and Ca2+-free medium) and of noncapacitated spermatozoa . The lack of added Ca2+ or capacitation reduced the percentage of spe rmatozoa that reacted in response to solubilized ZP but not to the kin ase activators. Ionophore A23187 stimulated acrosomal loss from noncap acitated spermatozoa to the same extent as capacitated spermatozoa in Ca2+-containing medium but had no effect on capacitated spermatozoa in Ca2+-free medium. Conclusions: Human spermatozoa do not require incub ation under capacitating conditions or the presence of extracellular C a2+ before they can undergo the acrosome reaction in response to certa in agonists. Therefore, Ca2+ influx and/or preincubation to induce cap acitation are not absolute requirements for the in vitro agonist-induc ed acrosome reaction. However, these conditions can optimize the acros ome reaction response to zona proteins. The intracellular mechanisms l eading to the acrosome reaction appear to be functional in noncapacita ted spermatozoa but membrane changes probably are required before cert ain extracellular ligands such as zona proteins can exert their maxima l effect.