Thapsigargin stimulates acrosomal exocytosis in hamster spermatozoa

Thapsigargin (TG), a plant-derived sesquiterpene lactone, inhibits several isoforms of both the sarcoplasmic and endoplasmic reticulum Ca2+-ATPases. Thus, intracellular Ca2+ stores found in the endoplasmic reticulum can be released by this compound. The mammalian sperm acrosome reaction (AR) depends on influx of extracellular Ca2+. However, few reports have presented evidence for the involvement of putative Ca2+ stores and intracellular Ca2+ mobilization in the AR. Thus, we designed experiments to evaluate the effect of TG on the hamster sperm AR. Thapsigargin stimulated-in a dose-dependent manner-the AR of spermatozoa previously capacitated for at least 3 hr, not affecting sperm motility. A maximal stimulatory effect was apparent 3 min after addition of TG to spermatozoa previously capacitated for 4 hr and was dependent on external Ca2+ since ethyleneglycol-bis-(b-amino-ethyl ether) N,N'-tetra-acetic acid added 1 min before TG completely inhibited AR stimulation. The Ca2+ channel blockers diltiazem and nifedipine also abolished the TG-stimulatory effect when added to capacitated spermatozoa 10 min before the inhibitor. In addition, the trypsin inhibitors p-nitrophenyl-p'-guanidine-benzoate hydrochloride and benzamidine added to the sperm suspensions 10 min before TG inhibited by 70-80% the TG-induced AR. These results indicate that putative Ca2+ stores release may be involved in stimulation of extracellular Ca2+ influx required for the occurrence of the AR. In addition, a sperm trypsin-like protease may be part of the mechanism by which TG induces the hamster sperm AR.