THAPSIGARGIN STIMULATES ACROSOMAL EXOCYTOSIS IN HAMSTER SPERMATOZOA

Thapsigargin (TG), a plant-derived sesquiterpene lactone, inhibits sev eral isoforms of both the sarcoplasmic and endoplasmic reticulum Ca2+- ATPases. Thus, intracellular Ca2+ stores found in the endoplasmic reti culum can be released by this compound. The mammalian sperm acrosome r eaction (AR) depends on influx of extracellular Ca2+. However, few rep orts have presented evidence for the involvement of putative Ca2+ stor es and intracellular Ca2+ mobilization in the AR. Thus, we designed ex periments to evaluate the effect of TG on the hamster sperm AR. Thapsi gargin stimulated-in a dose-dependent manner-the AR of spermatozoa pre viously 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 exte rnal Ca2+ since ethyleneglycol-bis-(b-amino-ethyl ether) N,N'-tetra-ac etic acid added 1 min before TG completely inhibited AR stimulation. T he Ca2+ channel blockers diltiazem and nifedipine also abolished the T G-stimulatory effect when added to capacitated spermatozoa 10 min befo re the inhibitor. In addition, the trypsin inhibitors p-nitrophenyl-p' -guanidinebenzoate hydrochloride and benzamidine added to the sperm su spensions 10 min before TG inhibited by 70-80% the TG-induced AR. Thes e results indicate that putative Ca2+ stores release may be involved i n 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. Mol. Reprod. D ev. 51:84-91, 1998. (C) 1998 Wiley-Liss, Inc.