SAMMA induces premature human acrosomal loss by Ca2+ signaling dysregulation

SAMMA is licensed for development as a contraceptive microbicide. Understanding mechanisms of its biological activity is prerequisite to designing more active second generation products. This study examined Ca2+ involvement in SAMMA-induced premature acrosomal loss (SAL) in noncapacitated human spermatozoa. SAMMA causes acrosomal loss (AL) in a dose-dependent manner (ED50 = 0.25 mu g/mL). SAL requires extracellular Ca2+ (ED50 = 85 mu M). SAL is inhibited by verapamil (nonspecific voltage-dependent Ca2+ channel blocker; IC50 = 0.4 mu M), diphenylhydantoin and NiCl2 (T-type [Ca(v)3.x] channel blockers; IC50 210 mu M and 75 mu M, respectively). Verapamil blockade of L-type (Ca(v)1.x) channels is use-dependent; activated channels are more sensitive to inhibition. However, verapamil inhibition of SAL does not increase after repeated SAMMA stimulation. SAL is unaffected by 10 mu M nifedipine (selective L-type channel blocker). This contrasts to 40% inhibition (P < .001) of AL induced by 1 mu M thapsigargin (Ca2+-ATPase inhibitor; releases intracellular Ca2+ stores, promotes capacitative Ca2+ entry). SAL is unaffected by 1 mu M BAPTA-AM (intracellular Ca2+ chelator), and 50 mu M 2-APB (blocks InsP3 receptors and store-operated channels). This contrasts with thapsigargin-induced AL, inhibited nearly 65% by BAPTA-AM (P < .005) and 91% by 2-APB (P < .001). The results suggest that SAL is mediated by Ca2+ entry through channels pharmacologically similar to the T-type (Ca(v)3.2) class. This process appears distinct from that caused by physiological stimuli such as progesterone or zona pellucida-derived proteins. SAMMA's contraceptive activity may be caused by induction of premature AL through dysregulation of Ca2+ signaling.