SAMMA Induces Premature Human Acrosomal Loss by Ca2+ Signaling Dysregulation

Abstract

ABSTRACT: 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 μg/mL). SAL requires extracellular Ca²⁺ (ED₅₀ = 85 μM). SAL is inhibited by verapamil (nonspecific voltage-dependent Ca²⁺ channel blocker; IC₅₀ = 0.4 μM), diphenylhydantoin and NiCl₂ (T-type [Ca_v3.x] channel blockers; IC₅₀ 210 μM and 75 μM, respectively). Verapamil blockade of L-type (Ca_v1.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 μM nifedipine (selective L-type channel blocker). This contrasts to 40% inhibition (P < .001) of AL induced by 1 μM thapsigargin (Ca²⁺-ATPase inhibitor; releases intracellular Ca²⁺ stores, promotes capacitative Ca²⁺ entry). SAL is unaffected by 1 μM BAPTA-AM (intracellular Ca²⁺ chelator), and 50 μM 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_v3.2) 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.