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  • Progesterone-Induced Acrosome Reaction: DISCUSSION(8)

Progesterone and zona glycoprotein (ZP3) can induce the AR but do not exert their effects through the same receptor, as far as the receptor-mediated ions fluxes are concerned. Despite the molecular dissimilarity of progesterone and ZP3, each of them can activate at least two different types of receptors responsible for Ca2+ channels—onereactingrapidly,andthusprobablyactivateddi-rectly by the inducer, and the other responding after some delay and probably regulated by the inducer indirectly tia a signaling cascade. The nature of the stallion sperm progesterone receptor as well as the putative Ca2+ channel involved in signaling remains to be elucidated. In human spermatozoa, the molecular mass recognized by C-262 was 50-52 kDa protein, which is different from that of the A and B isoforms of human intracellular progesterone receptor, 94 and 120 kDa, respectively. A specific membrane protein with affinity for progesterone has been isolated from rat brain that has an estimated molecular mass of 40-50 kDa. flovent inhaler

Conclusively, an increasing amount of stallion spermatozoa expose a plasma membrane progesterone receptor during in vitro capacitation treatment. The coupling of progesterone to its sperm receptor is an important step toward the AR induction. Progesterone as an inducer of the AR is an alternative to the zona pellucida. Both inducers are present in the vicinity of the ovulated oocyte. Thus, progesterone might virtually participate in this biologically important acrosomal exocytosis in an additive or synergistic manner, leading to a successful fertilization. The exposure of the progesterone receptor and the subsequent coupling of progesterone are probably involved in sperm binding to the zona pellucida as well as zona-induced AR

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