Induction of the acrosome reaction in dog sperm cells is dependent on epididymal maturation: the generation of a functional progesterone receptor is involved.

In the current study we investigated the progesterone receptor exposure on the sperm from the testis and different parts of the epididymis, the relation to the sperm maturation stage, the functionality of the progesterone receptor and the capacity of sperm to undergo acrosome reaction. Exposed progesterone receptors on spermatozoa were detected using Progesterone-BSA conjugate labeled with fluorescein isothiocyanate (P-BSA-FITC) or a monoclonal antibody against progesterone receptor, C-262. Either progesterone or calcium ionophore was used to induce acrosome reaction. A high percentage (69 +/- 8%; mean +/- SD) of spermatozoa from the cauda epididymis showed P-BSA-FITC labeling at the onset of incubation, whereas only 0.1 +/- 1 and 4 +/- 2%, of spermatozoa from the testes, caput, and corpus epididymis, respectively, were labeled. There was no significant increase in P-BSA-FITC binding during the course of a 6 hr incubation. Treatment with either 10 microM progesterone or 5 microM calcium ionophore induced acrosome reaction in cauda epididymal sperm but not in testicular sperm, caput or corpus epipidymal sperm. It is concluded that the matured sperm of the dog from cauda epididymis and freshly ejaculated sperm demonstrate a functional membrane-bound progesterone receptor while less matured spermatozoa from the testicle, caput, and corpus epididymis fail to demonstrate such a receptor. Acrosome reaction of dog sperm can be induced using either progesterone or calcium ionophore; however, the maturation stages of spermatozoa influence this occurrence.