Isolation and characterization of the primary structure of testis-specific L-type calcium channel: Implications for contraception

Therapeutic administration of calcium channel-blocking medications has been correlated with reduced mannose receptor expression and iatrogenic human male infertility. In this report, we investigate whether the pharmacological activity of dihydropyridines, which block calcium influx through voltage-dependent calcium channels, contributes to the production of an infertile state. An influx of extracellular calcium is an absolute requirement for the initiation of a progesterone-stimulated acrosome reaction by human spermatozoa. To determine whether dihydropyridines could inhibit progesterone-induced acrosome loss, we have studied a protein expressed in rat and human spermatozoa which is related both antigenically and by cDNA sequence to the al subunit of the rat cardiac muscle voltage-dependent calcium channel, which forms the pore of the channel. Using reverse transcription-polymerase chain reaction, we have isolated a 2169 base clone from rat testis mRNA whose sequence was largely identical to that of the al subunit of the rat cardiac muscle calcium channel, but had an 84 base change, attributable to splicing and alternate exon usage. This change inserts a peptide cassette encoding an amphipathic membrane-spanning helix that constitutes part of the ionic pore of the skeletal muscle calcium channel regulating the kinetics of activation of the calcium channel and may serve as an intramembrane dihydropyridine binding site. In parallel, human spermatozoa from fertile donors were exposed to nifedipine in vitro. Nifedipine inhibited progesterone-stimulated calcium influx and subsequent acrosome reactions in human spermatozoa at concentrations effective in excitable cells, but required a prolonged time to do so. In contrast, progesterone ligand binding was unaffected by nifedipine treatment, These data demonstrate that human spermatozoa express an L-type calcium channel which is responsive to nifedipine. Assuming sperm calcium transport pathways are highly conserved, the slow kinetics by which the blockade of the human sperm channel was obtained can be correlated with alterations in channel activation and conductance associated with isoform diversity generated by alternate splicing as observed in the rat. These data provide unequivocal evidence for the presence of functional L-type voltage-dependent calcium channels in rat and human spermatozoa. The data also define an altered binding site for calcium entry antagonists in this channel and offer a unique target for the design of new male contraceptive agents.