Inhibitory effect of ghrelin on T-type Ca²⁺ channel currents in mouse spermatogenic cells

Ghrelin, a newly isolated brain-gut peptide, has been found to play important roles mainly in female reproductive system. However, to date, any roles of ghrelin in male reproduction remain still unknown. In the present study, we identified a novel functional role of ghrelin in modulating T-type Ca²⁺ channel currents (T-currents) in mouse spermatogenic cells. We found that ghrelin reversibly inhibited T-currents in a dose-dependent manner. Ghrelin at 0.1 μM reversibly inhibited T-currents by ~31.7%. This inhibitory effect was blocked by D-Lys3-GHRP-6, a selective growth hormone secretagogue receptor 1a (GHS-R1a) antagonist, or GDP-β-S, a non-phosphorylatable analogue of GDP, indicating the involvement of a G-protein coupled GHS-R. Pretreatment of the cells with H89, a protein kinase A (PKA) inhibitor, or intracellular application of PKI 5-24, blocked ghrelin-induced T-current inhibition, whereas inhibition of phospholipase C or protein kinase C elicited no such effects. Moreover, the inhibitory effect of ghrelin on T-currents was associated with a hyperpolarizing shift in the voltage-dependence of inactivation in mouse spermatogenic cells, whereas the activation of T-currents was not affected. Since T-type Ca²⁺ channels are a key component in acrosome reaction (AR), our data suggest that ghrelin might be a significant factor in male reproductive action and a potential contraceptive agent.