In addition to Ca2+ and K+ fluxes, angiotensin II (Ang II) has been shown to influence sperm motility. The present study investigated the involvement of angiotensin II type 1 receptor (AT1) in mediating the modulatory effect of Ang II on a sperm Ca2+-activated K+ channel expressed in Xenopus oocytes injected with RNAs of spermatogenic cells. Ang II at a concentration of 1 microM was found to potentiate the ionomycin-induced current, previously demonstrated to be mediated by a 'Maxi' Ca2+-activated K+ channel. However, at higher concentration, 20 microM, Ang II was found to suppress the ionomycin-induced current. Both potentiating and inhibitory effects of Ang II were blocked by losartan, a specific antagonist of AT1 receptors. Immunohistochemical studies further confirmed the presence of AT1 receptors in spermatogenic cells while expression of AT1 receptor mRNA was demonstrated by RT-PCR. These results suggest that Ang II may influence sperm motility as well as other sperm function by acting on AT1 receptors, and exerting potentiating and inhibitory effects on the Ca2+-activated K+ channels.