Functional expression of P2U receptors in rat spermatogenic cells: dual modulation of a Ca(2+)-activated K+ channel

Previous study has demonstrated functional expression of a sperm Ca(2+)-activated K+ channel in Xenopus oocytes injected with RNAs from the rat testes. Using the same expression system, the present study investigated the specific purinoceptor subtype involved in mediating the effect of extracellular ATP. The effect of ATP on an outwardly rectifying current, previously demonstrated to be mediated by a "Maxi" Ca(2+)-activated K+ channel, was compared to the current responses to different nucleotides using the two-electrode voltage-clamp technique. An order of relative effectiveness appeared to be UTP > ATP > ADP > adenosine, consistent with the pharmacological classification of P2U receptors. ATP was also found to exert an inhibitory effect on the Ca(2+)-activated K+ current, elicited by the Ca2+ ionophore ionomycin. A similar inhibitory effect was observed with UTP, again suggesting the involvement of P2U receptor. RT-PCR study also confirmed the expression of P2U receptor mRNA in isolated spermatogenic cells. The present results demonstrate the expression of P2U receptor and its dual role, both stimulatory and inhibitory, in the modulation of the Ca(2+)-activated K+ channel in spermatogenic cells. The present finding lends support to an important role of extracellular ATP in sperm functions.