Mechanisms Of Cytosolic Ca2+ Suppression By Prostaglandin E-2 Receptors In Rat Melanotrophs

We have previously reported that voltage-dependent Ca2+ (VDC) channels of rat melanotrophs are inhibited by prostaglandin E-2 (PGE(2)). In this study, mechanisms involved in the inhibitory actions of PGE(2) receptors of rat melanotrophs were analysed using reverse transcriptase-polymerase chain reaction (RT-PCR), Ca2+-imaging and whole-cell, patch-clamp techniques with recently developed EP agonists, each of which is selective for the known four subclasses of EP receptors (EP1-4). PGE(2) reversibly suppressed the cytosolic Ca2+ concentration ([Ca2+](i)). The maximum reduction in [Ca2+](i) by PGE(2) was comparable to that by dopamine or to that by extracellular Ca2+ removal. RT-PCR analysis of all four EP receptors revealed that EP3 and EP4 receptor mRNAs were expressed in the intermediate lobe. The effects of PGE(2) to suppress [Ca2+](i) were mimicked by the selective EP3 agonist, ONO-AE-248, whereas three other EP agonists, ONO-DI-004 (EP1), ONO-AE1-259 (EP2) and ONO-AE1-329 (EP4 ), had little or no effect on [Ca2+](i) . All four G-protein activated inward rectifying K+ (GIRK) channel mRNAs were identified in intermediate lobe tissues by RT-PCR. Dopamine concentration-dependently activated GIRK currents, whereas PGE(2) did not activate GIRK currents, even at the concentration causing maximal inhibition of VDC channels. These results suggest that PGE(2) acts on EP3 receptors to suppress Ca2+ entry of rat melanotrophs by selectively inhibiting VDC channels of these cells. We have compared the possible cellular and molecular mechanisms of inhibition by dopamine and PGE(2) .