Presynaptic BK type Ca 2+-activated K + channels are involved in prostanoid TP receptor-mediated inhibition of noradrenaline release from the rat gastric sympathetic nerves

Abstract

Previously, we reported that prostanoid TP receptor mediates the inhibition of electrically evoked noradrenaline release from gastric sympathetic nerves in rats. Prostanoid TP receptor has been shown to activate phospholipase C (PLC), which catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-triphosphate (IP 3) and diacylglycerol; IP 3 triggers the release of Ca 2+ from intracellular stores and diacylglycerol activates protein kinase C. In the present study, therefore, we examined whether these PLC-mediated mechanisms are involved in the TP receptor-mediated inhibition of gastric noradrenaline release using an isolated, vascularly perfused rat stomach. U-46619 (9,11-dideoxy-9α,11α-methanoepoxy PGF 2α) (a prostanoid TP receptor agonist)-induced inhibition of noradrenaline release from the stomach was reduced by U-73122 [1-[6-[[(17β)-3-methoxyestra-1,3,5(10)-trien-17-yl]-amino]hexyl]-1 H-pyrrole-2,5-dine] (a PLC inhibitor) and ET-18-OCH 3 (1- O-octadecyl-2- O-methyl- sn-glycero-3-phosphorylcholine) (a phosphatidylinositol-specific PLC inhibitor), respectively. 2-APB (2-aminoethyldiphenyl borate) (a putative IP 3 receptor antagonist) also abolished the U-46619-induced inhibition of noradrenaline release, but Ro 31-8220 [2-{1-[3-(amidinothio)propyl]-1 H-indol-3-yl}-3-(1-methylindol-3-yl)-maleimide] (a protein kinase C inhibitor) had no effect. Furthermore, a small dose of tetraethylammonium and charybdotoxin [blockers of BK type Ca 2+-activated K + channel] abolished the U-46619-induced inhibition, but apamin (a blocker of SK-type Ca 2+-activated K + channel) had no effect. These results suggest that BK type Ca 2+-activated K + channels are involved in prostanoid TP receptor-mediated inhibition of electrically evoked noradrenaline release from the gastric sympathetic nerve terminals in rats.