Novel G-protein-coupled receptors in rat arteries: potential targets for N-arachidonoyl glycine?

W.-S. Vanessa Ho, Shuk Yin M. Yeung. St Geroge’s, University of London, London, United Kingdom.

We have recently reported that the endogenous lipid, N-arachidonoyl glycine (NAGly) is a vasorelaxant, through activation of calcium-activated potassium channels and nitric oxide in rat small mesenteric arteries (Ho & Parmar, 2009). NAGly is structurally related to the endocannabinoid, anandamide; however, it does not activate cannabinoid receptors. Instead, NAGly has been reported to activate two novel G-protein-coupled receptors, GPR18 (Gi/o-coupled) and GPR92 (Gq-coupled). Here, we have explored the expression of these receptors, and the relaxant responses to NAGly, in small mesenteric and basilar arteries of the rat (male Wistar or Sprague-Dawley rats, 200-300g; Charles River). Total RNA was extracted and quantified from isolated arteries and spleen (used as a positive control). Conventional reverse transcriptase polymerase chain reaction (RT-PCR) experiments were undertaken using primers designed to specifically detect rat GPR18 and 92. All products were sequenced to confirm identity. Isolated arteries were also mounted in a wire myograph for tension recording; data are expressed as mean±s.e.m (n≥4 rats). We found that NAGly induced relaxation in either small mesenteric arteries (precontracted with the α1-adrenoceptor agonist, methoxamine: pEC50 = 5.6±0.2, Emax = 95±1%) or basilar arteries (precontracted with 5-HT: relaxation at 30μM = 31±2%). However, another putative GPR92 agonist, farnesyl pyrophosphate, which has reported to be 10-fold more potent than NAGly at GPR92 expressed in cultured cells (Oh et al. 2008), had no relaxant effects at concentrations up to 3μM in mesenteric arteries. On the other hand, GPR18 and 92 were detected in both mesenteric and basilar arteries. To conclude, this study, for the first time, report the presence of GPR18 and 92 in the vasculature. Their functional roles remain unclear, but it is possible that GPR18 acts as a vascular target for NAGly.

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