Mechanisms for the coupling of cannabinoid receptors to intracellular calcium mobilization in rat insulinoma beta-cells

In RIN m5F rat insulinoma ß-cells, agonists at cannabinoid CB1 receptors modulate insulin release. Here we investigated in these cells the effect of the activation of cannabinoid CB1 and CB2 receptors on intracellular Ca2+ ([Ca2+]i). The CB1 agonist arachidonoyl-chloro-ethanolamide (ACEA), and the CB2 agonist JWH133, elevated [Ca2+]i in a way sensitive to the inhibitor of phosphoinositide-specific phospholipase C (PI-PLC), U73122 (but not to pertussis toxin and forskolin), and independently from extracellular Ca2+. PI-PLC-dependent Ca2+ mobilization by ACEA was entirely accounted for by activation of inositol-1,3,4-phosphate (IP3) receptors on the endoplasmic reticulum (ER), whereas the effect of JWH133 was not sensitive to all tested inhibitors of IP3 and ryanodine receptors. ACEA, but not JWH133, significantly inhibited the effect on [Ca2+]i of bombesin, which acts via Gq/11- and PI-PLC-coupled receptors in insulinoma cells. The endogenous CB1 agonists, anandamide and N-arachidonoyldopamine, which also activate transient receptor potential vanilloid type 1 (TRPV1) receptors expressed in RIN m5F cells, elevated [Ca2+]i in the presence of extracellular Ca2+ in a way sensitive to both CB1 and TRPV1 antagonists. These results suggest that, in RIN m5F cells, CB1 receptors are coupled to PI-PLC-mediated mobilization of [Ca2+]i and might inhibit bombesin signaling.