Intracellular calcium signaling through the cADPR pathway is agonist specific in porcine airway smooth muscle.

Cyclic ADP-ribose (cADPR) induces intracellular Ca2+ ([Ca2+]i) release in airway smooth muscle, and the cADPR antagonist, 8-amino-cADPR, abolishes [Ca2+]i oscillations elicited by acetylcholine (ACh), suggesting that cADPR is involved during muscarinic receptor activation. Whether the cADPR signaling pathway is common to agonists acting through different G protein-coupled receptors is not known. Using digital video imaging of Fura2-AM loaded porcine airway smooth muscle cells, we examined the effects of the membrane-permeant cADPR antagonist, 8-bromo-cADPR (8Br-cADPR), on the [Ca2+]i responses to ACh, histamine and endothelin-1 (ET-1). In cells preincubated with 100 microM 8Br-cADPR, the [Ca2+]i responses to ACh and ET-1 were significantly attenuated, whereas responses to histamine were not, suggesting agonist specificity of cADPR signaling. The effects of 8Br-cADPR were concentration dependent. We further examined whether muscarinic receptor subtypes specifically couple to this pathway, because in porcine airway smooth muscle cells, ACh activates both M2 and M3 muscarinic receptors coupled to Gai and Gaq, respectively. Methoctramine, an M2-selective antagonist, attenuated the [Ca2+]i responses to Ach, and there was no further attenuation by 8Br-cADPR. In airway smooth muscle, the CD38/cADPR signaling pathway is involved in [Ca2+]i responses to contractile agonists in an agonist-specific manner.