IRAG is essential for relaxation of receptor-triggered smooth muscle contraction by cGMP kinase

Signalling by cGMP-dependent protein kinase type I (cGKI) relaxes various smooth muscles modulating there-by vascular tone and gastrointestinal motility. cGKI-dependent relaxation is possibly mediated by phosphorylation of the inositol 1,4,5-trisphosphate receptor 1 (IP3RI)-associated protein (IRAG), which decreases hormone-induced IP3-dependent Ca2+ release. We show now that the targeted deletion of exon 12 of IRAG coding for the N-terminus of the coiled-coil domain disrupted in vivo the IRAG-IP 3RI interaction and resulted in hypomorphic IRAG Δ12/Δ12 mice. These mice had a dilated gastrointestinal tract and a disturbed gastrointestinal motility. Carbachol- and phenylephrine-contracted smooth muscle strips from colon and aorta, respectively, of IRAGΔ12/Δ12 mice were not relaxed by cGMP, while cAMP-mediated relaxation was unperturbed. Norepinephrine-induced increases in [Ca2+]i were not decreased by cGMP in aortic smooth muscle cells from IRAGΔ12/Δ12 mice. In contrast, cGMP-induced relaxation of potassium-induced smooth muscle contraction was not abolished in IRAGΔ12/Δ12 mice. We conclude that cGMP-dependent relaxation of hormone receptor-triggered smooth muscle contraction essentially depends on the interaction of cGKI-IRAG with IP 3RI.