Calcitonin gene-related peptide relaxes rabbit iris dilator smooth muscle via cyclic AMP-dependent mechanisms: cross-talk between the sensory and sympathetic nervous systems

Abstract

PURPOSE. The purpose of these studies is to determine whether or not cyclic AMP is involved in the relaxant action of calcitonin gene-related peptide (CGRP) in rabbit iris dilator muscle. METHODS. Iris dilator muscle isolated from rabbit was used. Accumulation of cAMP and cGMP in the tissue extracts was measured by radioimmunoassay (RIA), IP 3 production was measured by ion-exchange chromatography, and changes in tension were recorded isometrically. RESULTS. CGRP, vasoactive intestinal peptide, prostaglandin E2, isoproterenol and forskolin (1 µM of each) increased cAMP accumulation by 136, 256, 78, 141 and 315%, respectively. CGRP dose-dependently increased cAMP accumulation (EC 50 = 5.25 nM), inhibited IP 3 production (EC 50 = 5.4 nM) and induced relaxation (EC 50 = 10 nM) in muscle precontracted with norepinephrine (NE) (10 µM). Prostaglandin E 2, isoproterenol and forskolin also induced relaxation. CGRP stimulated cAMP formation either in the presence or absence of 3-isobutyl-1-methylxanthine (IBMX), a cAMP phosphodiesterase inhibitor, in a time – and concentration-dependent manner. The neuropeptide had no effect on cGMP accumulation. CGRP(8-37), a CGRP receptor antagonist, reversed the relaxant action of the neuropeptide and inhibited CGRP-induced cAMP accumulation in a concentration-dependent manner (IC 50 = 12.5 nM). 2',5'-dideoxyadenosine (DDA), a specific adenylate cyclase inhibitor, significantly reduced the inhibitory actions of CGRP on NE-induced contraction and IP 3 production and inhibited CGRP-induced cAMP accumulation in a concentration-dependent manner (IC 50 = 6.9 nM). CONCLUSIONS. These results strongly suggest that cAMP mediates the relaxant action of CGRP in rabbit iris dilator. The mechanism of cAMP inhibition of NE-induced IP 3 production and contraction is unclear. Modulation of a 1 -adrenergic function in the iris dilator by CGRP-induced cAMP formation is yet another example of cross-talk between the cAMP and IP 3 -Ca 2+ second messenger systems, it demonstrates a cross-talk between the sympathetic and sensory nervous systems. CGRP-containing sensory nerve fibers could play an important role in regulation of smooth muscle function in the iris-ciliary body.