CALCITONIN-GENE-RELATED PEPTIDE RELAXES RABBIT IRIS DILATOR SMOOTH-MUSCLE VIA CYCLIC-AMP-DEPENDENT MECHANISMS - CROSS-TALK BETWEEN THE SENSORY AND SYMPATHETIC NERVOUS SYSTEMS

Purpose. The purpose of these studies is to determine whether or not c yclic AMP is involved in the relaxant action of calcitonin gene-relate d 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), IP3 produ ction was measured by ion-exchange chromatography, and changes in tens ion were recorded isometrically. Results. CGRP, vasoactive intestinal peptide, prostaglandin E-2, isoproterenol and forskolin (1 mu M of eac h) increased cAMP accumulation by 136, 256, 78, 141 and 315%, respecti vely. CGRP dose-dependently increased cAMP accumulation (EC50 = 5.25 n M), inhibited IP3 production (EC50 = 5.4 nM) and induced relaxation (E C50 = 10 nM) in muscle precontracted with norepinephrine (NE) (10 mu M ). Prostaglandin E-2, isoproterenol and forskolin also induced relaxat ion. CGRP stimulated cAMP formation either in the presence or absence of 3-isobutyl-1-methylxanthine (LBMX), a cAMP phosphodiesterase inhibi tor, in a time- and concentration-dependent manner. The neuropeptide h ad no effect on cGMP accumulation. CGRP(8-37), a CGRP receptor antagon ist, reversed the relaxant action of the neuropeptide and inhibited CG RP-induced cAMP accumulation in a concentration-dependent manner (IC50 = 12.5 nM). 2',5'-dideoxyadenosine (DDA), a specific adenylate cyclas e inhibitor, significantly reduced the inhibitory actions of CGRP on N E-induced contraction and IP3 production and inhibited CGRP-induced cA MP accumulation in a concentration-dependent manner (IC50 = 6.9 nM). C onclusions. These results strongly suggest that cAMP mediates the rela xant action of CGRP in rabbit iris dilator. The mechanism of cAMP inhi bition of NE-induced IP3 production and contraction is unclear. Modula tion of alpha(1)-adrenergic function in the iris dilator by CGRP-induc ed cAMP formation is yet another example of cross-talk between the cAM P and IP3-Ca2+ second messenger systems, it demonstrates a cross-talk between the sympathetic and sensory nervous systems. CGRP-containing s ensory nerve fibers could play an important role in regulation of smoo th muscle function in the iris-ciliary body.