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 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), IP3 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 microM of each) increased cAMP accumulation by 136, 256, 78, 141 and 315%, respectively. CGRP dose-dependently increased cAMP accumulation (EC50 = 5.25 nM), inhibited IP3 production (EC50 = 5.4 nM) and induced relaxation (EC50 = 10 nM) in muscle precontracted with norepinephrine (NE) (10 microM). Prostaglandin E2, 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 (IC50 = 12.5 nM). 2',5'-dideoxyadenosine (DDA), a specific adenylate cyclase inhibitor, significantly reduced the inhibitory actions of CGRP on NE-induced contraction and IP3 production and inhibited CGRP-induced cAMP accumulation in a concentration-dependent manner (IC50 = 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 IP3 production and contraction is unclear. Modulation of alpha 1-adrenergic function in the iris dilator by CGRP-induced cAMP formation is yet another example of cross-talk between the cAMP and IP3-Ca2+ 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.