Calcitonin gene-related polypeptide stimulates c-AMP production in the iris/ciliary body complex

Binding of CGRP analogs and their effect on adenylate cyclase activity in porcine iris-ciliary body

The structure-activity relationship of the calcitonin gene-related peptide (CGRP) in the porcine iris-ciliary body was studied using different CGRP analogs. The receptor binding affinity is located mainly in the carboxyterminal end of the CGRP peptide while the ability to stimulate adenylate cyclase (AC) enzyme is mainly in the aminoterminal end of the peptide. The binding of CGRP analogs was also found to be temperature-dependent. Changes in the alpha-helical region or in the beta-turn, as well as replacements of threonine-4, asparagine-25 or asparagine-26, reduce the binding affinity already at +4 degrees C. Truncated aminoterminus, changes in the loop region between cysteines 2 and 7, and especially in threonine 6, have for their part an important role in maintaining AC-stimulating activity.

Calcitonin gene-related peptide as a GH secretagogue in human and rat pituitary somatotrophs

To elucidate the role of calcitonin gene-related peptide (CGRP) in regulating pituitary function, we investigated the effects of CGRP and the related peptide adrenomedullin (AdM) on the secretion of growth hormone (GH) in vitro from human pituitary adenoma cells, rat pituitary tumor (GH3) cells, and normal rat pituitary cells. In 3 of 5 human somatotroph adenomas, GH secretion was stimulated by CGRP (1-100 nM). In one case of somatotroph adenoma, GH release was observed following the addition of 10 nM GHRH and 10 nM CGRP. The addition of CGRP or AdM (1 pM-10 nM) evoked GH secretion from GH3 cells with a bell-shaped distribution curve. CGRP (100 pM) caused the maximum increase of GH secretion (172+/-14 (mean+/-S.D.)% of control). The addition of CGRP8-37, an antagonist of CGRP type 1 receptors, inhibited the stimulatory effect of AdM but did not inhibit the effect of CGRP. The addition of CGRP and AdM evoked moderate GH secretion from normal rat pituitary cells. These results suggested that CGRP is a new GH secretagogue in human and rat pituitary tumor cells.

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.

Calcitonin gene-related peptide induced relaxation of the rabbit iris dilator muscle

Calcitonin gene-related peptide (CGRP) and Substance P (SP) -immunoreactive nerves have been found in the anterior uvea of various mammalian species. Although SP is known to play a major role in control of pupil motility in rabbits, little is known about the effect of CGRP on the iris smooth muscles. We isolated iris sphincter and dilator muscles from rabbit eyes and investigated the mechanical responses and intracellular cyclic AMP (cAMP) levels in these muscles. CGRP (up to 0.1 microM) had no effect on either the resting muscle tone or the amplitude of contraction evoked by field stimulation of the sphincter. On the other hand, CGRP (0.1 microM) relaxed dilator muscle which had been pre-contracted by phenylephrine and reduced the amplitude of contraction evoked by field stimulation. These responses were antagonized by CGRP (8-37), a CGRP antagonist. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), dose-dependently inhibited the contraction evoked by field stimulation. However, 3 microM IBMX had no effect on CGRP inhibition of twitch contraction in this preparation. CGRP had little effect on cAMP production in dilator muscle either with or without IBMX. In conclusion, the miosis which occurs during an ocular inflammatory response, when both CGRP and SP are thought to be released from terminals of sensory neurons, results from CGRP relaxation of the dilator and from the strong contractile effect of SP on the sphincter. Adenylate cyclase activation does not seem to be involved in the relaxant effect of CGRP on the dilator.

Intraocular effects of ruthenium red in responses to compound 48/80 and topical formaldehyde in rabbit

We have previously shown the presence and localization of mast cells and the intraocular effects of compound 48/80 in the rabbit eye. In the present study we have evaluated the mechanism of action of compound 48/80 using ruthenium red as a blocker of sensory axon reflexes in the rabbit eye and by measuring the intraocular pressure (IOP), the pupil size, the blood pressure, the protein and cAMP content in the aqueous humour. Topical neutral formaldehyde was used as a topical inducer of neuronally mediated response in a separate series of experiment. Intracamerally-injected ruthenium red suppressed the compound 48/80-induced elevation intraocular pressure and prevented miosis, while having little if any effect on the breakdown of the blood-aqueous barrier and on the increase in the cAMP concentration in aqueous humour. Ruthenium red also inhibited the irritative response in eyes treated with topical 1% formaldehyde. As the blood-aqueous barrier in the rabbit is an extremely sensitive system higher doses of ruthenium red causes damage of the barrier in the ruthenium red treated eyes. The results demonstrate that compound 48/80 not only has a mast cell degranulating effect but also a sensory nerve- stimulating effect.

Peripheral neural circuits regulating IOP? A review of its anatomical backbone

The peripheral nervous system is classically separated into a somatic division containing both afferent and efferent pathways and an autonomic division composed of efferents only. The somatic afferent division is divided in A- and B-neurons. The B-neurons are supposed to be autonomic afferents as part of a reflex system involved in homeostasis. Recent data obtained by neuronal tracing and immunohistochemical experiments concerning the eye related peripheral nervous system endorse the existence of these peripheral reflex systems. Somatic afferents of trigeminal origin synaptically innervate parasympathetic neurons in the pterygopalatine ganglion. This probably represents a pathway mediating autonomically regulated ocular activity in response to sensory stimulation. In addition, it has been hypothesized that trigeminal sensory nerve fibres have an efferent function in response to noxious stimuli e.g. the ocular injury response. Sympathetic nerve fibres originating in the superior cervical ganglion course through the trigeminal and pterygopalatine ganglion without forming direct synaptic contacts. These fibres, however, contain clusters of vesicles suggesting some kind of interneural communication. Parasympathetic nerve fibres of pterygopalatine origin course through the ciliary ganglion. These nerve fibre terminals also contain clusters of vesicles without direct synaptic contacts. Experimental data concerning the distribution of neuropeptides revealed a more detailed knowledge of the anterior eye segment innervation. These experimental data are subject to some debate. The pros and cons of different techniques are discussed. Neural circuits regulating IOP have long been postulated. The possible role of peripheral reflex systems in the regulation of IOP is discussed.

Amylin (islet amyloid polypeptide) inhibition of insulin release in the perfused rat pancreas: implication of the adenylate cyclase/cAMP system

Amylin inhibits glucose-induced insulin secretion in the rat pancreas. To study the mechanism by which amylin acts on the B-cell, we have investigated, in the perfused rat pancreas, the effect of synthetic rat amylin (75 pM) on insulin release elicited by secretagogues acting on the B-cell via the adenylate cyclase/cAMP system, i.e., glucagon (10 nM), gastric inhibitory polypeptide (GIP, 1 nM), forskolin (1 microM) and isobutylmethylxanthine (IBMX, 75 microM). In addition, we examined the effect of amylin on GIP-induced insulin release in pancreata from rats pretreated with pertussis toxin, an agent which inactivates certain Gi proteins coupled to adenylate cyclase. Amylin inhibited the insulin response to glucagon (approx. 70%), GIP (approx. 90%), IBMX (approx. 75%) as well as the early phase of forskolin-induced insulin output (approx. 74%). However, amylin failed to modify GIP-induced insulin release in pancreata obtained from pertussis toxin pretreated rats. These results would indicate that the inhibitory effect of amylin on insulin secretion could be, at least in part, attributed to its interfering with the adenylate cyclase/cAMP system. Furthermore, prevention of the inhibitory effect of amylin on GIP-induced insulin output by pertussis toxin pretreatment, supports the concept that amylin can inhibit insulin release via a pertussis toxin-sensitive Gi protein coupled to the adenylate cyclase system.