Ruthenium red antagonism of the effects of capsaicin mediated by extrinsic sensory nerves on myenteric plexus neurons of the isolated guinea-pig ileum

Mechanism of acid-induced mesenteric hyperemia in rats

The mesenteric hyperemia induced by intraduodenal application of hydrochloric acid (HCl) is mediated in part by capsaicin-sensitive afferent nerves. Antagonist of capsaicin-sensitive receptors (capsazepine) and blocker of capsaicin-sensitive cation channels (ruthenium red) have been described. We employed these tools to dissect the mechanism of regulation of mesenteric hyperemia induced by intraduodenal administration of HCl. Subcutaneous 100 micromol/kg capsazepine or intraduodenal 0.1% ruthenium red was administered to pentobarbital anesthetized rats. Then, 2.5 ml/kg of 640 microM capsaicin or 0.1 N HCl was administered intraduodenally. The mesenteric hyperemic responses were recorded. The results demonstrated that in a dose that decreased the mesenteric hyperemia induced by intraduodenal capsaicin, capsazepine failed to attenuate the mesenteric vasodilatory effect of intraduodenal HCl. Ruthenium red significantly attenuated the mesenteric hyperemia after intraduodenal capsaicin and HCl. These in vivo data provide the first functional evidence for the existence of capsazepine-sensitive capsaicin receptors and cation channel complexes in the rat duodenal and intestinal mucosa. The capsaicin- and HCl-sensitive receptors are unlikely to be functionally identical in these locations. The ruthenium red-sensitive cation channels appear to mediate the capsaicin- and HCl-induced mesenteric hyperemia.

An investigation into the mechanism of capsaicin-induced oedema in rabbit skin

1. Oedema formation induced by intradermal capsaicin has been studied in rabbit skin. The effect of the anti-inflammatory steroid dexamethasone and also of a range of known inhibitors of oedema formation have been investigated in order to elucidate mechanisms involved in capsaicin-induced oedema formation. 2. Oedema formation, in response to intradermally-injected test agents, was measured by the local extravascular accumulation of intravenously injected 125I-labelled albumin. In separate experiments skin blood flow was assessed by the clearance of intradermally-injected 133xenon. 3. Oedema formation induced by intradermal histamine (3 nmol) and bradykinin (1 nmol), when in the presence of vasodilator doses of calcitonin gene-related peptide (CGRP) (3 pmol) or prostaglandin E1, (PGE1) (10 pmol), was significantly inhibited (P < 0.01) in rabbits pretreated with intravenous dexamethasone (3 mg kg-1, -4 h). In contrast dexamethasone had no effect on capsaicin (3 mumol)-induced oedema formation or, on capsaicin (30-100 nmol)-induced blood flow. 4. Oedema formation observed in response to intradermal capsaicin (3 mumol) was significantly inhibited (P < 0.01) when the selective capsaicin antagonist, ruthenium red (3 nmol) was co-injected. This suggests that the mechanism of capsaicin-induced oedema involves activation of sensory nerves. However, oedema was not inhibited when capsaicin was co-injected with the neurokinin NK1 receptor antagonist, RP67580 (10 nmol), the NK2 antagonist SR48960 (10 nmol) or the CGRP antagonist CGRP8-37 (300 pmol). 5. Oedema formation induced by capsaicin was not inhibited when co-injected with the histamine HI receptor antagonist, mepyramine (3 nmol), the PAF antagonist, WEB 2086 (100 nmol), the bradykinin B2 receptor antagonist, Hoel4O (1 nmol), or the cyclo-oxygenase inhibitor, indomethacin (10 nmol),suggesting that these mediators do not play a major role in the capsaicin-induced response.6. Histological analysis of capsaicin-treated skin sites revealed undamaged, intact microvessels and lack of haemorrhage. Further, co-injection of capsaicin with the hydrogen peroxide remover, catalase(2,200 u), had no effect on oedema formation. This suggests that capsaicin does not induce oedema formation secondary to free radical-induced damage.7. These results indicate that capsaicin-induced oedema in rabbit skin involves activation of sensory nerves. However, the oedema is not inhibited by pretreatment with the anti-inflammatory steroid,dexamethasone. Further the mechanisms which lead to the oedema formation observed after intradermal capsaicin remain unknown.

Adenosine inhibits efferent function of extrinsic capsaicin-sensitive sensory nerves in the enteric nervous system

Capsaicin (1-3 microM) and electrical stimulation of mesenteric nerves in the presence of hexamethonium and guanethidine antidromically stimulate extrinsic sensory nerve fibers to produce a specific slow depolarizing response of myenteric neurons and a contractile response of muscles in the isolated guinea-pig ileum, mediated by release of substance P and acetylcholine. Adenosine (1-100 microM) inhibited the response to mesenteric nerve stimulation. Adenosine (10-100 microM) suppressed the contractile response to a threshold concentration of capsaicin (1 microM) while leaving the contractile response to a submaximal concentration of substance P (1 nM) and acetylcholine (0.1 microM) intact. Adenosine (1-10 microM) inhibited dose dependently the capsaicin 10 microM)-induced depolarization of myenteric neurons, but did not inhibit the depolarizing response to exogenous substance P. The adenosine P1 receptor antagonist, 8-phenyltheophylline (1-10 microM), antagonized the inhibitory effect of adenosine (1-10 microM) on the mechanical responses. We conclude that adenosine-induced prejunctional inhibition of the mechanical responses is mediated by adenosine P1 receptors.

Possible mechanism of ruthenium red antagonism of capsaicin-induced action in the isolated guinea pig ileum

Ruthenium red (3-5 microM) antagonism of the inhibitory effect of capsaicin (1 microM) on the contractile response to mesenteric nerve stimulation in the presence of hexamethonium (50 microM) and guanethidine (2 microM) was reversed significantly by sialic acid (2 mM) or neuraminidase (0.1 U/ml). These results suggested that ruthenium red at low concentrations inhibits the capsaicin-induced desensitization of activated Ca2+ influx into sensory nerves at least in part by binding to sialic acid residues.