El sistema óxido nítrico/GMPc como mecanismo de señalización de las endotelinas en el sistema nervioso / The system nitric oxide/GMPc as signaling mechanism of the endothelins in te nervous system

En el presente trabajo investigamos el mecanismo de señalización mediado por receptores de las endotelinas (ETs), en dos estructuras del sistema nervioso relacionadas con la función neuroendocrina, como son la eminencia media y la médula suprarrenal de la rata. Las tres isoformas de ETs incrementaron la producción de GMPc, en una magnitud similar y dependiente de la concentración. El IRL 1620, un agonista selectivo del receptor ETB, también incrementó la formación de GMPc en ambas estructuras. El antagonista del receptor ETB, BQ-788, inhibió completamente el incremento de GMPc inducido por ET-1 o ET-3. Adicionalmente, la ET-1 estimuló la actividad de la sintetasa del óxido nítrico, tanto en la eminencia media como en la médula suprarrenal. En la médula suprarrenal, el incremento en la producción de GMPc inducido por las ETs o por el IRL fue bloqueado por el análogo de la L-arginina, N-nitro-L-arginina (L-NAME) y por dos inhibidores de la guanililciclasa soluble, el azul de metileno y el ODQ. Los resultados demuestran que en dos estructuras del sistema nervioso, como la eminencia media y la médula suprarrenal, la estimulación del receptor ETB de las endotelinas se encuentra asociada a la activación de la sintetasa del óxido nítrico y al consecuente incremento de la formación de GMPc. Estos hallazgos sugieren un papel funcional de las ETs en estructuras del sistema nervioso mediante la formación de óxido nítrico y la activación de la guanililciclasa soluble

Nitric oxide/cGMP mediates the spasmolytic action of 3,4'-dihydroxy-5,5'-dimethoxybibenzyl from Scaphyglottis livida.

Five aromatic compounds, 3,4'-dihydroxy-5,5'-dimethoxybibenzyl (1), batatasin III (2), coelonin (3), 3,7-dihydroxy-2,4-dimethoxyphenanthrene (4), and 3,7-dihydroxy-2,4,8-trimethoxyphenanthrene (5) were isolated from the orchid Scaphyglottis livida (Lindley) Schltr. Compounds 1-5 induced a concentration-dependent inhibition of the spontaneous contractions of the rat ileum with potencies comparable or higher to that of papaverine. The relaxation evoked by compounds 1-4 was blocked by L-NAME, an inhibitor of nitric oxide synthase. It was also demonstrated that 1 increased cyclic GMP content in rat ileum rings. Compound 1-induced elevation of cGMP was inhibited by L-NAME and ODQ, inhibitors of nitric oxide synthase and soluble guanylyl cyclase, respectively. These results indicate that nitric oxide/cGMP formation constitute the signaling pathway in the spasmolytic action of compound 1.

Thioperamide, a histamine H3-receptor blocker, facilitates vasopressor response to footshocks.

OBJECTIVE AND DESIGN: We assessed the functional role of the histamine H3-receptor in conscious intact rats during activation of the sympathoadrenal axis. MATERIAL: Male Sprague-Dawley rats, with or without cerebroventricular cannula, were subjected to mild footshocks and mean arterial pressure (MAP) and heart rate were determined using a tail-cuff plethysmograph. TREATMENTS: Saline, phentolamine (3 mg/kg, i.p.), (R)-alphafluoromethylhistidine (AFMH) (100 mg/kg, i.p., or 100 microg/5 microl, i.v.t.), (R)-alphamethylhistamine (AMH) (2 mg/kg, i.p. or 100 microg/5 microl, i.v.t.), thioperamide (THIO) (1 or 2 mg/kg, i.p., or 100 microg/5 microl, i.v.t.), mepyramine (10 mg/kg, i.p.), cimetidine (2 mg/kg, i.p.). METHODS: Urinary catecholamines were determined by fluorometry. Statistical differences between experimental groups were evaluated by Student's t-test or one-way ANOVA. RESULTS: Footshocks increased both MAP and heart rate. The vasopressor response to footshocks was facilitated (p < 0.001) by i.p. administration of AFMH, a histidine decarboxylase inhibitor, or THIO, a H3-receptor antagonist, but not by i.v.t. injection of these drugs. AMH, a H3-receptor agonist, given i.p., decreased the vasopressor response to footshocks (p < 0.001). This action of AMH was abolished by THIO but not by mepyramine or cimetidine. The MAP response to exogenous norepinephrine was not altered by i.p. administration of either AFMH or THIO. CONCLUSIONS: Our results demonstrate an involvement of peripheral histamine H3 prejunctional receptors in the inhibitory modulation of peripheral noradrenergic responses during stress.

Endothelin ET(B) receptor subtype mediates nitric oxide/cGMP formation in rat adrenal medulla.

We investigated the effect of endothelins (ETs) on receptor-mediated cGMP formation in whole rat adrenal medulla. ET-3 increased cGMP formation in a concentration-dependent manner; in addition, all three isoforms of ETs, at equimolar doses, increased cGMP levels in similar degree. IRL-1620, a selective ET(B) receptor agonist, also increased cGMP formation, mimicking the effects of ETs, but the increase was higher than those produced by ETs. L-arginine analogue, N-nitro-L-arginine (L-NAME), and methylene blue and OQD, two inhibitors of soluble guanylyl cyclase, significantly inhibited the increase in cGMP production induced by ETs or IRL-1620. Likewise, the selective ET(B) receptor antagonist, BQ-788, significantly inhibited ET-1- or ET-3-induced cGMP generation. Our results demonstrate that in whole rat adrenal medulla, endothelins stimulate NO-induced cGMP generation through ET(B) receptors, and they support the concept that endothelins could play a role in the regulation of adrenal medulla function.

Adrenalectomy reduces atrial natriuretic peptide stimulated guanylate cyclase activity in rat paraventricular nucleus.

Atrial natriuretic peptide (ANP) or an Atriopeptin III analog (PL 058) stimulated cGMP formation in the membrane fraction of rat olfactory bulb, median eminence and paraventricular nucleus, in a dose-dependent manner. The effect of the Atriopeptin III analog was 20-40% greater than that of ANP. Bilateral adrenalectomy, with or without mineralo- or glucocorticoid-replacement, on ANP-stimulated cGMP formation was investigated in rat paraventricular nucleus. 11 days after bilateral adrenalectomy a reduced responsiveness to ANP- or PL 058-induced cGMP production was observed. This effect was prevented by deoxycorticosterone, but not by dexamethasone administration. Our results further support the presence of guanylate cyclase-coupled ANP receptors in brain localized target sites; and they provide evidence suggesting that guanylate cyclase-linked ANP binding sites in the PVN are susceptible to regulatory changes after adrenalectomy-induced activation of the hypothalamus-hypophyso-adrenocortical system.

Brain natriuretic peptide stimulates particulate guanylate cyclase activity in selected areas of the rat brain.

The effect of porcine brain natriuretic peptide (pBNP) on cyclic guanosine monophosphate (cGMP) production was investigated in localized rat brain areas by radioimmunoassay procedure. Porcine BNP activated particulate guanylate cyclase in the median eminence, subfornical organ, choroid plexus, olfactory bulb, paraventricular nucleus and pineal gland in a concentration-dependent fashion and its action was comparable to that of rat atrial natriuretic peptide (alpha-ANP), with ED50 values ranging from 5 to 7 x 10(-7) M for both peptides. Our results suggest that the activation of a specific receptor coupled to the guanylate cyclase system and the subsequent elevation of cGMP levels constitutes the common mechanism of the central action of BNP and ANP.