In the isolated rat mesenteric bed, the 1 min perfusion with 100 nM anandamide, a concentration that did not evoke vasorelaxation, elicited an acute release of 165.1 ± 9.2 pmol nitric oxide (NO) that was paralleled by a 2-fold increase in cGMP tissue levels. The rise in NO released was mimicked by either (R)-(+)-methanandamide or the vanilloid receptor agonists resiniferatoxin and (E)-capsaicin but not by its inactive cis-isomer (Z)-capsaicin. The NO release elicited by either anandamide or capsaicin was reduced by the TRPV1 receptor antagonists 5 -iodoresiniferatoxin, SB 366791 and capsazepine as well as by the cannabinoid CB 1 receptor antagonists SR 141716A or AM251. The outflow of NO elicited by anandamide and capsaicin was also reduced by endothelium removal or NO synthase inhibition, suggesting the specific participation of endothelial TRPV1 receptors, rather than the novel endothelial TRPV4 receptors. Consistently, RT-PCR showed the expression of the mRNA coding for the rat TRPV1 receptor in the endothelial cell layer, in addition to its expression in sensory nerves. The participation of sensory nerves on the release of NO was precluded on the basis that neonatal denervation of the myenteric plexus sensory nerves did not modify the pattern of NO release induced by anandamide and capsaicin. We propose that low concentrations of anandamide, devoid of vasorelaxing effects, elicit an acute release of NO mediated predominantly by the activation of endothelial TRPV1 receptors whose physiological significance remains elusive.

1 To assess the involvement of endothelial a 2 -adrenoceptors in the clonidine-induced vasodilatation, the mesenteric artery of Sprague Dawley rats was cannulated and perfused with Tyrode solution (2 ml min 71 ). We measured perfusion pressure, nitric oxide (NO) in the perfusate using chemiluminescence, and tissue cyclic GMP by RIA. 2 In phenylephrine-precontracted mesenteries, clonidine elicited concentration-dependent vasodilatations associated to a rise in luminal NO. One hundred nM rauwolscine or 100 mM L o -nitro-Larginine antagonized the clonidine-induced vasodilatation. Guanabenz, guanfacine, and oxymetazoline mimicked the clonidine-induced vasorelaxation. 3 In non-contracted mesenteries, 100 nM clonidine elicited a maximal rise of NO (123+13 pmol); associated to a peak in tissue cyclic GMP. Endothelium removal, L o -nitro-L-arginine, or rauwolscine ablated the rise in NO. One hundred nM aminoclonidine, guanfacine, guanabenz, UK14,304 and oxymetazoline mimicked the clonidine-induced surge of NO. Ten mM ODQ obliterated the clonidineinduced vasorelaxation and the associated tissue cyclic GMP accumulation; 10 ± 100 nM sildena®l increased tissue cyclic GMP accumulation without altering the clonidine-induced NO release. 4 a 2 -Adrenergic blockers antagonized the clonidine-induced rise in NO. Consistent with a preferential a 2D -adrenoceptor activation, the K B s for yohimbine, rauwolscine, phentolamine, WB-4101, and prazosin were: 6.8, 24, 19, 165, and 1489 nM, respectively. 5 Rat pretreatment with 100 mg kg 71 6-hydroxydopamine reduced 95% tissue noradrenaline and 60% neuropeptide Y. In these preparations, 100 nM clonidine elicited a rise of 91.9+15.5 pmol NO. Perfusion with 1 mM guanethidine or 1 mM guanethidine plus 1 mM atropine did not modify the NO surge evoked by 100 nM clonidine. 6 Clonidine and congeners activate endothelial a 2D -adrenoceptors coupled to the L-arginine pathway, suggesting that the antihypertensive action of clonidine involves an endothelial vasorelaxation mediated by NO release, in addition to presynaptic mechanisms. British Journal of Pharmacology (2001) 134, 957 ± 968

The effects of isoprenaline, propranolol and phentolamine, were studied on tritiated noradrenaline overflow elicited by postganglionic nerve stimulation in guinea‐pig isolated atria. Isoprenaline (1.2 × 10−8) increased while propranolol (1.0 × 10‐7 M) reduced the overflow of tritiated noradrenaline evoked by nerve stimulation. These effects were less than those of phentolamine (3.1 × 10‐6 M), which increased by approximately three‐fold the overflow of [3 H] ‐noradrenaline elicited by nerve stimulation. Neuronal accumulation of tritiated noradrenaline in guinea‐pig atria was not affected by isoprenaline, propranolol or phentolamine at the concentrations employed in this study. Isoprenaline (1.2 × 10‐8 M) induced a positive chronotropic effect of about 80% of the maximum. On the other hand, propranolol produced a shift to the right in the frequency‐response curve to nerve stimulation and in the concentration‐response curve to exogenous noradrenaline in guinea‐pig atria. In the isolated nictitating membrane of the cat, the frequency‐response curve to nerve stimulation was not modified by propranolol, while in the presence of 3.9 × 10−6 m of N,‐2‐(2,6‐dimethylphenoxy)propyl‐N,N,N ‐trimethylammonium (β‐methyl‐TM 10) there was a shift to the right and a depression of slope. Neither propranolol nor β‐methyl‐TM 10 affected responses to exogenous noradrenaline. The effects of isoprenaline and of propranolol on transmitter release are compatible with the view that in addition to the presynaptic negative feed‐back mechanism for noradrenaline release by nerve stimulation mediated via α‐adrenoceptors a positive feed‐back mechanism exists in adrenergic nerve endings which is triggered through the activation of presynaptic β‐adrenoceptors.

We examined the effects of angiotensin II (Ang II) and Ang-(l-7) on the release of [