1 Some animal studies suggest that b-adrenoceptor-mediated vasorelaxation is in part mediated through nitric oxide (NO) release. Furthermore, in humans, we have recently shown that forearm blood¯ow is increased by infusion of b 2 -adrenergic agonists into the brachial artery, and the nitric oxide synthase (NOS) inhibitor N G -monomethyl-L-arginine (L-NMMA) inhibits this response. 2 The purpose of the present study was to determine whether stimulation of human umbilical vein endothelial b-adrenoceptors causes vasorelaxation and nitric oxide generation, and whether this might be mediated by cyclic adenosine-3',5'-monophosphate (cyclic AMP). 5 Isoprenaline relaxed umbilical vein rings, and this vasorelaxation was abolished by b 2 -(but not b 1 -) adrenergic blockage, and by endothelium removal or 1 mM L-NMMA. In addition, vasorelaxant responses to dibutyryl cyclic AMP were inhibited by 1 mM L-NMMA, with a reduction in E max from 90.0+9.3% to 50.5+9.9% (P50.05). 6 Isoprenaline 1 mM increased NOS activity in HUVEC (34.0+5.9% above basal, P50.001). Furthermore, isoprenaline increased adenylyl cyclase activity in a concentration-dependent manner; this response was inhibited by b 2 (but not b 1 -) adrenergic blockade. Forskolin 1 mM and dibutyryl cyclic AMP 1 mM each increased NOS activity in HUVEC, to a degree similar to isoprenaline 1 mM. The increase in L-arginine to L-citrulline conversion observed with each agent was abolished by coincubation with NOS inhibitors. 7 These results indicate that endothelial b 2 -adrenergic stimulation and cyclic AMP elevation activate the L-arginine/NO system, and give rise to vasorelaxation, in human umbilical vein.

1 Noradrenaline induces a meagre vasoconstriction in small muscular pulmonary arteries compared to large conduit pulmonary arteries. We have examined whether this may be partially related to di erences in the b-adrenoceptor-mediated vasorelaxation component and, in particular, b-adrenoceptor-mediated NO release. 2 Noradrenaline induced a bell-shaped concentration-response in large (1202+27 mm) and small (334+12 mm) pulmonary arteries of the rat. In large arteries tension increased to 95.6+1.8% of 75 mM KCl (KPSS; n=8) at 2 mM, above which tension declined. The response in small arteries was meagre (12+1.5% KPSS, n=9), peaking at 0.2 mM. N G -monomethyl-L-arginine (L-NMMA; 100 mM) abolished the decline in tension induced by higher concentrations of noradrenaline in large arteries, and increased maximum tension (117+3.5% KPSS, n=5, P50.05). In small arteries peak tension doubled (22.0+3.4% KPSS, n=6, P50.01), but still declined above 0.2 mM. 3 Propranolol (1 mM) abolished the decline in tension at higher concentrations of noradrenaline in both groups, but increased tension substantially more in small (37.4+3.7% KPSS, n=5, P50.001) than in large arteries (112.2+3.7% KPSS, n=9, P50.05). In the presence of L-NMMA, propranolol had no additional e ect on large arteries, whereas in small arteries there was greater potentiation than for either agent alone (67.8+5.9% KPSS, n=4). 4 b-Adrenoceptor-mediated relaxation was examined in arteries constricted with prostaglandin F 2a (50 mM). In the presence of propranolol isoprenaline caused an unexpected vasoconstriction, which was abolished by phentolamine (10 mM). In the presence of phentolamine, isoprenaline caused a maximum relaxation of 43.3+2.1% (n=6) in large, and 49.0+4.5% (n=6) in small arteries. L-NMMA substantially reduced relaxation in large arteries (7.4+1.5%, n=6, P50.01), but was less e ective in small arteries (26.8+5.8, n=5, P50.05). 5 Atenolol (b 1 -antagonist, 5 mM) reduced relaxation to isoprenaline (large: 34.8+4.5%, n=5; small: 35.0+1.9%,n=6), but in combination with L-NMMA had no additional e ect over L-NMMA alone. ICI 118551 (b 2 -antagonist, 0.1 mM) reduced isoprenaline-induced relaxation more than atenolol (large: 18.0+4.6%, n=6, P50.05; small: 25.6+10.7%, n=6, P50.05). ICI 118551 in combination with L-NMMA substantially reduced relaxation (large: 4.8+2.6%, n=9; small: 6.5+3.6%, n=5). 6 Salbutamol-induced relaxation was reduced substantially by L-NMMA in large arteries (control: 34.7+6.4%, n=6; +L-NMMA: 8.3+1.3%, n=5, P50.01), but to a lesser extent in small arteries (control: 50.9+7.5%, n=6; +L-NMMA: 23.0+0.7%, n=5, P50.05). Relaxation to forskolin was also partially antagonized by L-NMMA. 7 These results suggest that the meagre vasoconstriction to noradrenaline in small pulmonary arteries is partially due to a greater b-adrenoceptor-mediated component than in large arteries. b-Mediated vasorelaxation in large arteries was largely NO-dependent, whereas in small arteries a signi®cant proportion was NO-independent. Noradrenaline stimulation was also asso...

St Thomas' Campus, London SEl 7EH 1 Ligustrazine (tetramethylpyrazine, TMP) is a vasodilator that has been reported to have pulmonary selective properties in vivo, but not in vitro. Although TMP is generally described as being endotheliumindependent, we provide evidence here that TMP may have an endothelium-dependent and nitric oxide (NO)-mediated mechanism in pulmonary arteries that could predominate at concentrations used therapeutically in China. Similar effects were seen in small arteries. L-Arginine had no effect in the absence of an endothelium. D-Arginine was ineffective, and inhibition of L-arginine uptake with L-lysine blocked the action of L-arginine. L-Arginine (400 gM) had no significant effect on TMP-induced relaxation in mesenteric arteries (n = 5). L-Arginine itself caused a concentration-dependent relaxation in intrapulmonary arteries(639 + 34 gM) constricted with PE, reaching a maximum relaxation around 100 -400 gM (42.4 + 3.0%, n = 16), but this was independent of the endothelium. TMP (10 and 100 pM) significantly enhanced the relaxation to L-arginine, with a maximum relaxation in the presence of 100 gM TMP of 81.7 + 6.2%(n = 5, P<0.01), but the effect of TMP was entirely dependent on the endothelium. A similar effect was observed in PGF2,-constricted pulmonary arteries.6 These results show that TMP stimulates NO production at low concentrations in pulmonary arteries, via an apparently novel endothelium-resident mechanism that is dependent on exogenous L-arginine.Normal plasma L-arginine levels of around 150 gM would allow this mechanism to be maximally activated. As mesenteric arteries do not seem to express the mechanism to any significant extent, at low concentrations TMP would be effectively selective to the pulmonary vasculature, and may thus have potential as a therapeutic agent in pulmonary vascular disease.

1 a 1 -adrenoceptor agonists may potentiate relaxation to b-adrenoceptor agonists, although the mechanisms are unclear. We compared relaxations induced by b-adrenoceptor agonists and cyclic AMP-dependent vasodilators in rat pulmonary arteries constricted with prostaglandin F 2a (PGF 2a ) or the a 1 -adrenoceptor agonist phenylephrine (PE). In addition, we examined whether di erences were related to cyclic AMP-or nitric oxide (NO) and cyclic GMP-dependent pathways. 2 Isoprenaline-induced relaxation was substantially potentiated in arteries constricted with PE compared with PGF 2a . Methoxamine was similar to PE, whereas there was no di erence between PGF 2a and 30 mM KCl. The potentiation was primarily due to a marked increase in the NOindependent component of relaxation, from 9.1+1.7% for PGF 2a to 55.1+4.4% for PE. NOdependent relaxation was also enhanced, but to a lesser extent (*50%). Relaxation to salbutamol was almost entirely NO-dependent in both groups, and was potentiated *50% by PE. 3 Relaxation to forskolin (activator of adenylate cyclase) was also enhanced in PE constricted arteries. Part of this relaxation was NO-dependent, but the major e ect of PE was to increase the NO-independent component. Propranolol diminished but did not abolish the potentiation. There was no di erence in response to CPT cyclic AMP (membrane permeant analogue) between PE and PGF 2a , suggesting that mechanisms distal to the production of cyclic AMP were unchanged. 4 Relaxation to sodium nitroprusside (SNP) was the same for PE and PGF 2a , although relaxation to acetylcholine (ACh) was slightly depressed. This implies that potentiation by PE does not involve the cyclic GMP pathway directly. 5 Mesenteric arteries constricted with PE did not show potentiation of isoprenaline-induced relaxation compared to those constricted with PGF 2a , suggesting that this e ect may be speci®c to the pulmonary circulation. 6 These results clearly show that PE potentiates both the NO-independent and -dependent components of cyclic AMP-mediated relaxation in pulmonary arteries of the rat, although the e ect on the former is more profound. We suggest that potentiation of both components is largely due to direct activation of adenylate cyclase via a 1 -adrenoceptors, within the smooth muscle and endothelial cells respectively.