Endothelium-dependent relaxation of rat aorta to a histamine H(3) agonist is reduced by inhibitors of nitric oxide synthase, guanylate cyclase and Na,K-ATPase

The possible involvement of different effector systems (nitric oxide synthase, guanylate cyclase, beta-adrenergic and muscarinic cholinergic receptors, cyclooxygenase and lipoxygenase, and Na(+),K(+)-ATPase) was evaluated in a histamine H(3) receptor agonist-induced ((R)alpha-methylhistamine, (R)alpha-MeHA) endothelium-dependent rat aorta relaxation assay. (R)alpha-MeHA (0.1 nM - 0.01 mM) relaxed endothelium-dependent rat aorta, with a pD(2) value of 8.22 +/- 0.06, compared with a pD(2) value of 7.98 +/- 0.02 caused by histamine (50% and 70% relaxation, respectively). The effect of (R)alpha-MeHA (0.1 nM - 0.01 mM) was competitively antagonized by thioperamide (1, 10 and 30 nM) (pA(2) = 9.21 +/- 0.40; slope = 1.03 +/- 0.35) but it was unaffected by pyrilamine (100 nM), cimetidine (1 muM), atropine (10 muM), propranolol (1 muM), indomethacin (10 muM) or nordthydroguaiaretic acid (0.1 mM). Inhibitors of nitric oxide synthase, L-N(G)-monomethylarginine (L-NMMA, 10 muM) and N(G)-nitro-L-arginine methylester (L-NOARG, 10 muM) inhibited the relaxation effect of (R)alpha-MeHA, by approximately 52% and 70%, respectively). This inhibitory effect of L-NMMA was partially reversed by L-arginine (10 muM). Methylene blue (10 muM) and ouabain (10 muM) inhibited relaxation (R)alpha-MeHA-induced by approximately 50% and 90%, respectively. The products of cyclooxygenase and lipoxygenase are not involved in (R)alpha-MeHA-induced endothelium-dependent rat aorta relaxation nor are the muscarinic cholinergic and beta-adrenergic receptors. The results also suggest the involvement of NO synthase, guanylate cyclase and Na(+),K(+)-ATPase in (R)alpha-MeHA-induced endothelium-dependent rat aorta relaxation.     

Thiopental inhibits nitric oxide production in rat aorta

We studied whether thiopental affects endothelial nitric oxide dependent vasodilator responses and nitrite production (an indicator of nitric oxide production) elicited by acetylcholine, histamine, and A23187 in rat aorta (artery in which nitric oxide is the main endothelial relaxant factor). In addition, we evaluated the barbiturate effect on nitric oxide synthase (NOS) activity in both rat aorta and kidney homogenates. Thiopental (10-100 microg/mL) reversibly inhibited the endothelium-dependent relaxation elicited by acetylcholine, histamine, and A23187. On the contrary, this anesthetic did not modify the endothelium-independent but cGMP-dependent relaxation elicited by sodium nitroprusside (1 nM - 1 microM) and nitroglycerin (1 nM - 1 microM), thus excluding an effect of thiopental on guanylate cyclase of vascular smooth muscle. Thiopental (100 microg/mL) inhibited both basal (87.8+/-14.3%) and acetylcholine- or A23187-stimulated (78.6+/-3.9 and 39.7+/-5.6%, respectively) production of nitrites in aortic rings. In addition the barbiturate inhibited (100 microg/mL) the NOS (45+/-4 and 42.8+/-9%) in aortic and kidney homogenates, respectively (measured as 14C-labeled citrulline production). In conclusion, thiopental inhibition of endothelium-dependent relaxation and nitrite production in aortic rings strongly suggests an inhibitory effect on NOS. Thiopental inhibition of the NOS provides further support to this contention.     

Mechanism of vasorelaxant activity of a fraction of root extract of Sesamum indicum Linn

The petroleum ether soluble fraction (SIPE) of the root extract of S. indicum was evaluated for the vasorelaxant activity using isolated rat aorta. SIPE up to 180 microg/ml concentration significantly inhibited phenylephrine- and KCl-induced contraction to the extent of 98.13 +/- 6.37 and 70.19 +/- 3.43% respectively in isolated rat aorta in a concentration dependent manner. The vasorelaxant activity was not blocked by propranolol (10 microM), atropine (1 microM) indomethacin (10 microM) and glibenclamide (10 microM). Influence of SIPE on phenylephrine-induced contractions in aortic preparations in absence of functional endothelium and on pre-incubating the tissue with L-NAME (300 microM) or methylene blue (10 microM) was also studied. SIPE at 180 microg/ml concentration could elicit partial relaxation in presence of L-NAME or methylene blue to the extent of 34.26 +/- 6.13 and 25.66 +/- 10.95% respectively. However, in absence of functional endothelium, SIPE exhibited little relaxation to the extent of 6.70 +/- 4.87%. These studies revealed that the vasorelaxant activity of SIPE was chiefly mediated through endothelium-dependent pathway.     

Comparative affinities of adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) for [125I] AM and [125I] CGRP specific binding sites in porcine tissues.

We have investigated the binding characteristics of rat [125I] adrenomedullin (AM) and human [125I] calcitonin gene-related peptide (CGRP) to membranes prepared from a number of porcine tissues including atrium, ventricle, lung, spleen, liver, renal cortex and medulla. These membranes displayed specific, high affinity binding for [125I] rat AM and [125I] human CGRP. Porcine lung displayed the highest density of binding sites for radiolabeled AM and CGRP followed by porcine renal cortex. Competition experiments performed with [125I] rat AM indicated that the rank order of potencies of various peptides for inhibiting [125I] rat AM binding to various tissues were rat AM > or = human AM > or = human AM(22-52) > h alpha CGRP > or = h alpha CGRP(8-37) > sCT except spleen, atrium, renal cortex and renal medulla where rAM and hAM were 20-300 fold more potent than hAM (22-52). When the same experiments were performed using [125I] h alpha CGRP as the radioligand, the rank order potencies for various peptides were rAM = hAM > h alpha CGRP > h alpha CGRP(8-37) in most of the tissues except in spleen and liver where h alpha CGRP was the most potent ligand. In lung, h alpha CGRP was almost as potent as rAM and hAM in displacing [125I] h alpha CGRP binding. These data suggest the existence of distinct CGRP and AM specific binding sites in contrast to previous reports that showed that both peptides interact differently in rat tissues.     

Two selective rat adrenomedullin (AM)-receptor antagonists: AM20-50 and AM24-50.

Adrenomedullin (AM) is a hypotensive peptide, which is produced in several organs and tissues, the functions of which it regulates in a autocrine-paracrine manner. Rat (r) and human (h) AM are 50- and 52-amino acid peptides, which differ for 2-amino acid deletions and six substitutions and contain a disulfide bridge-formed six-membered ring between adjacent cysteine residues in the 14 and 19 and 16 and 21 positions, respectively. The amidated C-terminal sequence is needed for AM to bind its receptors, and the ring structure (but not t he N-terminal sequence) seems to be required for AM to activate its receptors. Hence, we examined the effectiveness of some N-terminus and ring-lackingAM fragments as AM-receptor antagonists in the rat zona glomerulosa (ZG), whose cells are provided with abundant AM binding sites and display an AM-induced inhibition of K+-stimulated aldosterone secretion. Quantitative autoradiographic studies showed that cold rAMI-50, rAM20-50 and rAM24-50 displaced [125I]AM1-50 binding from rat ZG with the same potency and efficacy, which were significantly higher than those of hAM1-52, hAM22-52 and hAM26-52. Accordingly, rAM20-50 and rAM24-50 reversed the inhibitory effect of 10(-8) M rAMI-50 on aldosterone response of dispersed rat ZG cells to 10(-2) M K+ with significantly higher potency and efficacy than hAM22-52 and hAM26-52. Taken together, our findings confirm that CONH2-terminal AM fragments, lacking the six-membered ring structure, act as antagonists of AM receptors in the rat ZG. Moreover, they provide the first evidence that rAMI-50 and its fragments should be used in the investigations carried out in the rat.     

Adrenomedullin release in the rat mesenteric resistance artery

Adrenomedullin (AM) is a potent vasodilator peptide whose major source is the vascular wall. In the present study, the mechanism of release of AM was investigated in the rat mesenteric resistance artery. The isolated mesenteric vascular bed was perfused with Krebs solution at a constant flow rate (5 ml/min) and AM in the perfusate was measured by a highly sensitive enzyme immunoassay (Immunoenzymometric assay; IEMA) method. In preparations without endothelium, spontaneous release of AM was detected in the perfusate (68.7+/-5.8 fmol/ml, n=45). Periarterial nerve stimulation (PNS, 4 and 8 Hz) caused 11.4+/-3.9% (4 Hz) and 9.1+/-3.5% (8 Hz) decreases in the spontaneous release of AM. Removal of Ca2+ from the medium did not affect the spontaneous AM release, but abolished the PNS-induced inhibition of spontaneous AM release. Perfusion of 10nM calcitonin gene-related peptide (CGRP) or 0.1 microM capsaicin (inducer of CGRP release) inhibited significantly the spontaneous AM release. PNS (8 Hz)-induced inhibition of spontaneous AM release was antagonized by CGRP(8-37) (CGRP receptor antagonist). These results suggest that AM is mainly released from vascular smooth muscle cells of the rat mesenteric artery and endogenous or exogenous CGRP inhibits AM release.     

Effect of tumor necrosis factor on hypoxic pulmonary vasoconstriction.

The effects of tumor necrosis factor (TNF) on hypoxic pulmonary vasoconstriction (HPV) and endothelium-dependent relaxation were examined in a blood-perfused rat lung preparation. Lungs from TNF-treated rats (0.26 mg/kg iv 12 h before experimentation) had a significantly greater HPV and a reduced vasorelaxant response to the endothelium-dependent vasodilator acetylcholine (ACh) but a similar vasorelaxant response to the endothelium-independent vasodilator nitroprusside compared with lungs from control rats (pretreated with 0.1 ml saline iv). Pentoxifylline (20 mg/kg iv and ip 20 min before administration of TNF) had no detectable effect on either HPV or ACh-induced relaxation but completely negated the augmentation on HPV and the inhibiting action on ACh-induced relaxation caused by TNF. The TNF effect on ACh relaxation was unaffected by pretreatment with L-arginine. These results indicate that TNF induces endothelial dysfunction and enhances HPV, effects that are inhibited by pentoxifylline.     

The adrenomedullin receptor acts as clearance receptor in pulmonary circulation

Adrenomedullin (AM) is a powerful pulmonary vasodilator with antimitogenic properties. We investigated the role of the AM receptor (AMR) and the calcitonin gene-related peptide type-1 receptor (CGRP1R) in regulating pulmonary vascular AM levels. The AMR antagonist hAM(22-52) (120 nmol/L) significantly elevated AM release compared with controls to 250% after 2 h in isolated rat lungs and to 830% after 4 h in pulmonary artery endothelial cells (PAEC). CGRP1R blockade had no effect. AMR blockade did not influence prepro-AM mRNA levels nor did inhibition of protein synthesis by cycloheximide (0.01 mg/mL) abolish the effect of the AMR antagonist. Radioligand-binding studies with PAEC membranes revealed a decrease by 44% of the AMR density in response to AMR antagonism. Altogether, the pulmonary vascular AMR represents not only a functionally active, but also a clearance receptor; its expression is constitutively stimulated by basal AM. This identifies a novel mechanism for controlling pulmonary AM levels.     

Characterization of the vasorelaxation to methanandamide in rat gastric arteries

In the present study, the relaxant effect of the cannabinoid methanandamide was explored in rat gastric arteries. Since in some vessels cannabinoids have been shown to release calcitonin gene-related peptide (CGRP) from perivascular nerves, the influence of methanandamide was compared with that of exogenous CGRP. Methanandamide and CGRP elicited concentration-dependent, endothelium-independent relaxations. Methanandamide-induced relaxations were unaffected by the CB1 receptor antagonist AM251, the CB2 receptor antagonists AM630 and SR144528, and combined pre-exposure to AM251 and SR144528. Pre-exposure to O-1918, an antagonist of a novel nonCB1/nonCB2 cannabinoid receptor, did not influence the relaxations to methanandamide. Capsaicin or capsazepine treatment slightly inhibited methanandamide-induced relaxations. Preincubation with 30 mmol/L extracellular K+ or 3 mmol/L TEA had no significant effect on the responses elicited by methanandamide, but reduced CGRP-induced relaxations. Relaxation to 10(-5) mol/L methanandamide was significantly blunted by Bay K8644 and by preincubation with nifedipine. Furthermore, 10(-5) mol/L methanandamide significantly inhibited CaCl2-induced contractions in norepinephrine-stimulated vessels previously depleted of intra- and extracellular Ca2+. Finally, preincubation with 10(-5) mol/L methanandamide almost completely abolished high K+-induced contractions. These findings suggest that the vasorelaxant action of methanandamide in rat gastric arteries is not mediated by stimulation of known cannabinoid receptors and only partly related to stimulation of TRPV1 receptors on perivascular nerves. At high concentrations, methanandamide might induce relaxation by reducing calcium entry into the smooth muscle cells.     

Chronic ouabain treatment increases the contribution of nitric oxide to endothelium-dependent relaxation

The aim of this study was to analyze the contribution of nitric oxide, prostacyclin and endothelium-dependent hyperpolarizing factor to endothelium-dependent vasodilation induced by acetylcholine in rat aorta from control and ouabain-induced hypertensive rats. Preincubation with the nitric oxide synthase inhibitor N-omega-nitro-l-arginine methyl esther (L-NAME) inhibited the vasodilator response to acetylcholine in segments from both groups but to a greater extent in segments from ouabain-treated rats. Basal and acetylcholine-induced nitric oxide release were higher in segments from ouabain-treated rats. Preincubation with the prostacyclin synthesis inhibitor tranylcypromine or with the cyclooxygenase inhibitor indomethacin inhibited the vasodilator response to acetylcholine in aortic segments from both groups. The Ca²⁺-dependent potassium channel blocker charybdotoxin inhibited the vasodilator response to acetylcholine only in segments from control rats. These results indicate that hypertension induced by chronic ouabain treatment is accompanied by increased endothelial nitric oxide participation and impaired endothelium-dependent hyperpolarizing factor contribution in acetylcholine-induced relaxation. These effects might explain the lack of effect of ouabain treatment on acetylcholine responses in rat aorta.     

Comparative Affinities of Adrenomedullin (AM) and Calcitonin Gene-Related Peptide (CGRP) for [125I] AM and [125I] CGRP Specific Binding Sites in Porcine Tissues

Abstract

We have investigated the binding characteristics of rat [¹²⁵I] adrenomedullin (AM) and human [¹²⁵I] calcitonin gene-related peptide (CGRP) to membranes prepared from a number of porcine tissues including atrium, ventricle, lung, spleen, liver, renal cortex and medulla. These membranes displayed specific, high affinity binding for [¹²⁵I] rat AM and [¹²⁵I] human CGRP. Porcine lung displayed the highest density of binding sites for radiolabeled AM and CGRP followed by porcine renal cortex. Competition experiments performed with [¹²⁵I] rat AM indicated that the rank order of potencies of various peptides for inhibiting [¹²⁵I] rat AM binding to various tissues were rat AM ≥ human AM ≥ human AM(22–52) > hαCGRP ≥ hαCGRP(8–37) <<<< sCT except spleen, atrium, renal cortex and renal medulla where rAM and hAM were 20–300 fold more potent than hAM(22–52). When the same experiments were performed using [¹²⁵I] hαCGRP as the radioligand, the rank order potencies for various peptides were rAM = hAM > hαCGRP > hαCGRP(8–37) in most of the tissues except in spleen and liver. where hαCGRP was the most potent ligand. In lung, hαCGRP was almost as potent as rAM and hAM in displacing [¹²⁵I] hαCGRP binding. These data suggest the existence of distinct CGRP and AM specific binding sites in contrast to previous reports that showed that both peptides interact differently in rat tissues.     

TIC4]endomorphins, analogues of endomorphins, have significantly enhanced vasorelaxant effects in rat aorta rings

[Tic(4)]EM1 and [Tic(4)]EM2, new endomorphins (EMs) analogues, caused relaxation of rat aorta rings precontracted with phenylphrine in a concentration-dependent manner and were 240- to 370-fold more potent than EMs. This effect was inhibited by endothelium removal or by incubation with NO synthase inhibitor L-NNA or opioid receptor antagonist naloxone. The results demonstrate that [Tic(4)]EMs have NO- and endothelium-dependent vasorelaxant effects which are mediated by the opioid receptor.     

Vasorelaxant activity of cyclic peptide, cyclosquamosin B, from Annona squamosa

A cyclic octapeptide, cyclosquamosin B (2), isolated from the seeds of Annona squamosa showed a vasorelaxant effect on rat aorta. It showed a slow relaxation activity against norepinephrine (NE)-induced contractions of rat aorta with/without endothelium. It showed inhibition effect on vasocontraction of depolarized aorta with high concentration potassium, but moderately inhibition effect on NE-induced contraction in the presence of nicardipine. These results showed that the vasorelaxant effect by 2 might be attributed mainly to inhibition of calcium influx from extracellular space through voltage-dependent calcium channels.     

Effects of hydralazine on guanosine cyclic 3', 5'-monophosphate levels in rat aorta

The mechanism of the vasodilator effect of hydralazine on isolated rat aorta was studied. Results demonstrated that the vasodilator effect of hydralazine was greater on intact aortas than on endothelium-denuded preparations, particularly at low concentrations of between 0.1 mM and 0.5 mM. In addition, hydralazine did not have any effect on cyclic GMP levels. We also found that methylene blue, an inhibitor of guanylate cyclase, completely abolished the vasorelaxant action of nitroglycerin but not that of hydralazine. These results indicate that the vasodilator effect of hydralazine was not due to elevating the cyclic GMP levels. On the other hand, hydralazine significantly inhibited both the contractions induced by norepinephrine and/or high-potassium. In conclusion, a part of the vasodilator effect of hydralazine seems to depend on the integrity of the vascular endothelium. However, this vasodilator effect was not associated with any elevation in cyclic GMP level. Thus, the direct vasodilator action of hydralazine may be related to its interference with the movement and/or translocation of calcium across the cell membrane.     

Mechanisms of the vasorelaxant effect of 1, 5-dihydroxy-2, 3-dimethoxy-xanthone, an active metabolite of 1-hydroxy-2, 3, 5-trimethoxy-xanthone isolated from a Tibetan herb, Halenia elliptica, on rat coronary artery

1, 5-Dihydroxy-2, 3-dimethoxy-xanthone (HM-5) is one of the naturally-occurring xanthones of a Tibetan medicinal herb Halenia elliptica. Recently, it has been shown that HM-5 is one of the phase I metabolites of 1-hydroxy-2, 3, 5-trimethoxy-xanthone (HM-1), the major active component of H. elliptica with potent vasorelaxant actions. This study investigated the vasorelaxant effect of HM-5 and its mechanism(s). HM-5 (0.35-21.9 microM) produced a concentration-dependent relaxation in rat coronary artery rings pre-contracted with 1 microM 5-hydroxytryptamine (5-HT), with an EC(50) of 4.40+/-1.08 microM. Unlike HM-1, the effect of HM-5 was endothelial-independent such that removal of the endothelium did not affect its vasodilator potency. Nitric oxide synthase (NOS) inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME, 100 microM), the soluble guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-alpha] quinoxalin-1-one (ODQ, 10 microM) did not affect the vasodilatory effects of HM-5, thus confirming the non-involvement of endothelium related mechanisms. In endothelium-denuded coronary artery rings, the vasorelaxant effect of HM-5 was inhibited by a potassium channel blocker, TEA (10 mM), and 4-aminopyridine (4-AP, a K(v) blocker; 1 mM) but not by other K+ channel blockers such as iberiotoxin (100 nM), barium chloride (100 microM) and glibenclamide (10 microM). The involvement of Ca2+ channel was studied in artery rings pre-incubated with Ca2+-free buffer (intact endothelium or endothelium-denuded) and primed with 1 microM 5-HT or 60 mM KCl prior to the addition of CaCl2 to elicit contraction. In the 5-HT-primed preparations, HM-5 (34.7 microM) significantly inhibited the CaCl(2)-induced vasoconstriction (89.9% inhibition in intact endothelium artery rings; 83.3% inhibition in endothelium-denuded rings). In the KCl-primed preparations, HM-5 (34.7 microM) produced a 34% inhibition in endothelium-denuded rings. The same concentration of HM-5 inhibited (by 62.3%) the contractile response to 10 microM phorbol 12, 13-diacetate (PDA), a protein kinase C activator, in Ca2+-free solutions. Taken together, this study showed that the mechanisms of the vasorelaxant effects of HM-5 were distinctly different from those of its parent drug HM-1. The vasorelaxant effect of HM-5 was mediated through opening of potassium channel (4-AP) and altering intracellular calcium by partial inhibition of Ca2+ influx through L-type voltage-operated Ca2+ channels and intracellular Ca2+ stores.     

Maturational modulation of endothelium-dependent vasodilatation in ovine cerebral arteries

To address the hypothesis that maturation enhances endothelial vasodilator function in cerebral arteries, relaxant responses to ADP and A-23187 were determined in ovine carotid and cerebral arteries harvested from 25 newborn lambs (3-7 days) and 23 adult sheep. Maturation significantly increased pD(2) values for A-23187 (newborn range: 4.9 +/- 0.3 to 5.4 +/- 0.3; adult range: 6.0 +/- 0.2 to 7.1 +/- 0.2) and the maximal vasodilator response to A-23187 by 10-18%. In contrast, maturation decreased maximum responses to ADP by 5-25% with no change in pD(2). The magnitudes of endothelium-dependent relaxation were not affected by 10 microM indomethacin but were virtually abolished by 100 microM N(G)-nitro-L-arginine methyl ester/L-nitro arginine, indicating that nitric oxide (NO) is the primary endothelium-dependent vasodilator in these arteries. Maturation also modestly decreased endothelial NO synthase (eNOS) abundance in both carotid (32%) and cerebral (26%) arteries. Together, these findings reinforce the view that receptor coupling to endothelial activation is tightly regulated and may offset underlying changes in maximal endothelial vasodilator capacity. This capacity, in turn, appears to increase with postnatal age despite major growth and expansion of endothelial cell size and vascular wall volume. In ovine cerebral arteries, endothelial vasodilator capacity appears completely dependent on eNOS activity but not on cyclooxygenase activity. In turn, eNOS activity appears to be postnatally regulated by mechanisms independent of changes in eNOS abundance alone.     

Calcium mobilization as the endothelium-independent mechanism of action involved in the vasorelaxant response induced by the aqueous fraction of the ethanol extract of Albizia inopinata G. P. Lewis (AFL) in the rat aorta.

In a previous work, we demonstrated that, in normotensive rats, AFL induced a marked hypotension due to a decrease in total peripheral resistances (TPR), partially secondary to the release of NO by the endothelium. NO did not, however, account for the total vasodilation produced by AFL in these rats. The aim of this study was to determine the involvement of the intracellular calcium mobilization in the vasorelaxant action induced by AFL in the rat aorta. In aorta of normotensive rats AFL (10, 20, 40 and 80 microg/ml) inhibited the sustained contractions induced by KCl (80 and 30 mM) and phenylephrine (Phe, 1 microM) with similar IC50 values (54 +/- 6, 52 +/- 4 and 65 +/- 4 microg/ml, respectively). The relaxing response induced by AFL against Phe-induced contractions was modified significantly by the endothelium removal (IC50 = 132 +/- 23 and 65 +/- 4 microg/ml, endothelium removed and intact endothelium aortic rings, respectively). Nevertheless, removal of the endothelium did not significantly change IC50 values when KCl (30 and 80 mM) was used as the contractile agent. The inhibitory effect induced by AFL on high (64.5 mM) K+-induced contraction was potentiated slightly (p < 0.05) by the decrease (from 2.5 to 0.3 mM, Ca2+) and attenuated by the increase (from 2.5 to 7.5 mM Ca2+) in the external [Ca2+]. In addition, in aortas from normotensive rats, AFL antagonized transient contractions induced in Ca2+-free media induced by 1 microM noradrenaline in a concentration-dependent manner, but not those induced by 20 mM caffeine. It is suggested that the remaining vasodilator effect of AFL in normotensive rats is probably due to an inhibition of Ca2+ influx and/or inhibition of intracellular Ca2+ mobilization from the noradrenaline-sensitive stores.     

Differential patterns of relaxation by synthetic atrial peptide (APII) in the pulmonary artery, ascending and distal abdominal aorta

The vasodilatory effects of the synthetic rat atriopeptin (APII) have been studied in vitro in agonist-contracted, endothelium-denuded segments of the rat pulmonary artery, the ascending, and the distal abdominal aorta. In the pulmonary artery the contractures to methoxamine were inhibited more potently by APII (pD2 = 9.10 +/- 0.40, n = 6) than by the vasodilatory neuropeptide VIP (pD2 = 7.37 +/- 0.66, n = 6). The intrinsic activity of APII was 0.46 +/- 0.16 (n = 6). In segments previously exposed to either VIP or the beta 2-agonist salbutamol, APII was a near complete agonist (alpha = 0.82 +/- 0.17, n = 7 and 0.84 +/- 0.14, n = 6, respectively) without significant changes in the potencies. APII was a complete agonist also for the inhibition of the alpha-agonist-contracted segments of the aorta, however, with potencies 10-fold lower than in the pulmonary artery. VIP was without functionally significant effects in the aorta. The tachykinins (CGRP, SP, Neurokinins A and B) were without effects in all segments tested. In the ascending aorta, APII induced a long-lasting tachyphylaxis to the alpha-agonists, nearly completely abolishing the subsequent responsiveness to NA and methoxamine for more than 4 h.     

Repeated inhalation of adrenomedullin ameliorates pulmonary hypertension and survival in monocrotaline rats

Adrenomedullin (AM) is a potent vasodilator peptide. We investigated whether inhalation of aerosolized AM ameliorates monocrotaline (MCT)-induced pulmonary hypertension in rats. Male Wistar rats given MCT (MCT rats) were assigned to receive repeated inhalation of AM (n = 8) or 0.9% saline (n = 8). AM (5 mug/kg) or saline was inhaled as an aerosol using an ultrasonic nebulizer for 30 min four times a day. After 3 wk of inhalation therapy, mean pulmonary arterial pressure and total pulmonary resistance were markedly lower in rats treated with AM than in those given saline [mean pulmonary arterial pressure: 22 +/- 2 vs. 35 +/- 1 mmHg (-37%); total pulmonary resistance: 0.048 +/- 0.004 vs. 0.104 +/- 0.006 mmHg.ml(-1).min(-1).kg(-1) (-54%), both P < 0.01]. Neither systemic arterial pressure nor heart rate was altered. Inhalation of AM significantly attenuated the increase in medial wall thickness of peripheral pulmonary arteries in MCT rats. Kaplan-Meier survival curves demonstrated that MCT rats treated with aerosolized AM had a significantly higher survival rate than those given saline (70% vs. 10% 6-wk survival, log-rank test, P < 0.01). In conclusion, repeated inhalation of AM inhibited MCT-induced pulmonary hypertension without systemic hypotension and thereby improved survival in MCT rats.