A1-adenosine receptor antagonists block endotoxin-induced lung injury.

Endotoxin produces a variety of biological effects on different cell types, such as priming of neutrophils and macrophages, which then release a number of important mediators of endotoxin-induced lung injury. However, the specific mechanism by which endotoxin initiates its cascade of pathophysiological events in the lung has not been described. Both A1 adenosine receptor activation and endotoxin induce the release of thromboxane A2 from the lung and inhibit adenylate cyclase. By acting on A1 adenosine receptors, adenosine promotes neutrophil chemotaxis and adherence to endothelial cells. We hypothesized that A1 adenosine receptor activation is essential to endotoxin-induced lung injury, and we used the highly selective A1-adenosine receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 8-benzyl-7,[2-[ethyl(2-hydroxyethyl)amino]-ethyl] theophylline (bamiphylline), to investigate whether selective blocking of the A1 adenosine receptor would prevent endotoxin-induced acute lung injury. An intralobar arterial infusion of endotoxin (15 mg/kg) into the left lower lobe of the lung in intact-chest, spontaneously breathing cats produced lung injury characterized by the presence of neutrophils, macrophages, and red blood cells (RBCs) in alveoli, and alveolar edema and necrosis. Lower doses of endotoxin (5 or 10 mg/kg) produced less severe and dose-dependent lung injury. Endotoxin (15 mg/kg)-induced alveolar injury was blocked in a highly significant manner by A1-adenosine receptor antagonists, DPCPX and bamiphylline. An intravenous bolus of DPCPX 30 min before endotoxin infusion or a continuous intravenous infusion of bamiphylline 30 min before, during, and 30 min after endotoxin reduced the percent injured alveoli (defined as the presence of 2 or more inflammatory cells or RBCs, or edematous fluid) from 57 +/- 31% (endotoxin 15 mg/kg) to 9 +/- 1% (DPCPX) or 21 +/- 14% (bamiphylline), which were not significantly different from control (1-h perfusion only) (4 +/- 1%) (P < 0.05). These data represent the first evidence that A1-adenosine receptor antagonism blocks the capacity of endotoxin to cause lung injury. A1-adenosine receptor antagonists may be useful in preventing adult respiratory distress syndrome associated with septicemia.

A1 adenosine receptor antagonists block ischemia-reperfusion injury of the heart.

BACKGROUND: It has been reported that A1 adenosine receptor antagonists are highly effective for the prevention and early treatment of ischemia-reperfusion injury of isolated perfused cat lung, which suggests that activation of A1 adenosine receptors is important in ischemia-reperfusion injury of the lung. In addition, preconditioning ischemia reduces ischemia-reperfusion injury of the lung and heart. Moreover, activation of A1 adenosine receptors by adenosine and selective A1 adenosine receptor agonists mimics the protective effects of preconditioning ischemia in the heart. It has been reported that prior treatment with selective A1 adenosine receptor agonists results in a rapid uncoupling of A1 adenosine receptors from signal transduction mechanisms. In the heart, these effects of A1 adenosine receptor agonists have not been reported. However, if prior treatment of ischemia of the heart with adenosine or A1 adenosine receptor agonists results in uncoupling of A1 adenosine receptors from signal transduction mechanisms that produce injury after prolonged ischemia and reperfusion, A1 adenosine receptor antagonists should provide a protective effect similar to these treatments for ischemia-reperfusion injury of the heart. Therefore, it was the purpose of these experiments to investigate the effect of selective A1 adenosine receptor antagonists on ischemia-reperfusion injury of the heart. METHODS AND RESULTS: With the use of a regional infarct model in open-chest cats, the left anterior descending artery or first diagonal branch was occluded for 1 hour followed by 2 hours of reperfusion. Infarct size (area of necrosis/area at risk; AN/AR) was estimated with the use of nitroblue tetrazolium staining. The selective A1 adenosine receptor antagonists xanthine amine congener (XAC; 0.1 mg.kg-1.h-1), bamifylline (BAM; 10 mg.kg-1.h-1), 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 10 micrograms.kg-1.min-1) administered as continous intravenous infusions for 1 hour before ischemia [DPCPX (I)], or DPCPX 30 micrograms.kg-1.min-1 administered intravenously during 30 minutes of ischemia and 30 minutes of reperfusion [DPCPX (I/R)] significantly (P < .05) reduced AN/AR from 52.2 +/- 3.8% (control, n = 5) to 23.4 +/- 6.6% (XAC, n = 5), 34.9 +/- 3.6% (BAM, n = 5), 15.9 +/- 2.9% [DPCPX(I), n = 5], or 13 +/- 3.2% [DPCPX (I/R), n = 5]. CONCLUSIONS: A1 adenosine receptor antagonists significantly reduce ischemia-reperfusion injury of the heart. A1 adenosine receptor antagonists may be useful for the prevention or early treatment of ischemia-reperfusion injury of the heart after coronary artery bypass graft surgery or cardiac transplant surgery and during or after angioplasty or thrombolytic therapy of the heart.

P2X purinoceptors in the feline pulmonary vascular bed: distribution and selective in vivo pharmacological probes.

The distribution and identification of selective pharmacological probes for P2X purinoceptors in the pulmonary vascular (PV) bed of the cat have been investigated with autoradiographic and pharmacological techniques. Autoradiographic localization of the selective P2X purinoceptor ligand alpha, beta-[3H]methylene ATP (alpha, beta-MeATP) binding sites in cat lung shows that P2X purinoceptors are present in all vessels in the bed; high densities were present in large (2-mm diam) and small (0.5-mm diam) pulmonary arteries, bronchial arterioles (0.1-mm diam), and large pulmonary veins, whereas low density is characteristic of parenchymal arterioles and alveolar walls. Most of the binding is displaced with the P2X-purinoceptor agonist beta, gamma-methylene ATP and the putative selective P2X-purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) in all vessels; however, the binding is further displaced with the, P2Y-purinoceptor agonist 2-methylthio ATP (2-MeS-ATP), which suggests the presence of P2Y as well as P2X purinoceptors. P2X purinoceptors mediate potent vasoconstrictor actions in the PV bed. alpha, beta-MeATP is a selective agonist for P2X purinoceptors and does not act via serotonergic, histaminergic, adrenergic, or leukotriene vasoconstrictor receptors to produce an increase in PV resistance. The vasoconstrictor responses of alpha, beta-MeATP are attenuated by PPADS. However, PPADS has no effect on the vasodilation induced by ATP, adenosine, or 2-MeS-ATP. The diadenosine nucleotide AP5A also produced dose-dependent vasoconstrictor responses of the PV bed, which were approximately three times less potent than those of alpha, beta-MeATP and significantly reduced by PPADS. These data support that vasoconstrictor P2X purinoceptors are present on pulmonary vessels. The functional significance of these vascular P2X purinoceptors in the PV bed is not known; however, alpha, beta-MeATP, AP5A, and PPADS may be used in vivo to define their physiological role in health and disease in the lung.

Characterization and autoradiographic localization of [3H] alpha,beta-methylene ATP binding sites in cat urinary bladder.

1. The characteristics and distribution of [3H] alpha,beta-methylene ATP ([3H] alpha,beta-MeATP), a radioligand for P2X-purinoceptors, binding sites in cat urinary bladder detrusor were examined. 2. Saturation analysis revealed that, in cat bladder membrane preparations, only one population of binding sites with high affinity (Kd = 1.8 nM) was present, in contrast to other species where both high-and low-affinity binding sites are present. Another feature is that the density of the binding sites in the cat bladder (Bmax = 21.2 pmol/mg protein) is considerably higher (about 2-fold) than the high-affinity binding component in the rat bladder membrane preparations. 3. Displacement experiments with unlabelled purinoceptor ligands indicate that [3H] alpha,beta-MeATP mainly binds to P2X-purinoceptors. The order of binding displacement activity was: alpha,beta-methylene ATP, beta,gamma-methylene ATP > 2-methylthioATP > ATP > suramin and L-beta,gamma-methylene ATP > > adenosine. 4. Autoradiographic study demonstrated dense specific binding sites of [3H] alpha,beta-MeATP on detrusor smooth muscle of cat bladder. 5. The results of this study are consistent with pharmacological studies for the existence of P2X-purinoceptors in cat bladder.

Pharmacological probes for A1 and A2 adenosine receptors in vivo in feline pulmonary vascular bed.

Under conditions of controlled pulmonary blood flow and constant left atrial pressure, adenosine produces dose-dependent, tone-dependent responses in the pulmonary vascular (PV) bed of intact-chest, spontaneously breathing cats. The potency profile for adenosine receptor agonists to produce vasoconstriction at low baseline PV tone is 5'-(N-ethylcarboxamido)adenosine > or = CGS-21680 > or = 2-chloroadenosine (2-CADO) > or = [R]-N6-(2-phenylisopropyl)adenosine (R-PIA) > or = N6-cyclopentyladenosine > adenosine > > CV-1808. After an increase in PV tone with the use of an intralobar infusion of the thromboxane mimic U-46619, the potency profile for adenosine receptor agonists to produce vasodilation at elevated PV tone is 2-CADO > or = CV-1808 > or = CGS-21680 > R-PIA > or = adenosine. The selective A1 adenosine receptor antagonists xanthine amine congener (XAC) and 8-cyclopentyl-1,3-dipropylxanthine (DP-CPX) significantly antagonize the vasoconstrictor responses of adenosine and R-PIA at low baseline PV tone while having less effect on the vasodilator responses of adenosine, 2-CADO, and R-PIA at elevated PV tone. DPCPX antagonizes the vasoconstrictor responses of CGS-21680 at low baseline PV tone. The nonselective A1 and A2 adenosine receptor antagonist BWA-1433U significantly antagonizes vasoconstrictor responses of R-PIA and vasodilator responses of adenosine, 2-CADO, and R-PIA. These data support that adenosine produces vasoconstriction at low baseline PV tone and vasodilation at elevated PV tone in the feline PV bed by acting on A1 and A2 adenosine receptors, respectively. Compared with the adenosine receptor agonists tested in this in vivo model, R-PIA and CV-1808 are the most selective adenosine receptor agonists for A1 and A2 adenosine receptors, respectively, in the feline PV bed. R-PIA, CV-1808, DPCPX, and XAC may be used in this in vivo model to define the roles of A1 and A2 adenosine receptors in acute lung injury and pathophysiological changes in the pulmonary vasculature associated with pulmonary hypertension and edema formation in the same animal model.

Calcium blockage in pulmonary hypertension and hypoxic vasoconstriction.

Both extracellular and intracellular calcium (Ca2+) play important roles in hypoxic pulmonary vasoconstriction (HPV) and the vasoconstrictor responses to endogenous pulmonary vasoconstrictor substances, as evidenced by the effect of calcium-channel blockers on these vasoconstrictor responses and the measurement of changes in Ca2+ flux or intracellular Ca2+ concentrations in isolated cells. The more vasoselective the calcium-channel blocker, the greater its effect on pulmonary vasoconstriction. However, these drugs are not selective for the pulmonary vascular bed and are not as potent as pulmonary vasodilators when compared with other vasodilator drugs, including prostaglandin E1, isoproterenol, prostacyclin, or nitroglycerin. Moreover, the primary effect of vasoselective calcium-channel blockers on pulmonary vascular resistance is secondary to the effects of these agents on systemic vascular resistance and cardiac output. Although there is improvement in oxygen delivery, exercise tolerance, and survival in patients with primary pulmonary hypertension who respond to calcium-channel blockers, the response of individual patients to these drugs is difficult to predict because the extent of reversible versus irreversible changes in the pulmonary vasculature is not known. The use of these drugs in patients with chronic hypoxia-induced pulmonary vasoconstriction may be associated with a worsening of ventilation-perfusion mismatching secondary to inhibition of HPV.

Adenosine A1 and A2 receptors mediate tone-dependent responses in feline pulmonary vascular bed.

Adenosine produces tone-dependent pulmonary vascular responses; however, the adenosine receptor subtype mediating these responses is unknown. In the present study, the adenosine receptor subtypes mediating tone-dependent responses were investigated, Intralobar injections of adenosine,ATP, and analogues under low-tone conditions caused dose-related increases in lobar arterial pressure; the order of potency was alpha,beta-methylene ATP (alpha,beta-metATP) > N6-cyclopentyladenosine (CPA) > ATP > adenosine. Under low-tone conditions, pressor responses to adenosine, ATP, and CPA, an adenosine A1-receptor agonist, were reduced by KW-3902, an adenosine A1-receptor antagonist, whereas KW-3902 and meclofenamate had no effect on responses to alpha,beta-metATP, norepinephrine, serotonin, or angiotensin II. Under elevated-tone conditions, injections of adenosine, ATP, and analogues caused dose-related decreases in lobar arterial pressure, and adenosine was 10-fold less potent than 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA), an A2-receptor agonist, and ATP. KF-17837, an A2-receptor antagonist, reduced vasodilator responses to adenosine and CPCA, whereas responses to ATP, isoproterenol, diethylamine-NO, lemakalim, and bradykinin were not changed. The vasodilator responses to adenosine were not attenuated by Nw-nitro-L-arginine benzyl ester, methylene blue, or U-37883A. These results suggest that vasoconstrictor responses to adenosine are mediated by A1 receptors and the release of vasoconstrictor prostanoids, and that, under elevated-tone conditions, vasodilator responses are mediated by A2 receptors but not the release of nitric oxide or the activation of guanylate cyclase or K+ATP channels.

Pulmonary vasodilator responses to nicardipine: comparison with other vasodilators.

OBJECTIVES: To determine the direct effect of nicardipine on the pulmonary vascular bed of the intact-chest, spontaneously breathing cat, and to compare its effectiveness as a pulmonary vasodilator with the effectiveness of isoproterenol, nitroglycerin, and sodium nitroprusside. DESIGN: Prospective, controlled animal study. Each animal received all drugs in random order. SETTING: Animal laboratory at a university. SUBJECTS: Experiments were performed in vivo in ten intact-chest, spontaneously breathing cats with controlled pulmonary blood flow and constant left atrial pressure, during conditions of increased pulmonary vascular tone. INTERVENTIONS: Five animals received intralobar injections of nicardipine (0.1 to 100 micrograms), nitroglycerin (0.1 to 10 micrograms), sodium nitroprusside (0.1 to 100 micrograms), and isoproterenol (0.01 to 1 microgram). Injections were made only when lobar arterial pressure had returned to baseline value. In another five animals, nicardipine, nitroglycerin, and sodium nitroprusside were administered intravenously as a continuous drug infusion in incremental doses titrated to produce a 20% reduction in mean systemic arterial pressure. Each dose was infused until lobar and systemic arterial pressures stabilized. A minimum, 30-min interval was allowed between the infusions of these drugs. MEASUREMENTS AND MAIN RESULTS: When pulmonary vascular tone was increased with a thromboxane A2 mimetic (analog), U46619 (a stable prostaglandin endoperoxide analog), intralobar injections of nicardipine caused dose-related decreases in lobar arterial pressure without affecting left atrial pressure. When compared with the other vasodilator agents, the order of potency was isoproterenol >> nitroglycerin > nicardipine = sodium nitroprusside. Isoproterenol reduced mean systemic arterial pressure 10 to 100 times greater than nitroglycerin, nicardipine, or sodium nitroprusside. However, there were no significant differences between the latter three drugs in producing a decrease in mean systemic arterial pressure. When infused intravenously, nitroglycerin caused the largest amount of pulmonary vasodilation for a given amount of systemic vasodilation. There were no significant differences between the pulmonary vasodilator responses of nicardipine and sodium nitroprusside. CONCLUSIONS: In this feline model of increased pulmonary vascular resistance, nicardipine exerts a direct vasodilator effect in vivo on the pulmonary vascular bed. Nicardipine, nitroglycerin, and sodium nitroprusside caused similar decreases in systemic arterial pressure. However, the pulmonary vasodilator effect was greater with nitroglycerin, which suggests that nitroglycerin is more vasoselective for the pulmonary vascular bed than nicardipine or sodium nitroprusside.

A1 adenosine receptor antagonists block ischemia-reperfusion injury of the lung.

Ischemia-reperfusion (I-R) injury of the lung occurs after lung transplantation, pulmonary thromboembolectomy, or cardiopulmonary bypass. In the heart, adenosine, A1 adenosine receptor agonists, and a brief period of preconditioning ischemia attenuate I-R injury. Moreover, in the lung, thromboxane is released during ischemia and is an important mediator of I-R injury. We previously reported that adenosine produces vasoconstriction in the feline pulmonary vascular bed by acting on A1 receptors to induce the release of thromboxane and that these vasoconstrictor responses are desensitized by low doses of A1 receptor agonists. Because A1 receptor agonists mimic the effect of preconditioning ischemia, we hypothesized, in contrast to previously proposed mechanisms, that small amounts of adenosine released during preconditioning ischemia desensitize A1 receptors. Also, we hypothesized that greater amounts of adenosine are released after longer periods of ischemia, which activate A1 receptors. Thus if desensitization of A1 receptors is the mechanism by which preconditioning attenuates I-R injury of the heart and A1 receptor activation during ischemia plays an important role in I-R injury of the lung, A1 receptor antagonists should provide a protective effect in I-R injury of the lung. In this study, 2 h of ischemia and 2 h of reperfusion of the left lower lobe in intact-chest, spontaneously breathing cats caused lung injury characterized by the presence of neutrophils, macrophages, and RBCs in alveoli and caused alveolar edema, which was blocked in a highly significant manner by the A1 receptor antagonists xanthine amine congener (XAC) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). An intralobar arterial infusion of XAC (30 min before ischemia) reduced the %injured alveoli (defined as presence of 2 or more inflammatory cells or RBCs, or edematous fluid) from 60 +/- 10 to 7 +/- 2%, which was not significantly different from controls (5 +/- 1%; P < 0.0001). DPCPX (iv) reduced the %injured alveoli to 13 +/- 7% when administered 30 min before ischemia and to 6 +/- 2% when administered after 1 h of reperfusion, not significantly different from controls (P < 0.0001). Preconditioning ischemia (10-min ischemia +10-min reperfusion) also reduced the %injured alveoli after 2 h ischemia and 2 h reperfusion to 23 +/- 13%, almost identical to 2 h ischemia and 1 h reperfusion. These data support the hypothesis that A1 receptor antagonists block I-R injury of the lung. A1 receptor antagonists may be useful in preventing I-R injury after transplant surgery and during surgical procedures associated with I-R injury of the heart, brain, kidney, and spinal cord.

Tone-dependent responses of histamine in feline pulmonary vascular bed.

Under conditions of controlled pulmonary blood flow and constant left atrial pressure, histamine produced tone-dependent responses in the pulmonary vascular (PV) bed of intact-chest, spontaneously breathing cats. At low, baseline PV tone, histamine produced dose-dependent increases in mean lobar arterial pressure that were antagonized by the selective histamine H1-receptor antagonist, diphenhydramine. The cyclooxygenase inhibitor, meclofenamate, and the thromboxane A2 (TxA2) receptor antagonist, SQ-29548, had no effect on these vasoconstrictor responses of histamine. After an increase in PV tone with an intralobar arterial infusion of a TxA2 mimic, U-46619, histamine produced vasodilator responses at low doses, biphasic vasodilator/vasoconstrictor responses at midrange doses, and vasoconstrictor responses at high doses. Diphenhydramine antagonized vasoconstrictor responses and the vasodilator responses of low to midrange doses and enhanced vasodilator responses of high doses of histamine at elevated PV tone. Selective H2-receptor antagonists, ranitidine and meclofenamate, and selective H3-receptor antagonist, thioperamide, did not antagonize vasodilator responses of histamine. H1- and H2-receptor antagonism was more effective in reducing the vasodilator responses of histamine at elevated PV tone than H1-receptor antagonism alone. These data support that histamine produces vasoconstrictor responses at low baseline and elevated PV tone by acting on H1 receptors that do not induce the release of vasoconstrictor prostanoids. At elevated PV tone, histamine produces vasodilation by acting on H1 receptors that are not coupled to the release of vasodilator prostaglandins and also, in part, by acting on H2 receptors.

Fentanyl and propofol uptake by the lung: effect of time between injections.

The effect of time between the administrations of fentanyl and propofol on the first pass uptake of propofol in the cat lung was studied using double indicator dilution technique. The pulmonary first pass uptake of propofol (mean +/- s.e. mean) was 58 +/- 6% in six cats (control group) that had received no fentanyl prior to propofol (1 mg/kg) administration. The uptake was significantly reduced to 32 +/- 3% in animals pretreated with fentanyl (1 microgram/kg) 30 seconds before propofol administration (n = 6). However, when fentanyl was administered 3 minutes (n = 6) or 10 minutes (n = 6) prior to propofol, the pulmonary uptake of propofol (45 +/- 5%, 50 +/- 7% respectively) was not significantly reduced. The results demonstrate that the ability of fentanyl to inhibit pulmonary removal of propofol depends on its time of administration prior to propofol. These data may have clinical implication with respect to timing of the preinduction opiate injection.

Pulmonary uptake of propofol in cats. Effect of fentanyl and halothane.

BACKGROUND: Many drugs are removed by the lung. The pulmonary uptake of one drug can be inhibited when a second, highly accumulated drug is administered parenterally or as a chronic oral treatment. The effect of inhalational anesthetics on pulmonary drug uptake has not been extensively studied and may alter pharmacokinetics of intravenously administered drugs. METHODS: The uptake of propofol by the lung during a single passage through the pulmonary circulation was studied in four groups of anesthetized cats: spontaneously breathing cats (control group, n = 6), cats whose lungs were mechanically ventilated (n = 6), and cats whose lungs were mechanically ventilated and that were anesthetized with 1% (n = 6) or 1.5% (n = 6) halothane. In an additional group, the single-pass pulmonary uptake of propofol was studied in six spontaneously breathing cats pretreated with fentanyl. The amount of propofol taken up by the lung during the first pass was measured from double indicator-dilution outflow curves using indocyanine green (ICG) as the intravascular reference indicator. RESULTS: The first-pass uptake of propofol (mean +/- SEM) was 61.3 +/- 4.9% and 60 +/- 3.7% of the injected dose in control cats and in cats whose lungs were mechanically ventilated, respectively. Although exposure of the animals to 1% halothane had no significant effect on pulmonary extraction of propofol, the first-pass uptake decreased significantly to 38.8 +/- 6.9% in cats exposed to 1.5% halothane compared with control cats and to cats undergoing mechanical ventilation of the lungs without exposure to halothane. Also, in animals pretreated with fentanyl, propofol uptake was reduced to 40 +/- 5% compared with the control group. CONCLUSIONS: The results demonstrate a substantial extraction of propofol by the lung that is not affected by mechanical ventilation. Inhibition of propofol uptake by 1.5% halothane and by fentanyl provides a potential mechanism of drug-drug interaction that may interfere with the pharmacokinetic profile of propofol, and may alter the amount of propofol needed to achieve or supplement a given depth of anesthesia.

Tone-dependent responses of 5-hydroxytryptamine in the feline pulmonary vascular bed are mediated by two different 5-hydroxytryptamine receptors.

Mechanisms to explain tone-dependent responses of the feline pulmonary vascular (PV) bed to 5-hydroxytryptamine (5-HT) were investigated in intact-chest, spontaneously breathing cats under conditions of controlled pulmonary blood flow and constant left atrial pressure. At low (resting) PV tone, intralobar injections of 5-HT produced dose-dependent vasoconstrictor (VC) responses which were significantly blocked by the selective 5-HT2 receptor antagonist ketanserin and enhanced by the cyclooxygenase inhibitor meclofenamate. When PV tone was increased with 9,11-dideoxy-9 alpha 11 alpha epoxymethano prostaglandin F2 alpha, intralobar injections of 5-HT produced vasodilator (VD) responses at low doses, biphasic VC/VD responses at midrange doses and predominant VC responses at high doses. The VC responses at elevated PV tone were dose dependent and antagonized by ketanserin. The VD responses were antagonized by the mixed 5-HT1, 5-HT2 receptor antagonist methysergide. Meclofenamate had no effect on VC or VD responses of 5-HT at elevated PV tone. At both low (resting) and elevated PV tone, 5-HT and the selective 5-HT2 receptor agonist alpha-methyl-5-HT produced greater VC responses than the selective 5-HT1 receptor agonist 5-carboxyaminotryptamine and the selective 5-HT3 receptor agonist 2-methyl-5-HT. At elevated PV tone, 5-carboxyaminotryptamine and 5-HT produced greater VD responses than alpha-me-5-HT and 2-methyl-5-HT. Compared to low (resting) PV tone, VC responses of 5-HT and alpha-methyl-5-HT were enhanced at elevated PV tone. These data support that 5-HT-induced VC responses at both low (resting) and elevated PV tone are mediated by 5-HT2 receptors and 5-HT-induced VD responses at elevated PV tone are mediated by "5-HT1-like" receptors. A change in PV tone may alter receptor availability, affinity or receptor-effector coupling.

Adenosine and ATP produce vasoconstriction in the feline pulmonary vascular bed by different mechanisms.

It has been reported recently that adenosine and ATP produce dose- and tone-dependent responses in the feline pulmonary vascular (PV) bed. The present study was undertaken to investigate the mechanisms mediating vasoconstrictor (VC) responses to adenosine and ATP in the intact-chest, spontaneously breathing cat under conditions of controlled blood flow and constant left atrial pressure. The order of potency of adenosine receptor agonists to produce VC in the PV bed was the selective adenosine A1 receptor agonist R-phenylisopropyladenosine greater than the mixed A1, A2 receptor agonist, adenosine greater than the selective adenosine A2 receptor agonist, 2-phenylaminoadenosine. The dose-related increase in lobar arterial pressure in response to adenosine was blocked by an adenosine (P1) receptor antagonist, BWA1433U, the cyclooxygenase inhibitor, meclofenamate, and the thromboxane A2 receptor antagonist, SQ29548. The order of potency of ATP analogs to produce VC in the PV bed was alpha,beta-methylene ATP (alpha,beta-meATP) much greater than beta,tau-methylene ATP greater than ATP. BWA1433U inhibited VC responses to ATP without affecting responses to its degradation-resistant analogs beta,tau-methylene ATP and alpha,beta-meATP. In the presence of BWA1433U and a continuous intralobar infusion of the selective 5'-nucleotidase inhibitor, alpha,beta-methyleneadenosine-5'-diphosphate, ATP VC responses are significantly enhanced compared to those after BWA1433U. alpha,beta-Methyleneadenosine-5'-diphosphate had no effect on the VC response to U44069 after BWA1433U. Meclofenamate significantly inhibited the vasoconstrictor responses to ATP but not to alpha,beta-meATP.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine does not mediate the pulmonary vasodilator response of adenosine 5'-triphosphate in the feline pulmonary vascular bed.

Adenosine and ATP produce dose- and tone-dependent responses in the feline pulmonary vascular bed. That is, at baseline (low) pulmonary vascular tone adenosine and ATP produce vasoconstrictor responses and at elevated pulmonary vascular tone adenosine and ATP produce vasodilator responses. The mechanism mediating the vasodilator responses to adenosine and ATP was investigated in the intact-chest cat under conditions of controlled pulmonary blood flow and left atrial pressure. When lobar vascular resistance was raised with U46619, intralobar injections of adenosine and ATP caused dose-related decreases in lobar arterial pressure. The pulmonary vasodilator responses to ATP and adenosine were not altered by atropine, propranolol, meclofenamate or cimetidine indicating that these responses were not mediated by the release of vasodilator prostaglandins or to activation of beta adrenergic, muscarinic or histamine (H2) receptors. The decreases in lobar arterial pressure in response to adenosine were reduced significantly by BWA1433U, an adenosine (P1) receptor antagonist. BWA1433U induced a parallel shift of the adenosine dose-response curve to the right; however, it had no significant inhibitory effect on the decrease in lobar arterial pressure in response to ATP. The P1 receptor antagonist in doses of 10 and 30 mg/kg i.v. had no significant effect on the vasodilator response to nitroglycerin. The present data suggest that vasodilator responses to adenosine in the feline pulmonary vascular bed are mediated by adenosine (P1) receptors, whereas responses to ATP are mediated by a different mechanism that does not involve release of a vasodilator prostaglandin.

Methylprednisolone on circulating eicosanoids and vasomotor tone after endotoxin.

Acute pulmonary and systemic vasomotor changes induced by endotoxin in dogs have been related, at least in part, to the production of eicosanoids such as the vasoconstrictor thromboxane and the vasodilator prostacyclin. Steroids in high doses, in vitro, inhibit activation of phospholipase A2 and prevent fatty acid release from cell membranes to enter the arachidonic acid cascade. We, therefore, administered methylprednisolone (40 mg/kg) to dogs to see if eicosanoid production and the ensuing vasomotor changes could be prevented after administration of 150 micrograms/kg of endotoxin. The stable metabolites of thromboxane B2 (TxB2) and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) were measured by radioimmunoassay. Methylprednisolone by itself did not alter circulating eicosanoids but when given 2.5 h before endotoxin not only failed to inhibit endotoxin-induced eicosanoid production but actually resulted in higher circulating levels of 6-keto-PGF1 alpha (P less than 0.05) compared with animals receiving endotoxin alone. Indomethacin prevented the steroid-enhanced concentrations of 6-keto-PGF1 alpha after endotoxin and prevented the greater fall (P less than 0.05) in systemic blood pressure and systemic vascular resistance with steroid plus endotoxin than occurred with endotoxin alone. Administration of methylprednisolone immediately before endotoxin resulted in enhanced levels (P less than 0.05) of both TxB2 and 6-keto-PGF1 alpha but with a fall in systemic blood pressure and vascular resistance similar to the animals pretreated by 2.5 h. In contrast to the early steroid group in which all of the hypotensive effect was due to eicosanoids, in the latter group steroids had an additional nonspecific effect. Thus, in vivo, high-dose steroids did not prevent endotoxin-induced increases in eicosanoids but actually increased circulating levels of TxB2 and 6-keto-PGF1 alpha with a physiological effect favoring vasodilation.