Pulmonary vasodilator activity of prostacyclin (PGI2) in the cat

Nebivolol has a beneficial effect in monocrotaline-induced pulmonary hypertension

Pulmonary hypertension is a rare disorder that, without treatment, is progressive and fatal within 3–4 years. Current treatment involves a diverse group of drugs that target the pulmonary vascular bed. In addition, strategies that increase nitric oxide (NO) formation have a beneficial effect in rodents and patients. Nebivolol, a selective β1 adrenergic receptor-blocking agent reported to increase NO production and stimulate β3 receptors, has vasodilator properties suggesting that it may be beneficial in the treatment of pulmonary hypertension. The present study was undertaken to determine whether nebivolol has a beneficial effect in monocrotaline-induced (60 mg/kg) pulmonary hypertension in the rat. These results show that nebivolol treatment (10 mg/kg, once or twice daily) attenuates pulmonary hypertension, reduces right ventricular hypertrophy, and improves pulmonary artery remodeling in monocrotaline-induced pulmonary hypertension. This study demonstrates the presence of β3 adrenergic receptor immunoreactivity in pulmonary arteries and airways and that nebivolol has pulmonary vasodilator activity. Studies with β3 receptor agonists (mirabegron, BRL 37344) and antagonists suggest that β3 receptor-mediated decreases in systemic arterial pressure occur independent of NO release. Our results suggest that nebivolol, a selective vasodilating β1 receptor antagonist that stimulates β3 adrenergic receptors and induces vasodilation by increasing NO production, may be beneficial in treating pulmonary hypertensive disorders.

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.

Amblyomma americanum tick saliva insulin-like growth factor binding protein-related protein 1 binds insulin but not insulin-like growth factors

Silencing Amblyomma americanum insulin-like growth factor binding protein-related protein 1 (AamIGFBP-rP1) mRNA prevented ticks from feeding to repletion. In this study, we used recombinant (r)AamIGFBP-rP1 in a series of assays to obtain further insight into the role(s) of this protein in tick feeding regulation. Our results suggest that AamIGFBP-1 is an antigenic protein that is apparently exclusively expressed in salivary glands. We found that both males and females secrete AamIGFBP-rP1 into the host during feeding and confirmed that female ticks secrete this protein from within 24-48 h after attachment. Our data suggest that native AamIGFBP-rP1 is a functional insulin binding protein in that both yeast- and insect cell-expressed rAamIGFBP-rP1 bound insulin, but not insulin-like growth factors. When subjected to anti-blood clotting and platelet aggregation assays, rAamIGFBP-rP1 did not have any effect. Unlike human IGFBP-rP1, which is controlled by trypsinization, rAamIGFBP-rP1 is resistant to digestion, suggesting that the tick protein may not be under mammalian host control at the tick feeding site. The majority of tick-borne pathogens are transmitted 48 h after the tick has attached. Thus, the demonstrated antigenicity and secretion into the host within 24-48 h of the tick starting to feed makes AamIGFBP-rP1 an attractive target for antitick vaccine development.

Effect of chronic sodium nitrite therapy on monocrotaline-induced pulmonary hypertension

Pulmonary hypertension (PH) is a rare disorder that without treatment is progressive and often fatal within 3 years. The treatment of PH involves the use of a diverse group of drugs and lung transplantation. Although nitrite was once thought to be an inactive metabolite of endothelial-derived nitric oxide (NO), there is increasing evidence that nitrite may be useful in the treatment of PH, but the mechanism by which nitrite exerts its beneficial effect remains uncertain. The purpose of this study was to investigate the effect of chronic sodium nitrite treatment in a PH model in the rat. Following induction of PH with a single injection of monocrotaline, 60 mg; daily ip injections of sodium nitrite (3mg/kg) starting on day 14 and continuing for 21 days, resulted in a significantly lower pulmonary arterial pressure on day 35 when compared to values in untreated animals with monocrotaline-induced PH. In monocrotaline-treated rats, daily treatment with ip nitrite injections for 21 days decreased right ventricular mass and pathologic changes in small pulmonary arteries. Nitrite therapy did not change systemic arterial pressure or cardiac output when values were measured on day 35. The decreases in pulmonary arterial pressure in response to iv injections of sodium nitroprusside, sodium nitrite, and BAY 41-8543 were not different in rats with monocrotaline-induced pulmonary hypertension and rats with chronic nitrite therapy when compared to responses in animals in which pulmonary arterial pressure was increased with U46619. These findings are consistent with the hypothesis that the mechanisms that convert nitrite to vasoactive NO, activate soluble guanylyl cyclase and mediate the vasodilator response to NO or an NO derivative are not impaired. The present data are consistent with the results of a previous study in monocrotaline-induced PH in which systemic arterial pressure and cardiac output were not evaluated and are consistent with the hypothesis that nitrite is effective in the treatment of monocrotaline-induced PH in the rodent.

Targeting soluble guanylate cyclase for the treatment of pulmonary hypertension

Pulmonary arterial hypertension is a disease characterized by a sustained increase in pulmonary arterial pressure leading to right heart failure. Current treatments focus on endothelial dysfunction and an aberrant regulatory pathway for vascular tone. Unfortunately, a large proportion of patients are unresponsive to conventional vasodilator therapy. Investigations are ongoing into the effects of experimental therapies targeting the signal transduction pathway that mediates vasodilation. Here, we briefly discuss the pathophysiology of pulmonary hypertension and endothelial dysfunction, along with current treatments. We then present a focused review of recent animal studies and human trials examining the use of activators and stimulators of soluble guanylate cyclase for the treatment of pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension.

New approaches to the treatment of pulmonary hypertension: from bench to bedside

Pulmonary hypertension (PH) is a severe, life-threatening disease for which there are no effective curative therapies. A diverse group of agents such as prostacyclins, endothelin antagonists, phosphodiesterase inhibitors, calcium channel blockers, diuretics, inotropic agents, and anticoagulants are used to treat PH; however, none of these agents have a marked effect upon survival. Among the new agents that promise treatment of PH are rho-kinase inhibitors and soluble guanylate cyclase stimulators. Although these new classes of agents have beneficial effects in experimental animal models and clinical studies, they are not selective in their actions on the pulmonary vascular bed. This manuscript reviews the actions of rho-kinase inhibitors and soluble guanylate cyclase stimulators on the pulmonary vascular bed. It is our hypothesis that these new agents may be more effective than current therapies in the treatment of PH. Moreover, new methods in the delivery of these agents to the lung need to be developed so that their main effects will be exerted in the pulmonary vascular bed and their systemic effects can be minimized or avoided.

Analysis of responses to the Rho-kinase inhibitor Y-27632 in the pulmonary and systemic vascular bed of the rat

Responses to the Rho kinase inhibitor Y-27632 were investigated in the anesthetized rat. Under baseline conditions intravenous injections of Y-27632 decreased pulmonary and systemic arterial pressures and increased cardiac output. The decreases in pulmonary arterial pressures were enhanced when baseline tone was increased with U-46619, and under elevated tone conditions Y-27632 produced similar percent decreases in pulmonary and systemic arterial pressures. Injections of Y-27632 prevented and reversed the hypoxic pulmonary vasoconstrictor response. The increase in pulmonary arterial pressure in response to ventilation with a 10% O(2)-90% N(2) gas mixture was not well maintained during the period of hypoxic exposure. Treatment with the nitric oxide (NO) synthase (NOS) inhibitor nitro-l-arginine methyl ester (l-NAME) increased pulmonary arterial pressure and prevented the decline or fade in the hypoxic pulmonary vasoconstrictor response. The hypoxic pulmonary vasoconstrictor response was reversed by Y-27632 in control and in l-NAME-treated animals. The Rho kinase inhibitor attenuated increases in pulmonary arterial pressures in response to intravenous injections of serotonin, angiotensin II, and Bay K 8644. Y-27632, sodium nitrite, and BAY 41-8543, a guanylate cyclase stimulator, decreased pulmonary and systemic arterial pressures and vascular resistances in monocrotaline-treated rats. These data suggest that Rho kinase is involved in the regulation of baseline tone and in the mediation of pulmonary vasoconstrictor responses. The present data suggest that the hypoxic pulmonary vasoconstrictor response is modulated by the release of NO that mediates the nonsustained component of the response in the anesthetized rat. These data suggest that Rho kinase and NOS play important roles in the regulation of vasoconstrictor tone in physiological and pathophysiological states and that monocrotaline-induced pulmonary hypertension can be reversed by agents that inhibit Rho kinase, generate NO, or stimulate soluble guanylate cyclase.

Analysis of pulmonary vasodilator responses to SB-772077-B [4-(7-((3-amino-1-pyrrolidinyl)carbonyl)-1-ethyl-1H-imidazo(4,5-c)pyridin-2-yl)-1,2,5-oxadiazol-3-amine], a novel aminofurazan-based Rho kinase inhibitor

The effects of SB-772077-B [4-(7-((3-amino-1-pyrrolidinyl)carbonyl)-1-ethyl-1H-imidazo(4,5-c)pyridin-2-yl)-1,2,5-oxadiazol-3-amine], an aminofurazan-based Rho kinase inhibitor, on the pulmonary vascular bed and on monocrotaline-induced pulmonary hypertension were investigated in the rat. The intravenous injections of SB-772077-B decreased pulmonary and systemic arterial pressures and increased cardiac output. The decreases in pulmonary arterial pressure were enhanced when pulmonary vascular resistance was increased by U46619 [9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F(2alpha)], hypoxia, or N(omega)-nitro-L-arginine methyl ester. SB-772077-B was more potent than Y-27632 [trans-4-[(1R)-1-aminoethyl]-N-4-pyridinyl-cyclohexanecarboxamide dihydrochloride] or fasudil [5-(1,4-diazepane-1-sulfonyl)isoquinoline] in decreasing pulmonary and systemic arterial pressures. The results with SB-772077-B, fasudil, and Y-27632 suggest that Rho kinase is constitutively active and is involved in the regulation of baseline tone and vasoconstrictor responses. Chronic treatment with SB-772077-B attenuated the increase in pulmonary arterial pressure induced by monocrotaline. The intravenous injection of SB-772077-B decreased pulmonary and systemic arterial pressures in rats with monocrotaline-induced pulmonary hypertension. The decreases in pulmonary arterial pressure in response to SB-772077-B in monocrotaline-treated rats were smaller than responses in U46619-infused animals, and the analysis of responses suggests that approximately 60% of the pulmonary hypertensive response is mediated by a Rho kinase-sensitive mechanism. The observation that Rho kinase inhibitors decrease pulmonary arterial pressure when pulmonary vascular resistance is increased by interventions such as hypoxia, U46619, angiotensin II, nitric-oxide synthase inhibition, and Bay K 8644 [S-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3-pyridine carboxylic acid methyl ester] suggest that the vasodilatation is independent of the mechanisms used to increase intracellular calcium and promote vasoconstriction. The present results suggest that SB-772077-B would be beneficial in the treatment of pulmonary hypertensive disorders.

Rho kinase and Ca2+ entry mediate increased pulmonary and systemic vascular resistance in l-NAME-treated rats

The small GTP-binding protein and its downstream effector Rho kinase play an important role in the regulation of vasoconstrictor tone. Rho kinase activation maintains increased pulmonary vascular tone and mediates the vasoconstrictor response to nitric oxide (NO) synthesis inhibition in chronically hypoxic rats and in the ovine fetal lung. However, the role of Rho kinase in mediating pulmonary vasoconstriction after NO synthesis inhibition has not been examined in the intact rat. To address this question, cardiovascular responses to the Rho kinase inhibitor fasudil were studied at baseline and after administration of an NO synthesis inhibitor. In the intact rat, intravenous injections of fasudil cause dose-dependent decreases in systemic arterial pressure, small decreases in pulmonary arterial pressure, and increases in cardiac output. l-NAME caused a significant increase in pulmonary and systemic arterial pressures and a decrease in cardiac output. The intravenous injections of fasudil after l-NAME caused dose-dependent decreases in pulmonary and systemic arterial pressure and increases in cardiac output, and the percent decreases in pulmonary arterial pressure in response to the lower doses of fasudil were greater than decreases in systemic arterial pressure. The Ca++ entry blocker isradipine also decreased pulmonary and systemic arterial pressure in l-NAME-treated rats. Infusion of sodium nitroprusside restored pulmonary arterial pressure to baseline values after administration of l-NAME. These data provide evidence in support of the hypothesis that increases in pulmonary and systemic vascular resistance following l-NAME treatment are mediated by Rho kinase and Ca++ entry through L-type channels, and that responses to l-NAME can be reversed by an NO donor.

Pulmonary responses to selective phosphodiesterase-5 and phosphodiesterase-3 inhibitors

OBJECTIVE

To compare the direct pulmonary vasodilating activity and specificity of phosphodiesterase-5 (zaprinast) and phosphodiesterase-3 (milrinone) inhibitors on the pulmonary vascular (PV) bed of the spontaneously breathing cat with an intact chest.

DESIGN

Prospective, randomized animal study.

SETTING

Laboratory of university hospital.

SUBJECTS

Experiments were performed in vivo in intact-chest, spontaneously breathing cats with controlled pulmonary blood flow and constant left atrial pressure.

INTERVENTIONS

The responses to intralobar injections of zaprinast and milrinone were investigated at low PV tone. PV tone was then increased by intralobar arterial infusion of a thromboxane A(2) mimic, U46619. Animals received intralobar bolus injections of zaprinast or milrinone, followed by continuous IV infusion of the drug, which was administered in incremental doses titrated to produce a 20% reduction in mean systemic arterial pressure.

MEASUREMENTS AND MAIN RESULTS

At low PV tone, zaprinast, but not milrinone, decreased lobar arterial pressure (LoAP). At elevated PV tone, both drugs caused dose-dependent decreases in LoAP; however, milrinone caused significantly less pulmonary vasodilation. Dose-related decreases in mean systemic arterial pressure were observed with milrinone, but not with zaprinast. When the continuous IV infusion was titrated to produce a 20% reduction in mean systemic arterial pressure, the decreases in lobar arterial pressure with zaprinast infusion were significantly greater than those produced by milrinone.

CONCLUSIONS

These data show that zaprinast and milrinone exert a direct in vivo vasodilator effect on the PV bed at low (zaprinast) and elevated (zaprinast and milrinone) PV tone; however, at elevated PV tone, the pulmonary vasodilator effect was greater with zaprinast then with milrinone. This suggests that phosphodiesterase-5 inhibitors may potentially offer a therapeutic alternative in the management of acute pulmonary hypertension.

Influence of ACE-inhibition on salt-mediated worsening of pulmonary gas exchange in heart failure

AIMS

In congestive heart failure (CHF), pulmonary gas exchange, as evaluated by carbon monoxide diffusion (DLCO), is impaired. ACE-inhibition improves DLCO. Infusion of saline worsens DLCO, because of upregulated sodium and water transport to the alveolar interstitium, which thickens the alveolar-capillary interface and lengthens the diffusion path for gas exchange. We investigated whether enalapril can readjust the capillary permeability to sodium.

METHODS

In 10 NYHA class II-III CHF patients, we measured DLCO, its two subcomponents (VC, capillary blood volume available for gas exchange, and DM, alveolar-capillary membrane diffusion), left and right ventricular filling pressures, plasma noradrenaline, aldosterone and renin activity, at baseline and following saline infusion in the main pulmonary artery stem, before and after 1 week enalapril treatment (20 mg daily).

RESULTS

Saline (150 ml) significantly reduced DLCO (-9.1%) and DM (-9.8%) and augmented VC (+ 10.7%). Responses to 750 ml saline were somewhat greater and qualitatively similar. Enalapril produced a significant improvement of DLCO and DM at rest as well as after saline, that was not associated with variations in ventricular filling pressures, cardiac output and left ventricular ejection fraction, and was not accounted for by humoral changes.

CONCLUSIONS

In CHF, ACE-inhibition attenuates the deterioration of pulmonary gas transfer produced by saline infusion, suggesting an ability to readjust the upregulated sodium transport across the pulmonary microvascular endothelium.

Continuous intravenous epoprostenol for pulmonary hypertension due to the scleroderma spectrum of disease. A randomized, controlled trial

BACKGROUND

Pulmonary hypertension is a progressive and often fatal complication of the scleroderma spectrum of disease for which no treatment has been proven effective in a randomized trial.

OBJECTIVE

To determine the effect of epoprostenol on pulmonary hypertension secondary to the scleroderma spectrum of disease.

DESIGN

Randomized, open-label, controlled trial.

SETTING

17 pulmonary hypertension referral centers.

PATIENTS

111 patients with moderate to severe pulmonary hypertension.

INTERVENTION

Epoprostenol plus conventional therapy or conventional therapy alone.

MEASUREMENTS

The primary outcome measure was exercise capacity. Other measures were cardiopulmonary hemodynamics, signs and symptoms of pulmonary hypertension and scleroderma, and survival.

RESULTS

Exercise capacity improved with epoprostenol (median distance walked in 6 minutes, 316 m at 12 weeks compared with 270 m at baseline) but decreased with conventional therapy (192 m at 12 weeks compared with 240 m at baseline). The difference between treatment groups in the median distance walked at week 12 was 108 m (95% CI, 55.2 m to 180.0 m) (P < 0.001). Hemodynamics improved at 12 weeks with epoprostenol. The changes in mean pulmonary artery pressure for the epoprostenol and conventional therapy groups were -5.0 and 0.9 mm Hg, respectively (difference, -6.0 mm Hg [CI, -9.0 to -3.0 mm Hg), and the mean changes in pulmonary vascular resistance were -4.6 and 0.9 mm Hg/L per minute, respectively (difference, -5.5 mm Hg/L per minute [CI, -7.3 to -3.7 mm Hg/L per minute). Twenty-one patients treated with epoprostenol and no patients receiving conventional therapy showed improved New York Heart Association functional class. Borg Dyspnea Scores and Dyspnea-Fatigue Ratings improved in the epoprostenol group. Trends toward greater improvement in severity of the Raynaud phenomenon and fewer new digital ulcers were seen in the epoprostenol group. Four patients in the epoprostenol group and five in the conventional therapy group died (P value not significant). Side effects of epoprostenol therapy included jaw pain, nausea, and anorexia. Adverse events related to the epoprostenol delivery system included sepsis, cellulitis, hemorrhage, and pneumothorax (4% incidence for each condition).

CONCLUSIONS

Continuous epoprostenol therapy improves exercise capacity and cardiopulmonary hemodynamics in patients with pulmonary hypertension due to the scleroderma spectrum of disease.

Aspirin worsens exercise performance and pulmonary gas exchange in patients with heart failure who are taking angiotensin-converting enzyme inhibitors

BACKGROUND

Pulmonary function abnormalities participate in causing exercise disability in patients with congestive heart failure (CHF). Impaired pulmonary gas transfer is one of these abnormalities. Angiotensin-converting enzyme (ACE) inhibitors improve diffusion for carbon monoxide and exercise capacity, an effect that is seemingly mediated through prostaglandin activation because it is inhibited by cyclooxygenase blockade with aspirin. This suggests the possibility that aspirin may disturb the pulmonary function and exercise ability in CHF, at least in those patients who are taking ACE inhibitors. This study was aimed at probing this hypothesis.

METHODS

A dose of 325 mg aspirin was given daily for 8 weeks to 26 consecutive patients with primary dilated cardiomyopathy (New York Heart Association class II or III) whose current outpatient antifailure therapy included (group 1, 18 cases) or did not include (group 2, 8 cases) an ACE inhibitor in addition to digoxin and furosemide. During the study ACE inhibition was continued in group 1 by giving enalapril 20 mg daily.

RESULTS

Tests repeated at 8 weeks proved that aspirin was deleterious in group 1. Compared with run-in, rest carbon dioxide, peak exercise oxygen uptake (peak VO(2 )), and tidal volume levels were diminished in this group; the ratio of exercise minute ventilation to carbon dioxide production (VE/VCO(2)) was augmented and its variations were inversely related to those of peak VO(2). Similar results were not observed in group 2; however, once this part of the study was completed and enalapril was included in the current therapeutic regimen, an inhibitory effect of aspirin on carbon dioxide, peak VO(2), peak tidal volume, and VE/VCO(2) at 1 L levels became evident and was similar to that observed in group 1.

CONCLUSIONS

Aspirin does not affect ventilation efficiency and peak VO(2 ) in patients with CHF not taking ACE inhibitors, but it worsens the pulmonary diffusion for carbon monoxide, VO(2 ), and the ventilatory response to exercise in the presence of ACE inhibition. This may be relevant in patients with CHF from ischemic heart disease. Whether the same may be true of smaller aspirin doses was not investigated in this study.

Effects of anaphylaxis mediators on partitioned pulmonary vascular resistance during ragweed shock in dogs

We examined the effect of anaphylactic shock on the longitudinal distribution of pulmonary vascular resistance (PVR) in ragweed-sensitized dogs in which PVR was partitioned into an upstream arterial component (Rus) and a downstream venous and capillary component (Rds). We also assessed whether Rus and Rds would be reduced by pretreatment with histamine H1- and H2-receptor blocking agents and with cyclooxygenase and lipoxygenase pathway inhibitors. Anesthetized animals were examined on separate occasions 3 wk apart in which one of the treatments was randomly given. The pulmonary arterial occlusion technique was used to determine segmental pressure drops. During ragweed challenge, PVR increased approximately 4 times compared with the preshock value (3.04 vs. 12. 07 mmHg . l-1 . min; P < 0.05). Although both Rus and Rds increased postshock, the greatest relative increase occurred in Rds. None of the treatments reduced partitioned resistances compared with no treatment. Our results show that, under conditions of anaphylactic shock, increases in Rus and Rds could not be ascribed to release of histamine or products of the cyclooxygenase and lipoxygenase pathways.

Clinical trials in pulmonary hypertension

Progress in treatment of pulmonary hypertension has been impaired by the lack of formal clinical trials. This is now beginning to change, and the impact on our approach to treating patients with pulmonary hypertension in substantial. As with other relatively uncommon medical disorders, randomized, controlled, multi-center trials are needed to assess the safety and efficacy of potential therapeutic modalities. Treatments showing promise at the level of small pilot studies within a single center should be studied more rigorously.

Pulmonary vasoconstrictor effects of prostacyclin in rats: potential role of thromboxane receptors

Endogenous prostacyclin (PGI2; epoprostenol) is a potent endothelium-derived pulmonary vasodilator. However, the effects of exogenous PGI2 on isolated arteries could be either relaxant or contractile, depending on the species and organ studied. The present study investigated the distal pathways involved in the PGI2-induced contraction in rat intrapulmonary artery (PA) and relaxation in lamb PA. When vessels were precontracted with 30 mM K+, PGI2 (1 microM) induced relaxation in lamb PA but caused contraction in rat PA. Use of 30 mM K+, phenylephrine, serotonin, angiotensin II, or hypoxia to precontract the vessels did not alter the contractile effect of PGI2 in rat PA. Nevertheless, PGI2 produced a mild relaxation in rat PA precontracted by U-46619, a thromboxane A2 (TxA2)-receptor agonist, whereas the TxA2-receptor blocker SQ-29548 (0.1-0.5 microM) abolished the contractile response in rat PA. These data suggest that PGI2-induced contraction is mediated by activation of TxA2 receptors. The PGI2-induced modest relaxation in rat PA, which was only observed when TxA2 receptors were blocked by SQ-29548, suggests that the PGI2-mediated vasorelaxant pathway is diminished in these vessels. Simultaneous application of forskolin, an adenylate cyclase activator, and rolipram, a phosphodiesterase inhibitor, caused similar relaxation in both rat and lamb PA. This suggests that the adenosine 3',5'-cyclic monophosphate-dependent relaxing pathway is intact in rat PA and is comparable to that in lamb PA. On the basis of these data, we conclude that the pathways responsible for the paradoxical effects of PGI2 on rat and lamb PA are located upstream of the adenosine 3',5'-cyclic monophosphate-dependent relaxing pathway and that a paucity of PGI2 receptors in rat PA may be responsible.

The effects of epoprostenol on drug disposition. I: A pilot study of the pharmacokinetics of digoxin with and without epoprostenol in patients with congestive heart failure

The influence of epoprostenol on the pharmacokinetics of drugs administered concurrently to patients with congestive heart failure (CHF) receiving epoprostenol was evaluated as a secondary objective of a Phase II pilot study. A total of 278 blood samples were collected from 30 patients with end-stage CHF receiving conventional therapy alone or conventional therapy plus epoprostenol. Estimates of oral clearance (Cl), volume of distribution, and absorption rate constant of digoxin were generated from plasma digoxin concentrations using nonlinear mixed effects modeling, and the effect of epoprostenol on Cl of digoxin was evaluated by univariate analysis. Additional factors that were evaluated by univariate analysis included age, obesity, time since study entry, cardiac output, concomitant use of angiotensin-converting enzyme (ACE) inhibitor, concomitant dobutamine, and estimated creatinine clearance. Backward elimination was used to arrive at a final model that included concomitant epoprostenol as a covariate. The final model revealed an approximate 15% decrease in Cl of digoxin in response to short-term administration of epoprostenol that was no longer apparent by the end of the 12-week treatment phase. Simulations revealed that this effect, although statistically significant, would not be clinically significant in most patients; however, the potential exists for short-term elevation of digoxin concentrations in response to concurrent administration of epoprostenol.

The effects of epoprostenol on drug disposition. II: A pilot study of the pharmacokinetics of furosemide with and without epoprostenol in patients with congestive heart failure

The effect of epoprostenol on the pharmacokinetics of furosemide was investigated in 23 patients with end-stage congestive heart failure (CHF) receiving conventional therapy alone or conventional therapy plus epoprostenol. Estimates of the apparent oral clearance, volume of distribution, and absorption rate constant for furosemide were generated from 198 serum furosemide concentrations using nonlinear mixed effects modeling (NONMEM). Univariate analyses were performed to assess the effects of patient factors on the apparent oral clearance of furosemide. The final multivariate model determined by backwards elimination included concomitant digoxin therapy. When concomitant epoprostenol therapy was included in the final model, there was a 13% decrease in the apparent oral clearance of furosemide in response to short-term administration of epoprostenol. However, the effect of concomitant epoprostenol therapy was not statistically significant and was no longer apparent by the end of the 12-week study. These data suggest that epoprostenol may have a slight short-term effect on the pharmacokinetics of furosemide; the interaction between epoprostenol and furosemide is not clinically significant, however.

A comparison of continuous intravenous epoprostenol (prostacyclin) with conventional therapy for primary pulmonary hypertension

BACKGROUND

Primary pulmonary hypertension is a progressive disease for which no treatment has been shown in a prospective, randomized trial to improve survival.

METHODS

We conducted a 12-week prospective, randomized, multicenter open trial comparing the effects of the continuous intravenous infusion of epoprostenol (formerly called prostacyclin) plus conventional therapy with those of conventional therapy alone in 81 patients with severe primary pulmonary hypertension (New York Heart Association functional class III or IV).

RESULTS

Exercise capacity was improved in the 41 patients treated with epoprostenol (median distance walked in six minutes, 362 m at 12 weeks vs. 315 m at base line), but it decreased in the 40 patients treated with conventional therapy alone (204 m at 12 weeks vs. 270 m at base line; P < 0.002 for the comparison of the treatment groups). Indexes of the quality of life were improved only in the epoprostenol group (P < 0.01). Hemodynamics improved at 12 weeks in the epoprostenol-treated patients. The changes in mean pulmonary-artery pressure for the epoprostenol and control groups were -8 percent and +3 percent, respectively (difference in mean change, -6.7 mm Hg; 95 percent confidence interval, -10.7 to -2.6 mm Hg; P < 0.002), and the mean changes in pulmonary vascular resistance for the epoprostenol and control groups were -21 percent and +9 percent, respectively (difference in mean change, -4.9 mm Hg/liter/min; 95 percent confidence interval, -7.6 to -2.3 mm Hg/liter/min; P < 0.001). Eight patients died during the study, all of whom had been randomly assigned to conventional therapy (P = 0.003). Serious complications included four episodes of catheter-related sepsis and one thrombotic event.

CONCLUSIONS

As compared with conventional therapy, the continuous intravenous infusion of epoprostenol produced symptomatic and hemodynamic improvement, as well as improved survival in patients with severe primary pulmonary hypertension.

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.

Oxygen radicals stimulate thromboxane and prostacyclin synthesis and induce vasoconstriction in pig lungs

Reactive oxygen species have earlier been shown to induce vasoactive changes. In the present investigation we hypothesized that active oxygen intermediates would stimulate arachidonic acid metabolism and thereby influence the pulmonary circulation. Four groups of 8-week old pigs were studied after infusion of an oxygen radical generator. Haemodynamic changes were recorded, and thromboxane (TX)B2 (the stable metabolite of TXA2) and 6-keto-prostaglandin (PG)F1 alpha (the stable metabolite of prostacyclin, PGI2) measured by a radioimmunoassay technique after infusion of xanthine oxidase (XO) alone or in combination with pharmacological inhibitors. In the XO group pulmonary vascular resistance increased rapidly compared to baseline levels. Maximum resistance increase was 118.4 +/- 27.5%, 25 min after the XO infusion (p < 0.05 compared to baseline). The vasoconstriction was significantly attenuated after pretreatment with the cyclo-oxygenase inhibitor indomethacin. In this group the pulmonary resistance increase was 21.2 +/- 24.3% at 25 min (p < 0.01 vs. XO group). In a group given allopurinol (xanthine oxidase inhibitor), the resistance increased by 44.3 +/- 28.8% (p < 0.02 vs. XO group), and during catalase infusion (hydrogen peroxide scavenger), the increase was 52.9 +/- 24.2% (p < 0.01 vs. XO group). Along with the pulmonary vascular pressure augmentation, we measured 1.9 fold TXB2 and 2.2 fold 6-keto-PGF1 alpha concentration increases in the XO group. However, both TXB2 and 6-keto-PGF1 alpha formation was significantly inhibited by indomethacin (p < 0.01 respectively vs. XO group), allopurinol (p < 0.01 and p < 0.05 respectively vs. XO group) and catalase (p < 0.01 and p < 0.02 respectively vs. XO group).(ABSTRACT TRUNCATED AT 250 WORDS)

The transitional circulation: physiology and anesthetic implications

Over the past decade, several aspects of the physiology of the transitional circulation have been elucidated. The transitional circulation may be viewed as a process divided into four phases. The concept of these phases underscores the fact that the normal transitional circulation should be viewed as an orderly process and not a single event. Within these phases, several mechanisms seem to be involved in the control of pulmonary vascular resistance (PVR). Yet even these mechanisms do not completely explain the process of the normal transition, and very little is understood about why the transition occasionally fails. No doubt other as yet undescribed mechanisms also play a role. Much work remains to be done in the study of the normal and abnormal transitional circulation. The profound hypoxia that characterizes infants with failed transitional circulation from any cause is due to a persistently high PVR, causing right-to-left shunting at the ductal and foramental levels. Clinical care of these infants is based on efforts to simultaneously decrease PVR and increase systemic vascular resistance (SVR). Appropriate measures include the use of supplemental oxygen, hyperventilation, alkalinization, and sedation to decrease PVR and intravenous (IV) fluids and pressors to increase SVR. The rapidly fluctuating nature of the physiologic processes that cause failure of the transitional circulation must be kept in mind when caring for these infants, both during initial stabilization in the delivery room and while administering anesthesia for surgical repair of congenital defects.

Influence of OKY-1581 on responses to arachidonic acid in the feline pulmonary vascular bed

The effects of OKY-1581, a thromboxane synthesis inhibitor, on pulmonary vascular responses to arachidonic acid (AA) were investigated under baseline and elevated tone conditions in the intact chest cat. Under conditions of controlled blood flow at baseline tone, intralobar injections of AA increased lobar arterial pressure in a dose-related manner. These pressor responses were reduced by OKY-1581, and a small vasodilator response was unmasked. The administration of indomethacin to these same animals abolished all responses to AA. When baseline tone in the pulmonary vascular bed was elevated by infusion of U46619, intralobar injections of AA caused a biphasic change in lobar arterial pressure characterized by an initial increase followed by a secondary fall in pressure. Treatment with OKY-1581 attenuated the pressor component of the response and enhanced the depressor component of the response. All responses to AA at elevated tone were also blocked by indomethacin. Pressor responses to intralobar injections of U46619 were not altered by OKY-1581 or indomethacin and were similar under baseline and high pulmonary vascular tone conditions. The results of this study suggest that the pulmonary pressor response to AA in the cat is dependent in large part on the formation of TXA2 and also suggest that TXA2, PGI2, and vasoconstrictor prostaglandins (PGF2 alpha, PGD2, PGE2) are formed from AA in the cat lung.

Pathology of the lung in near-drowning

In order to study the initial pathological changes that occur in drowning, the authors developed an experimental model that closely simulates the actual changes in the nearly drowned patient. Adult male rabbits were anesthetized and intubated, and 6 ml/kg of fresh or salt water was instilled directly into the endotracheal tube. The animals were killed after 29 minutes, and the heart and lungs were then examined microscopically. The authors found that in the first 30 minutes, the brunt of the damage is borne by the vascular endothelium and not the alveolar cells.

The use of oxygen and prostacyclin as pulmonary vasodilators in congenital heart disease

We have obtained dose-response curves for the effects of prostacyclin on the pulmonary and systemic circulations of 10 patients with pulmonary hypertension secondary to congenital heart disease. With the subjects breathing air, prostacyclin caused statistically significant, pulmonary and systemic vasodilation. When the patients breathed 100% oxygen, pulmonary blood flow rose and pulmonary vascular resistance fell with no significant change in pulmonary artery pressure. Prostacyclin had a small additional vasodilator effect, but this did not reach statistical significance. Prostacyclin may be of some use in the assessment of the reversibility of an elevated pulmonary vascular resistance when surgery for the underlying congenital heart defect is being contemplated.

Species dependent relaxation of intrapulmonary arteries (IPA) of rabbits, dogs and humans by prostacyclin

Helically cut strips of successive IPA segments of rabbits, dogs and human patients were set up for isometric recording in vitro. High tone was produced by norepinephrine (NE, 3 microM). This tone was markedly reduced by prostacyclin (PGI2) in the secondary, tertiary and quaternary branches of human and canine pulmonary trunk. The IC50 values for PGI2 ranged from 22 to 503 nM, the human vessels being more sensitive to prostacyclin than canine IPA. Under these conditions, the primary and secondary branches of the rabbit pulmonary trunk were not relaxed by PGI2. The contractile potency of NE was determined in each pulmonary vessel studied. The secondary segments of rabbit IPA were about ten times as sensitive to NE (EC50 for NE: 38 +/- 7 nM) as compared to the secondary IPA from dogs and humans (EC50 values: 370 +/- 84 and 440 +/- 50, respectively). When high tone was induced by equieffective contractile concentrations of NE (3 microM for canine and human IPA and 0.3 microM for rabbit vessels), PGI2 was still less effective (P less than 0.01) in relaxing secondary IPA of rabbits (IC25: 220 +/- 55) than the corresponding segments of dogs and humans (IC25: 51 +/- 12 and 17 +/- 4, respectively). The difference between canine and human vessels was also significant (P less than 0.02). These results indicate that there is an interspecies difference in the sensitivity of IPA to NE and PGI2.

Nafazatrom (Bay g 6575) blunts canine hypoxic pulmonary vasoconstriction: evidence for a prostaglandin-mediated mechanism

The eicosanoids are circulating vasoactive substances which may serve as modulators of pulmonary vasomotor tone. Nafazatrom (Bay g 6575) has been reported to raise circulating levels of prostaglandin I2 (PGI2) either by stimulating its synthesis or inhibiting its metabolism, and may also decrease leukotriene synthesis by inhibiting lipoxygenase. We evaluated the hemodynamic effects of nafazatrom administered intravenously in doses of 2,5, and 10 mg/kg in intact, anesthetized dogs ventilated with 21% and 10% oxygen. The hypoxia-induced increase in pulmonary vascular resistance was blunted by nafazatrom in an inverse dose-dependent fashion. Pretreatment with the cyclooxygenase inhibitor indomethacin (5 mg/kg subcutaneously twice daily for two days) abolished the response to low-dose nafazatrom. The pulmonary vasodilator response to the infusion of arachidonic acid (100 micrograms/kg/min) during hypoxic ventilation was blocked by the 10 mg/kg dose of nafazatrom. We conclude that nafazatrom blunts hypoxic pulmonary vasoconstriction in a manner consistent with a drug-induced increase in PGI2 activity, and that the inverse dose-dependent effects of the drug which we observed are due to an inhibition of cyclooxygenase at higher doses.

Influence of cyclooxygenase blockade on responses to isoproterenol, bradykinin and nitroglycerin in the feline pulmonary vascular bed

We investigated the effect of indomethacin on responses to isoproterenol, bradykinin and nitroglycerin in the feline pulmonary vascular bed when pulmonary vascular resistance was actively increased by infusion of U46619 in order to determine if vasodilator responses to these agents were dependent on the integrity of the cyclooxygenase pathway. Since pulmonary blood flow and left atrial pressure were held constant, changes in lobar arterial pressure directly reflect changes in lobar vascular resistance. Intralobar injections of isoproterenol, bradykinin, and nitroglycerin decreased lobar arterial pressure in a dose-related manner. Pulmonary vasodilator responses to the lower and midrange doses of bradykinin and nitroglycerin were unchanged in the presence of indomethacin whereas pulmonary responses to the highest doses of nitroglycerin and bradykinin were increased by cyclooxygenase blockade. In contrast, pulmonary vasodilator responses to isoproterenol were significantly attenuated in the presence of propranolol, whereas pulmonary vasodilator responses to bradykinin and nitroglycerin were unchanged after beta blockade. The present data indicate that isoproterenol, bradykinin, and nitroglycerin have significant vasodilator activity in the cat when pulmonary vascular tone is actively increased. These data suggest that the formation of vasodilator cyclooxygenase products such as PGI2 do not mediate vasodilator responses to isoproterenol, bradykinin, and nitroglycerin in the feline pulmonary vascular bed.

Nitroglycerin therapy in the management of pulmonary hypertensive disorders

Vasodilator therapy has not been effective in patients with pulmonary hypertension because most of the drugs that have been utilized in treating this disorder do not exert selective effects on the pulmonary circulation. Nonselective agents may cause predominant systemic vasodilation and lead to severe hypotension; they may elicit reflex activation of the sympathetic nervous system and further elevate pulmonary artery pressures; or they may exert depressant effects on right ventricular function and aggravate right-sided heart failure. Nitroglycerin has theoretic appeal as a vasodilator drug in patients with pulmonary hypertension because it exerts a direct effect on the pulmonary circulation in doses that do not affect systemic resistance vessels or the myocardium and do not activate neurohumoral reflexes. Furthermore, the drug uniquely reduces pulmonary artery pressures in addition to pulmonary vascular resistance due to its ability to dilate venous capacitance vessels. Preliminary studies with sublingual and intravenous nitroglycerin in patients with pulmonary hypertension have shown that the drug produces marked hemodynamic improvement and that clinical benefits follow long-term therapy with transcutaneous or oral nitrates. However, treatment may provoke hypotensive events in some patients and systemic hypoxemia in others; still others may fail to benefit because the pulmonary vasculature is unresponsive to any vasodilator stimulus. Further work is needed to define the benefits and risks of nitroglycerin therapy in patients with pulmonary hypertension.

Prostaglandins and thromboxane in the delayed phase of shock induced by Serratia marcescens endotoxin

The cardiovascular and metabolic effects of an endotoxin derived from Serratia marcescens were examined in anaesthetized, spontaneously-breathing cats. There was a marked initial elevation of right atrial pressure (the result of pulmonary vasoconstriction) and decreases in systemic arterial pressure and in arterial PO2. The 'delayed' effects of endotoxin shock in this species (1-8 h) consisted of a reduced cardiac output and decreased urinary excretion. Blood pressure and myocardial contractility (assessed from measurement of left ventricular (LV) dP/dt and LV end-diastolic pressure) were maintained throughout this phase. There was evidence of a metabolic (lactic) acidosis largely compensated by hyperventilation. Plasma levels (both arterial and mixed venous blood samples) of prostaglandin (PG)E2, PGF2 alpha, 6-keto PGF1 alpha and thromboxane (TX)B2 were measured by radioimmunoassay techniques. Endotoxin administration caused substantial increases in the plasma levels of all four derivatives of arachidonic acid, especially between 1 and 6 h. Separation of the endotoxin-treated cats into survivors and non-survivors showed that the non-survivors had significantly higher circulating levels of PGE2, TXB2 and PGF2 alpha. It is suggested that TXB2 and PGF2 alpha might contribute to some of the detrimental effects of endotoxin (e.g. pulmonary, mesenteric, renal vasoconstriction; platelet aggregation with resulting organ failure) and that prostacyclin may be beneficial in endotoxin shock in this species.

Nonadrenergic effects of isoproterenol in dogs with hypoxic pulmonary vasoconstriction. Possible role of prostaglandins

To determine whether the pulmonary vasodilation produced by isoproterenol is mediated solely by its beta adrenergic effects, we studied the hemodynamic responses to isoproterenol in three groups of dogs with pulmonary vasoconstriction produced by continuous ventilation with 10% oxygen: (a) hypoxia alone, (b) hypoxia and propranolol 0.3 mg/kg i.v. bolus followed by an infusion of 5 micrograms/kg per min, and (c) hypoxia after pretreatment with an inhibitor of cyclooxygenase, either indomethacin or meclofenamate 5 mg/kg s.c. twice daily for 2 d prior to study. All groups had similar values for mean pulmonary artery pressure (PAPm) and pulmonary vascular resistance (PVR) during room air and hypoxic ventilation. Isoproterenol in doses of 0.0025, 0.005, and 0.05 micrograms/kg per min produced a dose-related decline in PAPm and PVR during hypoxia in group 1. Despite beta-blockade with propranolol (group 2), isoproterenol at all three doses significantly reduced PAPm and PVR. The responses to isoproterenol were comparable in the presence or absence of propranolol; at 0.05 micrograms/kg per min the effects of isoproterenol were blunted, but not abolished, by propranolol. Similar results were observed even when five times the dose of propranolol was given. Isoproterenol at all three doses had no effect, however, on PAPm and PVR in the cyclooxygenase inhibitor-pretreated group. These data suggest that the pulmonary vasodilator effects of isoproterenol are not mediated solely by pulmonary vascular beta adrenergic receptors, and that vasodilator prostaglandins may play a role in the responses to this drug.

The effects of arachidonic acid, PGI2, and 6-keto-PGF1 alpha on cyclic nucleotide concentrations in a ventilated and artificially perfused isolated dog lung

In 35 isolated dog lung preparations, the pulmonary circulation of the right lung was completely separated from that of the left so that 1 lung of each preparation could serve as a control. The lungs were ventilated with 14% O2, 6% CO2 and 80% N2 and the pulmonary circulations were perfused with a dextran-salt-bicarbonate solution containing theophylline. Samples of perfusate were assayed for cyclic AMP and cyclic GMP (radioimmunoassay). Infusions of arachidonic acid (n=6) and PGI2 (n=4) but not 6-keto-PGF1 alpha (n=3) into the pulmonary circulation led to increases in cyclic AMP compared to control. Cyclic GMP levels were unchanged by the various infusions. Indomethacin (n=4) and acetylsalicylic acid (n=4) (prostaglandin (PG) synthesis inhibitors), and tranylcypromine (n=4) (a PGI2 synthetase inhibitor), prevented the cyclic AMP increases from arachidonic acid. This prevention was not the result of interference with the ability of cells to produce or release cyclic AMP since indomethacin (n=3), acetylsalicylic acid (n=3), and tranylcypromine (n=4) did not prevent cyclic AMP increases from PGI2. We conclude that infusion of arachidonic acid into the canine lung elevated pulmonary cyclic AMP but not cyclic GMP; that part or all of this increase most likely resulted from conversion of arachidonic acid to products of PG synthesis, particularly PGI2; that infusion of PGI2 mimicked arachidonic acid in that pulmonary cyclic AMP but not cyclic GMP was elevated, and that 6-keto-PGF1 alpha, a metabolite of PGI2, is unlikely to account for the cyclic AMP increases in this study.

Reversal of oleic acid-induced respiratory distress by prostacyclin (PGI2)

The effects of Prostacyclin (PGI2) on gas exchange were investigated in 13 adult mongrel dogs with acute pulmonary edema induced by oleic acid (0.09 ml/kg). The time course of changes in gas exchange was measured during spontaneous ventilation with room air. In the Control group, with oleic acid induced respiratory distress, there were marked decreases in arterial PO2 and PCO2, and marked increases in respiratory frequency, expired minute volume, alveolar (end tidal)-arterial PO2 difference, arterial-alveolar (end tidal) PCO2 difference and physiological dead space to tidal volume ratio. In the Test group, respiratory distress was also induced by the oleic acid injection, however, subsequent infusion of PGI2 (200 ng/kg/min) reversed the values of these parameters, which were significantly different from those in the Control group. The results suggest that PGI2 has a protective action against impairment of gas exchange in the presence of increased pulmonary vascular permeability.

Prostacyclin: a potent antimetastatic agent

Metastasis is the principal cause of failures to cure human cancers. Prostacyclin is a powerful antimetastatic agent against B16 amelanotic melanoma cells. This effect, which may result from the platelet antiaggregatory action of prostacyclin, is potentiated by a phosphodiesterase inhibitor. Inhibitors of prostacyclin synthesis increase metastasis. Prostacyclin and agents that may increase endogenous prostacyclin production or prolong its activity are suggested as new antimetastatic agents.

Metabolism of prostaglandin endoperoxide by microsomes from cat lung

It has been reported that the prostaglandin (PG) precursor, arachidonic acid, produces divergent hemodynamic responses in the feline pulmonary vascular bed. However, the pattern of arachidonic acid products formed in the lung of this species is unknown. In order to determine the type and activity of terminal enzymes in the lung, prostaglandin biosynthesis by microsomes from cat lung was studied using the prostaglandin endoperoxide, PGH2, as a substrate. The major products of incubations of PGH2 with microsomes were thromboxane (TX) B2 (the major metabolite of TXA2), 6-keto-PGF1 alpha (the breakdown product of PGI2) and 12L-hydroxy-5,8,10-heptadecatrienoic acid (HHT). Formation of TXB2 was markedly reduced by imidazole. Tranylcypromine decreased the formation of TXB2 and HHT and inhibited the formation of 6-keto-PGF1 alpha. At low PGH2 concentrations, equal production of TXB2 and 6-keto-PGF1 alpha was observed. However, as PGH2 concentration increased, 6-keto-PGF1 alpha production approached early saturation while TXB2 production increased in a linear fashion. These results suggest that enzymatic formation of TXA2 and PGI2 is a function of substrate availability in the lung. These findings provide a possible explanation for the divergent hemodynamic responses to arachidonic acid infusions at high and low concentrations in the feline pulmonary vascular bed.

Haemodynamic effects of prostacyclin (PGI2) in pulmonary hypertension

An infusion of prostacyclin (PGI2) in four patients with pulmonary hypertension resulted in a dose-related decrease in the systemic and pulmonary vascular resistance and an increase in cardiac output. No selectivity for the pulmonary vasculature was observed and the drug was not therapeutically beneficial.

Pulmonary vasodilator responses to nitroprusside and nitroglycerin in the dog

The objective of this study was to determine the direct actions of nitroprusside and nitroglycerin on the pulmonary vascular bed in the intactchest dog. These widely used nitrogen oxide-containing vasodilator agents decreased pulmonary arterial pressure and increased cardiac output without altering left atrial pressure. Reductions in pulmonary arterial pressure and pulmonary vascular resistance were small under resting conditions, but were enhanced when pulmonary vascular tone was elevated by infusion of a stable prostaglandin analog that increases pulmonary vascular resistance by constricting intrapulmonary veins and upstream segments. In studies in which pulmonary blood flow to the left lower lobe was maintained constant, nitroprusside and nitroglycerin caused small but significant reductions in lobar arterial and small-vein pressures without significantly affecting left atrial pressure. With constant blood flow, lobar vascular pressures that were reduced in response to the vasodilators were more greatly reduced when lobar vascular resistance was increased by infusion of the prostaglandin analog or serotonin. However, when lobar vascular pressures were elevated by passive obstruction of lobar venous outflow, vasodilator responses to nitroprusside and nitroglycerin were not enhanced. These data suggest that nitroprusside and nitroglycerin decrease pulmonary vascular resistance by dilating intrapulmonary veins and upstream segments. These responses were minimal under control conditions but were enhanced when vascular tone was increased. This vasodilator action is independent of passive factors such as changes in pulmonary blood flow or left atrial pressure and is not secondary to an effect of these agents on the systemic circulation. Pulmonary vasodilator responses to nitroprusside and nitroglycerin were, however, found to be dependent on the existing level of vasomotor tone in the pulmonary vascular bed.

6-keto-PGE1 exhibits more potent bronchodilatory activity in the cat than its precursor, PGI2

In anesthetized, vagotomized an mechanically ventilated cats, we investigated the bronchodilatory activity of the PGI2 metabolite, 6-keto-PGE1, relative to PGI2 and PGE2. In a range of doses from 0.3-10.0 microgram, i.v. injection of 6-keto-PGE1 produced a dose-related decrease in central airway resistance (RL) in animals bronchoconstricted by 5-HT. This effect on RL was 3-10 times greater than that produced by i.v. PGI2. At the lower doses, 6-keto-PGE1 was also more potent than PGI2 in increasing dynamic lung compliance; their effects upon semi-static compliance were not significantly different. Comparison of the bronchopulmonary effects of the two prostanoids did not show any consistent difference in their temporal patterns. In contrast to PGI2 or PGE2, 6-keto-PGE1 had minimal pulmonary vasomotor activity. Inhibition of the cyclooxygenase pathway with sodium meclofenamate had no effect on the bronchopulmonary actions of 6-keto-PGE1 or on its duration of action. These data indicate that 6-keto-PGE1 is a more potent bronchodilator than PGI2 in the cat. They further suggest that conversion of PGI2 to 6-keto-PGE1, if it occurs to an appreciable extent in the lung in vivo, could enhance bronchodilatory activity.

Influence of prostaglandin synthesis inhibitors on pulmonary vasodilatory effects of hydralazine in dogs with hypoxic pulmonary vasoconstriction

TO DETERMINE WHETHER HYDRALAZINE, A SYSTEMIC VASODILATOR, EXERTED A SIMILAR EFFECT ON THE PULMONARY CIRCULATION, WE STUDIED THE CIRCULATORY CHANGES IN DOGS DURING THREE INTERVENTIONS: (a) the control state during room air ventilation; (b) during continuous hypoxic ventilation with 10% oxygen, and maintaining continuous hypoxic ventilation; and (c) after 1 mg/kg hydralazine intravenously. Ventilation with 10% oxygen caused the mean pulmonary artery pressure to increase from 10+/-1.2 to 23+/-2.4 mm Hg (P < 0.01) and the pulmonary arteriolar resistance to increase from 1.51+/-0.19 to 5.87+/-1.10 U (P < 0.01). Hydralazine significantly lowered the pulmonary artery pressure (23.0+/-2.4 to 14.3+/-1.5 mm Hg, P < 0.01) and the pulmonary arteriolar resistance (5.87+/-1.10 to 2.87+/-0.52 U, P < 0.01). Femoral artery pressure, pulmonary artery wedge pressure, heart rate, and cardiac output remained unchanged throughout. To ascertain the contribution of the prostaglandin system to the pulmonary vasodilator effects of hydralazine, we pretreated a group of dogs with the prostaglandin synthetase inhibitor, indomethacin, 5 mg/kg s.c., twice daily for 2 d. These animals then underwent identical studies.The pretreated dogs had comparable base-line and hypoxia hemodynamic data. However, hydralazine had no effect on pulmonary artery pressure (23.3+/-1.6 vs. 21.7+/-2.3 mm Hg, NS) or pulmonary arteriolar resistance (8.03+/-1.09 vs. 7.14+/-1.42, NS) during continuous hypoxic ventilation in the indomethacin-pretreated group. Pretreatment with indomethacin did not, however, block the pulmonary vasodilator effects of intravenous prostacyclin (PGI(2)). Pretreatment with meclofenamate, a cyclo-oxygenase inhibitor structurally unrelated to indomethacin, also blocked the effects of hydralazine during hypoxic ventilation. These data suggest that hydralazine exerts a pulmonary vasodilatory effect during hypoxia-induced pulmonary vasoconstriction, and that this vasodilator effect may be mediated by prostaglandins.