Selective and complete blockade of acetylcholine-induced relaxation in rabbit aortic rings by N omega-nitro-L-arginine but not by glybenclamide

This study addressed the possibility that acetylcholine-induced relaxation in the rabbit aorta is mediated by dual mechanisms: one N omega-nitro-L-arginine (NLA)-sensitive, the other glybenclamide-sensitive. Acetylcholine, nitroglycerin and BRL38227 (lemakalim), an activator of glybenclamide-sensitive potassium channels, were added to an organ bath containing rabbit aortic rings in a cumulative manner in the absence or presence of NLA and/or glybenclamide. NLA inhibited acetylcholine-induced relaxation and potentiated the relaxant response to nitroglycerin. BRL38227 caused a dose-dependent relaxation in rabbit aortic rings, and 30 microM glybenclamide produced essentially complete inhibition of this relaxation. Glybenclamide alone produced no inhibition of acetylcholine-induced relaxation. These results indicate that glybenclamide-sensitive potassium channels in the rabbit aorta play no role in mediating the relaxant response to acetylcholine, while NLA can produce a selective and essentially complete blockade of the relaxant response to acetylcholine in the rabbit aorta.

Differential effects of nitric oxide synthesis inhibitors on vascular resistance and responses to acetylcholine in cats

The effects of dose and the duration of treatment with N omega-nitro-L-arginine (L-NNA) and N omega-nitro-L-arginine methyl ester (L-NAME) on vascular resistance and the vasodilator response to acetylcholine (ACh) were investigated in the hindquarters vascular bed of the cat under constant flow conditions. L-NNA and L-NAME increase perfusion pressure and reduce vasodilator responses to ACh in the hindquarters vascular bed; however, the dose and time of exposure required to produce these effects are different. When L-NNA (2.5-5 mg/min) was infused into the hindquarters vascular bed, the increase in perfusion pressure was observed 10 min after onset of the infusion, at which time responses to ACh were not changed. The time of exposure for 50% of the maximal change in hindquarters perfusion pressure was significantly less than the time of exposure for 50% of the maximal decrease in the vasodilator response to ACh during infusion of L-NNA. A similar pattern was observed after the intravenous administration of 3-300 mg/kg L-NAME, while 1 mg/kg L-NAME produced a large but submaximal pressor response and lesser decreases in responses to ACh. The arginine analogues did not completely inhibit the vasodilator response to ACh in the hindquarters vascular bed, with a 30-fold increase in dose producing no additional blockade; however, L-NNA completely inhibited the relaxant response to ACh in the isolated aorta of the cat.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative effects of endothelin (ET-1) and U46619 on human saphenous vein and gastroepiploic artery, sources of human autologous grafts

The effects of endothelin (ET-1) on smooth muscle contractile activity were investigated and compared in human saphenous vein and gastroepiploic artery, vessels frequently used in revascularization procedures. ET-1 contracted saphenous vein and gastroepiploic artery in a concentration-dependent manner. The peptide produced a greater maximal effect in the vein than in the artery and, in both preparations, ET-1 was less efficacious than U46619, an agent which mimics the actions of thromboxane A2 at the thromboxane A2/prostaglandin H2 receptor. The contractile response to ET-1 declined spontaneously at a more rapid rate in the artery than in the vein. The present data indicate that ET-1 has significant contractile activity in both vessels which are used for coronary arterial bypass surgery and suggest that although, a weaker vasoconstrictor than U46619, the peptide could induce vasospasm in both graft vessels.

Analysis of responses to big endothelin in the hindquarters vascular bed of the cat

OBJECTIVE

To investigate vascular responses to the endothelin-1 (ET-1) precursor, human big endothelin 1-38 (big ET), in the peripheral vascular bed of the cat.

DESIGN

These studies were designed to investigate the hypothesis that bit ET is converted to an active peptide with properties similar to ET-1.

SETTING

Hindquarters vascular bed of the cat under conditions of controlled bloodflow; changes in perfusion pressure reflect changes in vascular resistance.

ANIMALS

Fifty-four adult mongrel cats.

INTERVENTIONS

Big ET, ET-1, the peptidases chymotrypsin, pepsin and cathepsin-D, and the metalloprotease inhibitor phosphoramidon.

MAIN RESULTS

Intra-arterial injections of big ET induced a slow-developing and sustained increase in hindquarters perfusion pressure which could be blocked by phosphoramidon. ET-1 (0.3 nmol), administered as a slow infusion over a 10-min period, produced a slowly developing increase in hindquarters perfusion pressure in a manner similar to that observed in response to injection of big ET. A bolus injection of ET-1 produced a biphasic response characterized by a transient decrease in pressure followed by an increase which was significantly greater in magnitude and more rapid in onset than the pressor response to big ET (0.3 nmol). After incubation of big ET with chymotrypsin, pepsin and cathepsin-D (each 5% weight/weight) for 30 mins at 37 degrees C, injection of activated big ET produced a biphasic response characteristic of the response to ET-1 with an initial transient decrease in pressure followed by a secondary increase in hindquarters perfusion pressure.

CONCLUSIONS

Big ET produces a phosphoramidon-sensitive pressor response which is similar to that produced by an infusion of ET-1. These data suggest that chymotrypsin, pepsin and cathepsin-D can convert big ET to an active peptide which elicits a biphasic response similar to that produced by ET-1.

Analysis of pulmonary and systemic vascular responses to platelet-activating factor in the cat

Pulmonary and systemic vascular responses to platelet-activating factor (PAF) were investigated in the anesthetized cat. Intravenous injections of PAF decreased arterial pressure, increased pulmonary arterial pressure, and caused small but significant decreases in right and left atrial pressures. A transient increase in cardiac output was followed by a secondary decrease, and heart rate was increased. Pulmonary vascular resistance (PVR) was increased, systemic vascular resistance (SVR) was reduced, and changes in PVR and SVR in response to PAF were blocked by the novel PAF receptor antagonist, BN 50730. Under constant-flow conditions PAF dilated the hindlimb vascular bed in a dose-related manner, whereas in the pulmonary lobar vascular bed, PAF caused dose-related increases in perfusion pressure. Hindlimb and lobar vascular responses to PAF were blocked by BN 50730 in a selective manner, whereas cyclooxygenase inhibitors had no effect on responses to the phospholipid mediator. Hindlimb vasodilator responses to PAF were reduced by N omega-nitro-L-arginine in a dose that blocked the response to acetylcholine but did not decrease responses to prostaglandin E1 or nitroprusside. Increases in lobar arterial pressure in response to PAF were not altered by treatment with a thromboxane receptor antagonist, when the lung was perfused with a low-molecular-weight dextran solution, or when ventilation to the lobe was interrupted. These data suggest that the release of cyclooxygenase products, activation of thromboxane A2 receptors, cellular aggregation, release of leukocyte or platelet mediators, or changes in bronchomotor tone do not contribute to the pulmonary vasoconstrictor response to PAF and that the hindlimb vasodilator response to the phospholipid mediator is dependent in part on the release of endothelium-derived relaxing factor.

Actions of SQ 29,548 on contractile responses and arachidonic acid metabolism of intrapulmonary arteries, in vitro

This study was undertaken to investigate the influence of SQ 29,548, a thromboxane (TX) A2 receptor antagonist, on contractile responses and arachidonic acid (AA) metabolism of bovine intrapulmonary arterial (IPA) rings. The contractile responses to 5-hydroxytryptamine (5-HT), histamine, phenylephrine, and potassium chloride (KC1) were not significantly altered by 10(-8) M SQ 29,548 in either endothelium-intact or denuded IPA. The concentration of SQ 29,548 was chosen as it reduced the response to 10(-7) M U46619, a TXA2 mimetic, by 50%. AA metabolism by IPA produced more PGI2 whereas that by intrapulmonary vein (IPV) produced more PGE2. SQ 29,548 in concentrations of 10(-8) to 10(-5) M did not affect the activity of PGI2 synthase or GSH-dependent PGE2 isomerase in IPA or IPV microsomal fractions. No microsomal TXA2 synthase activity was detectable. SQ 29,548 had no effect on PGH synthase activity of IPA or IPV. The data indicate the presence of a TXA2-mediated contractile response in the IPA which is endothelium-independent and is selectively antagonized by SQ 29,548. The data further indicate that the contractile responses of IPA to 5-HT, histamine, phenylephrine, and KCl do not have a TXA2-mediated component. It is suggested that SQ 29,548 is a pharmacological probe to determine the role of TXA2 n pathophysiologic states in the pulmonary vascular bed and may be a therapeutic agent to treat pulmonary hypertensive disorders in which TXA2 is involved.

Differential effects of PACAP and VIP on the pulmonary and hindquarters vascular beds of the cat

Responses to pituitary adenylate cyclase-activating polypeptide (PACAP), a novel peptide derived from ovine hypothalamus with 68% sequence homology with vasoactive intestinal polypeptide (VIP), were investigated in the pulmonary and hindquarters vascular beds of the anesthetized cat under conditions of controlled blood flow. Injection of the peptide into the perfused lung lobe under elevated tone conditions produced dose-dependent decreases in lobar arterial pressure that were accompanied by biphasic changes in systemic arterial pressure characterized by an initial decrease followed by a secondary increase in pressure. When compared with other vasodilator agents in the pulmonary vascular bed, the relative order of potency was isoproterenol greater than PACAP greater than acetylcholine greater than calcitonin gene-related peptide greater than VIP. In the hindquarters vascular bed, intra-arterial injections of PACAP produced biphasic changes in hindquarters perfusion pressure characterized by initial decreases followed by secondary increases, which were accompanied by biphasic changes in systemic arterial pressure. In terms of relative vasodilator activity in the hindlimb, the order of relative potency was isoproterenol greater than acetylcholine greater than calcitonin gene-related peptide greater than VIP greater than PACAP. PACAP was the only agent that caused a secondary vasoconstrictor response in the hindlimb and produced biphasic changes in systemic arterial pressure. D-Phe2-VIP, a VIP receptor antagonist, blocked the hindquarters vasodilation in response to VIP but had no effect on responses to PACAP. The present investigation shows that PACAP produces pulmonary vasodilation, as well as dilation, and vasoconstriction in the systemic (hindlimb) vascular bed.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of SQ 29,548 on vasoconstrictor responses in the hindquarters vascular bed of the cat

The effects of SQ 29,548, a thromboxane (TX) receptor blocking agent, on vasoconstrictor responses were investigated under conditions of controlled blood flow in the hindquarters vascular bed of the cat. Intravenous injection of SQ 29,548 at a dose of 0.1 mg/kg had no significant effect on systemic arterial pressure but caused a significant reduction in hindquarters perfusion pressure. Injection of the TXA2 mimics, U44069 and U46619, into the perfusion circuit caused dose-dependent increases in hindquarters perfusion pressure with U46619 being approximately 3 times more potent than U44069. Following the administration of SQ 29,548, pressor responses to both U44069 and U46619 were reduced significantly, and the dose-response curves for both TXA2 mimics were shifted to the right in a parallel fashion. SQ 29,548 had no significant effect on the dose-dependent increases in hindquarters perfusion pressure in response to angiotensin II or BAY K8644, a nifedipine analog which promotes calcium entry. The TXA2 receptor blocking agent had no significant effect on increases in hindquarters perfusion pressure in response to angiotensin II or BAY K8644, a nifedipine analog which promotes calcium entry. The TXA2 receptor blocking agent had no significant effect on increases in hindquarters perfusion pressure in response to sympathetic nerve stimulation or injections of norepinephrine and tyramine. These findings suggest that SQ 29,548 blocks responses to the TXA2 mimics in a competitive manner, and that this inhibitory effect is selective.(ABSTRACT TRUNCATED AT 250 WORDS)

N omega-nitro-L-arginine selectively inhibits vasodilator responses to acetylcholine and bradykinin in cats

The effects of N omega-nitro-L-arginine (nitroarginine), an inhibitor of endothelium-dependent relaxing factor (EDRF) production, on vascular tone and responses to vasodilator and vasoconstrictor agents were investigated in the hindquarters vascular bed of the cat. Under constant flow conditions, infusion of nitroarginine into the hindquarters vascular bed caused a significant increase in systemic arterial and hindquarters perfusion pressures. During infusion of nitroarginine, hindquarters vasodilator responses to acetylcholine and bradykinin were reduced significantly whereas vasodilator responses to isoproterenol, PGE1, nitroprusside, and 8-bromoguanosine 3',5'-cyclic monophosphate were not altered. Infusion of nitroarginine significantly enhanced vasoconstrictor responses to the thromboxane receptor agonist U 46619 and to phenylephrine. The results of these studies are consistent with the hypotheses that EDRF production may involve the formation of nitric oxide or a nitroso compound from L-arginine, and that EDRF production may play a role in the regulation of vascular tone and in the mediation of responses to the endothelium-dependent vasodilators, acetylcholine and bradykinin, in resistance vessels in the hindquarters. These data support the concept that EDRF is very likely an endogenous nitrovasodilator derived from L-arginine in the hindquarters vascular bed of the cat.

The effects of alcoholism and smoking on platelet eicosanoid production in vitro

Ethanol induces changes in eicosanoid synthesis in blood platelets and brain tissue. Cigarette smoking also causes alterations in eicosanoid formation. This preliminary report examined in vitro platelet sonicate eicosanoid production using 14C-arachidonic acid (14C-AA) and in separate experiments, 14C-PGH2, as substrates. Radiometric thin layer chromatography (TLC) was used to identify the products formed. Eicosanoid product formation in platelet sonicates collected from 28 abstinent male alcoholics were compared to those from 11 male control subjects. All but one of the alcoholics were chronic smokers and all control subjects were non-smokers. All smokers abstained from smoking for 12 h prior to the blood collection to control for any acute effects of cigarette smoke on eicosanoid production. Significant reductions in platelet sonicate production of PGD2 and PGE2 in vitro were observed in alcoholic smokers when 14C-PGH2, but not 14C-AA, was the substrate. These reductions were predicted equally well by two variables, smoking and alcoholism, using several statistical models. This is the first investigation that controlled for the acute effects of smoking and accounted for the potential effects of cigarette smoking on platelet eicosanoid production in alcoholics. Because cigarette smoking is prevalent among alcoholics, future studies on the role of eicosanoids in alcoholism should control for smoking.

Influence of SK&F 95587 and BN 50730 on bronchoconstrictor responses in the cat

The effects of SK&F 95587 (4[2-(benzenesulfonamido)-ethyl] phenoxyacetic acid), a thromboxane (TX) receptor blocking agent, on bronchoconstrictor responses were investigated in paralyzed, anesthetized, mechanically ventilated cats. Intravenous injections of the TXA2 receptor mimics, U-46619 [(15S)-hydroxy-11 alpha,9 alpha-(epoxymethano)prosta5Z,13E-dienoic acid] and U-44069 (9,11-dideoxy-11 alpha,9 alpha-epoxymethano PGF2 alpha), produced dose-related increases in transpulmonary pressure and lung resistance and decreases in dynamic compliance. After administration of SK&F 95587, 5 mg/kg i.v., bronchoconstrictor responses to U-46619 and U-44069 were reduced markedly, whereas airway responses to prostaglandin (PG)F2 alpha, serotonin, PGD2 or the PGD2 metabolite, 11 beta-PGF2 alpha, were not altered. The duration of action of SK&F 95587 was greater than 3 hr, and the blockade was overcome when 10-fold larger doses of the TXA2 mimics were administered. Bronchoconstrictor responses to platelet-activating factor (PAF) were blocked by SK&F 95587 and by the novel PAF receptor antagonist, BN 50730. BN 50730 also blocked the fall in systemic arterial pressure in response to PAF. However, BN 50730 did not influence airway responses to U-46619, PGF2 alpha, PGD2 or serotonin and had no effect on baseline bronchomotor tone or arterial pressure. The PAF receptor antagonism with BN 50730 was overcome when 10-fold larger doses of PAF were administered and the dose-response curves for changes in lung resistance and dynamic compliance were shifted to the right in a parallel manner. The present data suggest that SK&F 95587 has selective TX receptor blocking activity, and that BN 50730 has selective PAF receptor blocking properties in the airways of the cat. The present data also provide support for the hypothesis that bronchoconstrictor responses to PAF are mediated by specific receptors, which are coupled to a phospholipase and, when activated, result in the release of TXA2 and contraction of airway smooth muscle.

Increased prostacyclin and thromboxane A2 formation in human varicose veins

Increased urinary metabolites of the antiaggregatory vasodilator prostacyclin (PGI2) and the proaggregatory vasoconstrictor thromboxane A2 (TXA2) have been reported in deep vein thrombosis; however, the tissue(s) of origin is uncertain. Because little is known about the formation of PGI2 or TXA2 from its common precursor, prostaglandin (PG) endoperoxide H2 (PGH2), by varicose veins, we determined the formation of 6-keto-PGF1 alpha (the stable metabolite of PGI2), TXB2 (the stable metabolite of TXA2), and PGE2. Segments of normal saphenous vein and varicose vein (nine and six patients, respectively) were incubated with 10 microM [14C]PGH2 for 2 min at 37 degrees C; products were separated by thin-layer chromatography. Surface area and mass of normal and varicose vascular segments were 19.5 +/- 0.8 versus 18.8 +/- 0.6 mm2 and 11.6 +/- 1.4 versus 10.7 +/- 0.7 mg, respectively. Formation of 6-keto-PGF1 alpha and TXB2 by the segments of varicose vein was significantly increased over that of normal vein: 157 +/- 14 versus 243 +/- 17 pmole of 6-keto-PGF1 alpha (P less than 0.005) and 22 +/- 3 versus 35 +/- 5 pmole of TXB2 (P less than 0.01). The formation of PGE2 by segments of varicose vein was not significantly different from that of normal vein (201 +/- 9 vs 219 +/- 11, respectively). Deoxyribonucleic acid (DNA) content of normal and varicose vein was 1.69 +/- 0.12 and 1.51 +/- 0.13 mg per gram of tissue, respectively. The data suggest that the increased PGI2 formation may reflect increased activity or content of PGI2 synthase. The increase in TXA2 formation may reflect increased productivity or an increased presence of residual platelets or microemboli.

Concentration-activity profile of the modulation of cyclooxygenase product formation by reduced glutathione in microsomal fractions from the goat lung

Age-related changes in pulmonary formation of arachidonic acid (AA) metabolites are thought to play an important role in regulating cardiopulmonary function. This study addresses the potential role of reduced glutathione (GSH) in modulating cyclooxygenase product formation in the developing lung. Prostaglandin H2 (PGH2) metabolism was studied in microsomal fractions isolated from the lungs of unventilated fetal, neonatal and adult goats. GSH-dependent PGH2 to PGE2 isomerase activity in microsomal fractions from the perinatal (fetal and neonatal) goat lung was not saturable with respect to GSH and can respond to changes in GSH concentration over the range of 0.01 to 30 mM, which encompasses the full range the intracellular GSH levels reported in the literature. However, in fractions from the adult, a lower rate of PGE2 formation is observed at higher GSH concentrations. In addition, the tissue levels of GSH exhibited developmental stage-related differences with fetal being higher than neonatal or adult. The present observations may have physiologic relevance, in that decreases in pulmonary GSH levels after birth may contribute to decreases in plasma PGE2 levels by decreasing pulmonary PGE2 synthesis, thereby contributing to closure of the ductus arteriosus; conversely, increased GSH levels associated with hyperoxia may contribute to persistence of ductal patency. Formation of 6-keto-PGF1 alpha and of TXB2 (the stable metabolites of prostacyclin and TXA2) was decreased when PGE2 formation was increased by GSH activation of PGE2 isomerase in fractions isolated from all three developmental stages. A similar pattern of product formation was observed when AA was employed as substrate. These data suggest the possibility that changes in GSH concentration may modulate eicosanoid formation in cells that contain GSH-dependent PGE2 isomerase, as well as either or both prostacyclin or thromboxane synthase(s).

Analysis of cardiovascular and pulmonary responses to endothelin-1 and endothelin-3 in the anesthetized cat

Cardiovascular and pulmonary responses to endothelin (ET)-1, ET-3 and neuropeptide Y (NPY) were investigated in the anesthetized cat. ET-1, 0.1 to 1 nmol/kg i.v., decreased or elicited biphasic changes in arterial pressure (AP), whereas ET-3, in the same doses, decreased AP. Both ETs increased cardiac output (CO) and, at the highest doses, a secondary decrease in CO was observed. NPY, 0.3 to 3 nmol/kg i.v., increased AP and at the highest dose decreased CO. All three peptides had inconsistent effects on right ventricular contractile force and increased central venous pressure. ET-1 at lower doses increased heart rate (HR) and, at 1 nmol/kg, caused a biphasic change. ET-3 increased HR, whereas NPY decreased HR. Systemic vascular resistance (SVR) was increased by NPY and decreased by ET-3, whereas ET-1 elicited biphasic changes. ET-1 and ET-3 increased pulmonary arterial pressure, left atrial pressure and caused biphasic changes in pulmonary vascular resistance (PVR). NPY had no significant effect on PAP or PVR. When pulmonary blood flow was maintained constant, ET-1 and ET-3 had only pulmonary vasoconstrictor activity, whereas NPY and the ET analog had no significant effect. The increase in SVR in response to NPY, the decrease in response to ET-3 or the biphasic change in response to ET-1 were not modified by meclofenamate, hexamethonium or propranolol. Increases in HR in response to ET-1 and ET-3 were reduced by the beta receptor and ganglionic blocking agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of SQ 29,548 on vasoconstrictor responses in the mesenteric vascular bed of the cat

The effects of SQ 29,548 on vasoconstrictor responses were investigated in the feline mesenteric vascular bed. Injections of the thromboxane (TX) A2 mimics, U46619 and U44069, caused dose-related increases in mesenteric arterial perfusion pressure. After administration of SQ 29,548, 0.5 mg/kg i.v, vasoconstrictor responses to U46619 and U44069 were reduced markedly whereas responses to prostaglandin (PG) F2 alpha, angiotensin II, vasopressin and BAY K 8644, an agent which enhances calcium entry, were not altered. The duration of the TXA2 receptor blockade was greater than 2 h and SQ 29,548 had no significant effect on mesenteric vasodilator responses to PGE2, isoproterenol, nitroglycerin, acetylcholine or bradykinin. SQ 29,548, at a dose of 0.5 mg/kg i.v., significantly reduced the response to TXB2, which had modest vasoconstrictor activity in the mesenteric vascular bed. However, when the dose of SQ 29,548 was reduced to 0.05 mg/kg i.v., responses to TXB2 were not altered, whereas responses to U46619 were significantly decreased. SQ 29,548 had no significant effect on vasoconstrictor responses to norepinephrine or to sympathetic nerve stimulation. The TXA2 receptor antagonist blocked the vasoconstrictor component of the biphasic response to the PG precursor, arachidonic acid, and the endoperoxide, PGH2. The results of these studies suggest that SQ 29,548 is a specific TX receptor antagonist in the mesenteric vascular bed, that the vasoconstrictor component of the biphasic response to arachidonic acid and PGH2 is due to formation of TXA2, and that endogenously formed TXA2 does not modulate adrenergic responses in the mesenteric circulation of the cat.

Prostaglandin E2 formation by rat gastroduodenal tissue following intragastric acid perfusion

Rat gastroduodenal mucosa forms prostaglandin (PG) E2. However, little is known about regional differences in PGE2 formation or the effect of gastric hydrochloric acid (HC1) perfusion on regional PGE2 formation. In this study, the rats were divided into 3 groups. Group 1 received intravenous (i.v.), 1 Ml/h, and intragastric (i.g.), 8 ml/h, perfusions of saline simultaneously for 3 h. Group 2 received saline i.v. and 0.15 N HC1 i.g., 8 ml/h. Group 3 was injected with a bolus of asprin (ASA), 60 mg/kg, followed by ASA, 40 mg/kg/h i.v., and 0.15 N HC1 i.g.. The gastric aspirates were analyzed for volume and pH. Segments of gastroduodenal tissue from the fundus, corpus, antrum, and duodenum were minced and then incubated in 1 ml of 5 mM Tris buffer, pH 8.4, for 30 sec with mixing; the incubate was assayed for PGE2 by radioimmunoassay. Intragastric HC1 decreased the pH of aspirate without producing gastric mucosal lesions. However, when combined with i.v. ASA, ulcer formation was present in all animals (p less than 0.05). PGE2 was formed by isolated tissue from four different gastroduodenal regions. The duodenum formed significantly greater amounts than the fundus, antrum, or corpus, which were similar. Intragastric HC1 produced a trend toward increased PGE2 formation (pmol PGE2/mg tissue) in the fundus, 143 +/- 36 to 237 +/- 57; corpus, 87 +/- 13 to 200 +/- 57; antrum, 157 +/- 28 to 224 +/- 65; and duodenum, 235 +/- 56 to 338 +/- 51. However, statistical significance was not reached.

Modulation of prostaglandin biosynthesis in hypoxic murine mammary adenocarcinoma cells by misonidazole

Resistance of hypoxic cells to radiation and chemotherapy remains a major limitation to effective therapy of solid tumors. Misonidazole, a 2-nitroimidazole analogue, has been studied extensively as a radiosensitizer of hypoxic cells and has been shown to undergo bioreductive metabolism to exert preferential cytotoxicity against hypoxic cells. We have investigated the effects of misonidazole on the biosynthesis of prostaglandins (PGs) in a murine mammary adenocarcinoma cell line (No. 4526) under aerobic and hypoxic conditions in attempts to exploit modulation of PG levels under hypoxia as a means of improving therapeutic approaches for the treatment of solid tumors. We report a time-dependent inhibition of PG biosynthesis by the suspended cells under hypoxia induced by flushing sealed vials with N2 (1.5 liters/min). After 30 min of hypoxia, PG formation was inhibited by 50%. Indomethacin was able to further inhibit the PG formation in a concentration-dependent manner under hypoxia. Misonidazole, however, selectively increased the PGE2 biosynthesis under hypoxia by 49% at 100 microM. This increase was concentration dependent over the range of 25 to 100 microM and was blocked by indomethacin (0.1 microM). Imidazole, the heterocyclic moiety in misonidazole without the nitro function, had no effect on PG biosynthesis at these concentrations. These data suggest that arachidonic acid metabolism is sensitive to the differential oxygen levels which exist within solid tumors and that PG levels may be modulated by electron-affinic agents in hypoxic tumor cells.

Nisoldipine inhibits adrenergic responses in the hindquarters vascular bed of the cat

The effects of the calcium entry blocking agent nisoldipine on adrenergic vasoconstrictor responses were investigated in the hindquarters vascular bed of the cat under conditions of controlled blood flow. Nisoldipine dilated the hindquarters vascular bed and inhibited vasoconstrictor responses to Bay K 8644, a nifedipine analog which promotes calcium entry. During infusion of nisoldipine, vasoconstrictor responses to sympathetic nerve stimulation, norepinephrine, and tyramine were inhibited in a reversible manner. In addition to blocking responses to nerve-released and exogenous norepinephrine, the calcium entry antagonist decreased responses to methoxamine and BHT 933, alpha 1- and alpha 2-adrenoceptor agonists. Responses to methoxamine were reduced by prazosin, an alpha 1-adrenoceptor antagonist, but not by yohimbine, an alpha 2-adrenoceptor blocking agent, whereas responses to BHT 933 were decreased by yohimbine but not by prazosin. The results of these studies suggest that vasoconstrictor responses to neuronally released and exogenous norepinephrine, as well as to selective alpha 1- and alpha 2-adrenoceptor agonists, are dependent in part on an extracellular source of calcium in resistance vessels of the feline hindquarters vascular bed. The inhibitory effect of nisoldipine on vasoconstrictor responses to neuronally released norepinephrine may be important in the antihypertensive actions of calcium entry blocking agents.

Effects of porcine and rat endothelin and an analog on blood pressure in the anesthetized cat

Arterial responses to a wide range of doses of porcine and rat endothelin and a monocyclic analog were compared in the anesthetized cat. Injections of the porcine peptide in doses of 0.01-0.1 nmol/kg i.v. decreased systemic arterial pressure in a dose-related manner, whereas doses of 0.3 and 1 nmol/kg i.v. elicited biphasic responses. The rat peptide, in doses of 0.03-1 nmol/kg i.v., also decreased arterial pressure in a dose-related fashion, whereas injection at 3 nmol/kg i.v. caused a biphasic response. With both peptides the biphasic response was characterized by an initial short-lived decrease followed by a secondary sustained increase in pressure. The monocyclic porcine analog in doses of 3-30 nmol/kg i.v. had no significant effect on arterial pressure. Both peptides increased cardiac output, and changes in peripheral vascular resistance in response to both peptides were not altered by sodium meclofenamate. These data suggest that arterial depressor responses to porcine and rat endothelin are similar and dose-dependent. However, the porcine peptide has 3-fold greater pressor activity in the cat. The lack of effect with the monocyclic porcine analog suggests that the two disulfide linkages are necessary for activity.

Differential effects of ibuprofen, indomethacin, and meclofenamate on prostaglandin endoperoxide H2 metabolism

Previous studies have suggested the possibility that the non-steroidal antiflammatory drug (NSAID), ibuprofen, may inhibit thromboxane (TX) A2 synthase activity in addition to inhibiting cyclooxygenase activity. Microsomal fractions isolated from the cat lung contain cyclooxygenase as well as prostacyclin (PGI2) synthase, TX synthase, and a GSH-dependent prostaglandin (PG) E2 isomerase activities. When [1-14C] PG endoperoxide H2 (PGH2) was used as substrate, ibuprofen, indomethacin, and meclofenamate exhibited differential effects on terminal enzyme activities. Ibuprofen, at concentrations up to 1 mM, had no effect on the activities of PGI2 synthase, TXA2 synthase of GSH-dependent PGE2 isomerase, whereas indomethacin selectively inhibited PGI2 synthase activity at 5 X 10(-4) M and 10(-3) M. Meclofenamate selectively inhibited TXA2 synthase activity at 5 X 10(-4) M and 10(-3) M. At concentrations of 5 X 10(-3) M, this selectivity was not observed, and indomethacin and meclofenamate decreased the formation of both 6-keto-PGF1 alpha and TXB2. These data indicate that the choice of NSAID and the concentration employed may specifically alter PGH2 metabolism. This action may affect the physiologic consequences of the exchange of PGH2 between cells. The data further indicate that indomethacin has the potential for use as a tool to specifically attenuate PGI2 synthase activity in vitro.

Evidence for a bidirectional prostaglandin endoperoxide shunt between platelets and the bovine coronary artery

While platelets have been shown to be capable of supplying prostaglandin (PG) H2 to endothelial cells in culture for PGI2 synthesis, endothelial cells have been shown unable to supply PGH2 to platelets for thromboxane (TX) A2 synthesis. We incubated rings of the bovine coronary artery (BCAR) with human platelets treated with aspirin (to inhibit cyclooxygenase) or CGS 13080 (to inhibit TXA2 synthase) in the presence of 20 microM arachidonic acid. BCAR, with damaged endothelium, produced significantly less PGI2 than that with intact endothelium. However, co-incubation with CGS 13080-treated platelets resulted in an increase in PGI2 independent of endothelium, demonstrating a shunt of PGH2 from platelets to BCAR. Co-incubation of BCAR with aspirin-treated platelets resulted in a net increase in TXA2 demonstrating a shunt of PGH2 from BCAR to platelets. Employing [14C]PGH2 as substrate, BCAR with and without intact endothelium produced similar amounts of 6-keto-[14C]PGF1 alpha. Likewise, homogenates (50 micrograms protein) of intimal and subintimal regions of BCAR and BCAR converted similar amounts of PGH2 to 6-keto-PGF1 alpha. These data suggest that vascular production of PGH2 is more dependent on an intact endothelium than is the conversion of PGH2 to PGI2. These data also suggest a potential for a bidirectional exchange of PGH2 between platelets and vascular wall during platelet-vascular wall interactions.

Analysis of regional responses to endothelin in hindquarters vascular bed of cats

Regional responses to endothelin, a peptide derived from endothelial cells in culture, were investigated in the hindquarters vascular bed of cats, when flow varied naturally and when flow was maintained constant with a pump. Intravenous injections of endothelin at doses of 0.03 and 0.1 nmol/kg caused dose-dependent decreases in systemic arterial pressure and increases in distal aortic blood flow. Injection of endothelin at a dose of 0.3 nmol/kg iv caused a biphasic response characterized by an initial decrease in arterial pressure and an increase in blood flow, which was followed by a secondary rise in pressure and a fall in blood flow. When blood flow to hindquarters was maintained constant with a pump, intra-arterial injection of 0.03 nmol endothelin caused a decrease in perfusion pressure, whereas 0.1-1 nmol doses elicited biphasic responses characterized by an initial decrease followed by a secondary increase in perfusion pressure. When compared with other vasoactive peptides, the pressor activity of endothelin was less than angiotensin II by an order of magnitude but was threefold greater than that of neuropeptide Y in the hindquarters vascular bed. The pressor component of the response to endothelin and the response to the calcium agonist BAY K 8644 were decreased in a reversible manner by nisoldipine, a dihydropyridine calcium entry blocking agent. The results of these studies indicate that porcine-human endothelin has both vasodilator and vasoconstrictor activity in the hindquarters vascular bed of cats. The predominant response at a low concentration is vasodilation, whereas at higher concentrations a vasoconstrictor response that was dependent in part on the influx of extracellular calcium could be demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of pulmonary thromboxane A2 synthase activity and airway responses by CGS 13080

The effects of CGS 13080, a thromboxane (TXA2) synthase inhibitor, on airway responses to arachidonic acid (AA) were investigated in the anesthetized cat. Feline and human lung microsomal fraction exhibited prostaglandin I2 (PGI2, prostacyclin), and TXA2 synthase activities, and human platelet microsomal fractions exhibited TXA2 synthase activity. Cat and human lung microsomal fractions, but not human platelets, exhibited the presence of GSH-dependent PGE2 isomerase activity. CGS 13080 inhibited TXA2 synthase activity in all three microsomal fractions in a concentration-dependent manner. The increases in transpulmonary pressure and lung resistance and decreases in dynamic compliance in response to AA were decreased significantly by CGS 13080. These data suggest that the bronchoconstrictor actions of AA are mediated in large part by the formation of TXA2. The data further indicate that cyclooxygenase products other than TXA2 are involved in the bronchoconstrictor response to AA since meclofenamate had greater inhibitory activity than did CGS 13080. Moreover, the effects of CGS 13080 were due to inhibition of TXA2 synthase rather than an effect on TXA2 receptors, since airway responses to the TXA2 mimic, U46619, were not altered. The present data show that CGS 13080 inhibits TXA2 synthase activity without altering cyclooxygenase, PGI2 synthase, or GSH-dependent PGE2 isomerase activities. The data further indicate that in vivo administration of CGS 13080 may selectively increase PGI2 synthase activity.

Inhibition of GSH-dependent PGH2 isomerase in mammary adenocarcinoma cells by allicin

We have reported that allicin, a constituent of garlic oil, has no effect on the activities of platelet cyclooxygenase or thromboxane synthase, or vascular PGI2 synthase. The effect of allicin on glutathione (GSH) dependent PGH2 to PGE2 isomerase is unknown. We therefore studied the effect of allicin on PGE2 biosynthesis in a murine mammary adenocarcinoma cell line (No 4526). Intact or sonicated cells were incubated with either 14C-arachidonic acid (AA) or 14C-PGH2, respectively. Following metabolism, products were extracted, separated by TLC and analyzed by radiochromatographic scan. PGE2 was predominantly formed with minimal amounts of PGF2 alpha and PGD2. Formation of 6-keto-PGF1 alpha or TXB2 was not detected indicating the absence of TXA2 and PGI2 synthase activity. Indomethacin and ibuprofen inhibited the PGE2 formation (p less than 0.05). The enzymatic PGE2 formation in sonicates was blocked by depletion of the cellular non-protein thiols by buthionine sulfoximine and was shown to be dependent on GSH. Allicin, over the range of 10-1000 microM, inhibited the formation of PGE2 in cells exposed to 2.0 microM 14C-AA for 20 min. and in sonicated cells incubated with 20.0 microM 14C-PGH2 for 2 min (p less than 0.05). Allicin did not alter cyclooxygenase-mediated oxygen utilization in ram seminal vessicle microsomes, suggesting that allicin selectively inhibits the GSH-dependent PGH2 to PGE2 isomerase in this adenocarcinoma cell line.