In-vitro contraction of the equine aortic valve.

BACKGROUND AND AIM OF THE STUDY: The equine aortic valve is subject to non-inflammatory degenerative changes, associated with aortic valvular regurgitation (AR). This disease shares pathological and epidemiological features with AR in humans, and may serve as a useful model to study in-vitro functional responses associated with aging and disease. The study aim was to determine the contractile properties of the normal equine aortic valve. METHODS: The contractile responses of equine aortic valves to angiotensin II, the thromboxane-mimetic U44069, endothelin-1, 5-hydroxytryptamine and the alpha-adrenoceptor agonists medetomidine, norepinephrine and phenylephrine were studied in vitro in organ baths. Selective antagonists were used to confirm the receptors involved. RESULTS: The order of potency of the agents causing contraction of equine aortic valve segments was angiotensin II > endothelin-1 > U44069 > medetomidine norepinephrine phenylephrine. 5-Hydroxytryptamine did not cause contraction of the equine aortic valve. The contractile response to angiotensin II was abolished by the AT1 receptor antagonist Sar1-Ile8-Angiotensin II, and that of U44069 by the thromboxane TXA2 receptor (TP) antagonist SQ29548. The contractile effects of endothelin-1 were blocked by the ET(A) receptor antagonist BQ123, but not by the ET(B) receptor antagonist BQ788. Yohimbine inhibited the contractile effects of phenylephrine, suggesting an alpha-2 adrenoceptor-mediated response. CONCLUSION: Equine aortic valves contract in response to a number of physiologically important endocrine, paracrine and neuronal mediators. Regulation of valvular tone could therefore be important in the normal functioning of the valve, and further understanding of these mechanisms may lead to insights into the pathophysiology of naturally occurring equine aortic insufficiency. In this respect, the horse should be considered as a model of the human condition.

Efficiency of aerosolized nitric oxide donor drugs to achieve sustained pulmonary vasodilation.

Inhalation of nitric oxide (NO) causes selective pulmonary vasodilation, but demands continuous supply of the gaseous agent. We investigated the suitability of aerosolization of NO-donor drugs for achieving sustained reduction of pulmonary vascular tone. In buffer-perfused rabbit lungs, stable pulmonary hypertension was achieved by continuous infusion of the thromboxane-analogue U46619. The NO-donor drugs molsidomine, 3-morpholinosydnone-imine (SIN-1), sodium nitroprusside (SNP) and glyceryl-trinitrate reduced the pulmonary hypertension in a dose-dependent fashion, whether admixed to the perfusate or inhaled as alveolar-accessible aerosol particles (aerosolization time 3-6 min), with an efficiency ranking of SNP > SIN-1 >> molsidomine and glyceryl-trinitrate. Notably, nearly identical dose-response curves were obtained when corresponding molar quantities of the most potent agents, SNP and SIN-1, were applied either via transbronchial or via intravascular routes, with respect to rapidity of onset, extent (pressure reduction to near baseline) and duration (>90 min) of vasorelaxation. Appearance of sydnonimines in the perfusate after aerosolization and reduction of SIN-1 efficacy when nebulized in nonrecirculatingly perfused lungs demonstrated substantial entry of this prodrug into the vascular space after alveolar deposition. In contrast, undiminished vasodilatory efficacy of aerosolized SNP under conditions of non-recirculating perfusion suggested predominant efficacy via local NO release for this agent. We conclude that short aerosolization maneuvers of NO-donor drugs are suitable to achieve dose-dependent, extensive and sustained vasodilation in the pulmonary circulation, thus offering a new therapeutic approach in pulmonary hypertension.

Changes in reactivity towards 5-hydroxytryptamine in the renal vasculature of the diabetic spontaneously hypertensive rat.

BACKGROUND: Diabetes and hypertension are both associated with an increased risk of renal disease. The combination of these diseases produces marked acceleration of the problems. OBJECTIVE: To examine the reactivity in the renal vasculature of diabetic hypertensive rats. METHODS: We investigated the reactivity towards 5-hydroxytryptamine (5-HT) in Krebs solution-perfused kidneys of diabetic normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). In addition, the interaction between the thromboxane A2 mimetic U46619 and 5-HT was examined. RESULTS: Discrete dose-response curves were obtained for the response to 5-HT (0.1-30 micrograms/g kidney) in Krebs solution-perfused kidneys of control and diabetic WKY rats and SHR. The following order of reactivity was determined: control SHR > diabetic SHR > control WKY rats = diabetic WKY rats. The thromboxane A2 mimetic U46619 (10 ng/ml) potentiated responses to 5-HT significantly in kidneys of diabetic WKY rats and control SHR, but not in kidneys from control WKY rats and diabetic SHR. CONCLUSIONS: The differential affected reactivity towards 5-HT kidneys from diabetic and hypertensive rats might be due to previously documented differences in receptor number. The marked effect of U46619 on the reactivity towards 5-HT in kidneys of diabetic and control rats indicates that this interaction might be important given the increased levels of thromboxane A2 reported to occur in these diseases. The reason for the lack of effect of thromboxane/prostaglandin receptor stimulation on responses to 5-HT in the combined model (i.e. diabetic hypertensive rats) needs to be determined.

Thromboxane A2 acts at a site perfused by the carotid vasculature to mediate cardiovascular and adrenocortical responses.

Increases in thromboxane A2 (TxA2) synthesis are associated with hemodynamic responses and activation of the hypothalamus-pituitary-adrenal axis. This study tested the hypothesis that TxA2 acts on a site perfused by the carotid vasculature to mediate these responses. The TxA2 mimetic U46619 was infused for 30 min into the carotid artery or the vena cava of chronically instrumented adult sheep at doses of 0, 0.25, 0.5, and 1.0 microgram.kg-1.min-1. Mean arterial pressure increased in response to carotid or vena cava U46619 infusions of 0.5 and 1.0 microgram.kg-1.min-1. Heart rate was elevated in response to carotid (0.5 and 1.0 microgram.kg-1.min-1) or vena cava (1.0 microgram.kg-1.min-1) infusions of U46619. Paco2 declined and pH2 increased significantly in response to carotid infusions of 0.5 and 1.0 microgram.kg-1.min-1 but did not change in response to vena cava infusions. Adrenocorticotropic hormone (ACTH) increased in response to carotid infusions of 0.5 microgram.kg-1.min-1, while cortisol increased in response to infusions of 0.25, 0.5, and 1.0 microgram.kg-1.min-1. ACTH and cortisol did not change in response to vena cava infusions. Pao2, hematocrit, and arginine vasopressin did not change significantly. Pulmonary artery pressure and total peripheral resistance increased while cardiac output decreased in response to carotid or vena cava U46619 infusions of 1 microgram.kg-1.min-1; carotid and vena cava responses were not different from one another. We conclude that increases in blood pressure are mediated by peripheral PGH2/TxA2 receptor activation and that pituitary adrenal, blood gas, and heart rate responses are mediated by PGH2/TxA2 receptor activation at a site perfused by the carotid vasculature.

Direct evidence that thromboxane mimetic U44069 preferentially constricts the afferent arteriole.

The thromboxane A2 (TXA2) mimetic U44069 has been demonstrated to reduce the GFR and filtration fraction of the normal isolated perfused rat kidney markedly, suggesting a predominant constriction of preglomerular vessels. To assess this possibility directly, effects of U44069 on the renal microvessels of the isolated perfused hydronephrotic kidney were examined. At 10(-6) mol/L, U44069 elicited a 27 +/- 2% decrease in afferent arteriolar (AA) diameter (from 18.8 +/- 0.3 to 13.7 +/- 0.3 micron, P < 0.001). In contrast, efferent arteriolar (EA) diameter decreased by only 9 +/- 1% (from 16.4 +/- 0.5 to 15.0 +/- 0.5 micron, P < 0.001). These effects on both AA and EA were completely reversed by the TXA2 receptor antagonist SQ29548. The calcium antagonist diltiazem reversed U44069-induced AA constriction by 83 +/- 5%. The U44069-induced EA constriction was insensitive to the vasodilator action of diltiazem at concentrations from 10(-8) to 10(-6) mol/L, but at 10(-5) mol/L, diltiazem increased the EA diameter significantly, albeit modestly. Nifedipine also reversed the U44069-induced AA constriction (81 +/- 7%), but failed to inhibit the EA constriction at concentrations from 10(-9) to 10(-6) mol/L. These findings constitute the first direct evidence that a TXA2 agonist preferentially constricts the afferent arteriole. Furthermore, the ability of both the calcium antagonist and SQ29548 to reverse the renal microvascular actions of TXA2 agonists suggests a potential utility of these agents in ameliorating TXA2-induced renal hemodynamic abnormalities.

The in vitro effects of phosphodiesterase inhibitors on the human internal mammary artery.

The internal mammary artery (IMA) is the preferred conduit for myocardial revascularization, but it changes diameter in response to injury or thromboxane release to decrease myocardial blood supply. Papaverine, a phosphodiesterase (PDE) inhibitor, is injected in the IMA bed during surgery to prevent spasm. We evaluated the ability of papaverine and cyclic adenosine monophosphate PDE Type III (cAMP-PDE) inhibitors (amrinone, enoximone, and milrinone) in vitro to reverse the constriction of human IMA rings, induced by a thromboxane A2 analog, U46619, and evaluated amrinone's ability to modify the constricting effect of norepinephrine (NE). All cAMP-PDE inhibitors produced complete relaxation of U46619-induced contractions. The contractions necessary to produce 50% relaxation (EC50) were within therapeutic ranges. The vasodilatory potency of amrinone was greater after NE than after U46619 (EC50, 1.9 +/- 0.5 vs 4.3 +/- 2.2 x 10(-5)M; mean +/- SD; P < 0.05). Response to constriction after a submaximal dose of NE was attenuated to 38% (P < 0.001) from that observed in the control rings by a pretreatment with amrinone. These results suggest that cAMP-PDE inhibitors have the potential utility to reverse IMA spasm, and represent a potential therapeutic modality for IMA spasm after myocardial revascularization.

K+ channel-opening action contributes to the preventive effects of nicorandil on U46619-induced vasoconstriction of canine large coronary arteries in vivo.

The antispasmogenic effects of nicorandil on epicardial coronary artery vasoconstriction were compared with those of a K+ channel opener, cromakalim, and a nitrovasodilator, nitroglycerin, in open-chest dogs. Intracoronary administration of U46619 (0.5-1.0 micrograms), a stable thromboxane A2 analogue, reduced the external diameter of the left circumflex coronary artery with no marked alternations in systemic hemodynamics. This U46619-induced vasoconstriction of large epicardial coronary arteries was dose-dependently prevented by the intracoronary infusion of nicorandil (1-10 micrograms/kg/min), cromakalim (0.03 micrograms/kg/min) and nitroglycerin (1 micrograms/kg/min). After pretreatment with glibenclamide (3 mg/kg, i.v.), and ATP-sensitive K+ channel blocker, these effects of nicorandil and cromakalim were inhibited significantly, whereas the response to nitroglycerin remained unchanged. Nicorandil (3 micrograms/kg/min), cromakalim (0.03 micrograms/kg/min) and nitroglycerin (1 micrograms/kg/min) increased coronary blood flow. However, the inhibitory effects of each drug on the U46619-induced vasoconstriction were not influenced by the partial occlusion of the left circumflex coronary artery, which kept coronary blood flow constant. This indicates a direct antispasmogenic effect of K+ channel openers, which is independent of that mediated by the response to flow. Furthermore, our results suggest that, by this effect, nicorandil protects large coronary arteries from U46619-induced vasoconstriction.

Airway hyper- or hyporeactivity to inhaled spasmogens 24 h after ovalbumin challenge of sensitized guinea-pigs.

1. The aim of this study was to determine whether an inhalation of ovalbumin (OA, 10 or 20 mg ml-1) by conscious OA-sensitized guinea-pigs leads to airway hyperreactivity to spasmogens 24 h later. In contrast to most previous studies, the spasmogens (5-HT, methacholine (MCh), U-46619 and adenosine) were administered by inhalation and airway function was measured in conscious guinea-pigs. 2. Guinea-pigs were sensitized by i.p. injection of 10 micrograms OA and 100 mg aluminium hydroxide in 1 ml normal saline; 14-21 days later they were exposed to an inhalation of 5-HT, MCh, U-46619 or adenosine. Specific airway conductance (sGaw) was measured in conscious animals by whole body plethysmography. The spasmogens caused bronchoconstriction, measured as a reduction in sGaw from the pre-inhalation basal values. Dose-related bronchoconstrictions were observed with 5-HT, MCh and U-46619. 3. The effect of an ovalbumin macroshock challenge upon the responses to each spasmogen were examined by giving an inhalation of aerosolized OA at 24 h (or 7 days in the cause of adenosine) after an initial spasmogen challenge. Eighteen to twenty-four hours after the OA macroshock, the same guinea-pigs were exposed to a repeated inhalation of 5-HT, MCh, U-46619 or adenosine. 4. U-46619 was the only spasmogen to demonstrate hyperresponsiveness, the peak change in sGaw being increased from -12.3 +/- 9.9 to -38.8 +/- 5.0% by 10 mg ml-1 OA challenge. In contrast, the ovalbumin challenge (20 mg ml-1) inhibited the bronchoconstrictions to 5-HT (50 micrograms ml-1) and MCh (100 micrograms ml-1). Adenosine demonstrated bronchoconstriction in sensitized guinea-pigs but no significant change in the response was observed after an OA challenge. 5. All results were compared with a control group of sensitized guinea-pigs receiving a NaCl challenge. The bronchoconstrictor responses to 5-HT, MCh, U-46619 or adenosine did not differ significantly before and after the saline challenge, indicating reproducibility of the responses. 6. In further experiments, guinea-pigs were exposed to inhalation of 5-HT (50 micrograms ml-1) or MCh (300 micrograms ml-1) 24 h before atropine (10 micrograms, 100 micrograms or 1 mg ml-1) and again at 0.5 to 1.5 h afterwards. Atropine, antagonized the 5-HT- and MCh-induced bronchoconstrictions over the same antagonist dose-range. This suggests that the bronchoconstriction induced in the conscious guinea-pig by 5-HT is mediated primarily via muscarinic receptors, possibly by a vagal reflex. The inhibition of the responses to 5-HT and MCh by OA challenge would therefore appear to be related to interference with a common cholinergic pathway for these spasmogens. 7. In summary, airway hyperresponsiveness was evident at 24 h after OA challenge as measured by an enhanced bronchoconstrictor response to inhaled U-46619. When 5-HT or MCh were used as the spasmogens, an opposing decrease in responsiveness was observed. This was presumed to be due to an inhibition of cholinergic pathways by the OA challenge. Adenosine caused a bronchoconstriction in the sensitized animals but this was not enhanced by the OA challenge.

Effects of hypoxia and vascular tone on endothelium-dependent and -independent responses in developing lungs.

Both increases and decreases in endothelium-derived nitric oxide (EDNO) activity have been described in the developing pulmonary vasculature. We hypothesized that differences in baseline vasomotor tone and/or oxygen tension may contribute to this variability. Pulmonary arterial dose responses to endothelium-dependent and -independent vasodilators acetylcholine (ACh) and sodium nitroprusside (SNP), respectively, were measured in indomethacin-treated lungs of 1- to 2-day-old (2D) and 1-mo-old (1M) lambs. During 4% O2 ventilation, baseline pulmonary vascular resistance (PVR) and the dilator response to both ACh and SNP were greater in 2D lungs. However, when baseline PVR values were matched at both ages during either hypoxia or infusion of a thromboxane mimetic under normoxic conditions, developmental differences in ACh-induced vasodilation were minimal. Furthermore, hypoxia itself did not alter the responses to ACh in 2D lungs. In contrast, SNP caused greater vasodilation in 2D than in 1M lungs regardless of baseline PVR. These data and studies suggest that whereas high PVR enhances EDNO synthesis, responsiveness to ENDO decreases as synthesis of ENDO increases in developing lungs studied under basal conditions.

Endothelinb receptor agonists produce pulmonary vasodilation in intact newborn lambs with pulmonary hypertension.

The hemodynamic effects of endothelin-1 (ET-1) are mediated by at least two distinct receptors: ETa and ETb receptors. Recently, ETb receptor agonists (4 Ala ET-1 and IRL 1620) were developed. To investigate the role of ETb receptor activation on the pulmonary and systemic circulations, we studied the hemodynamic effects of intrapulmonary arterial injections of these receptor agonists in 10 intact newborn lambs. At rest, 4 Ala ET-1 (290-1,725 ng/kg) changed no hemodynamic variables. IRL 1620 (180-1,095 ng/kg) decreased mean pulmonary arterial pressure (PAP, 16.8% +/- 15.0 and 17.8% +/- 8.5, p < 0.05) and left pulmonary artery blood flow (21.6% +/- 22.1 and 33.4% +/- 27.7, p < 0.05) at the two highest doses only. During U46619-induced pulmonary hypertension, both 4 Ala ET-1 (3.2% +/- 8.0 to 15.9% +/- 6.4, p < 0.05) and IRL 1620 (8.7% +/- 6.3 to 21.9% +/- 4.1, p < 0.05) produced selective dose-dependent decreases in PAP. The decrease in mean PAP induced by 4 Ala ET-1 and IRL 1620 was attenuated by N omega-nitro-L-arginine [an inhibitor of endothelium-derived nitric oxide (EDNO) synthesis] (16.6% +/- 3.5 vs. 5.9% +/- 2.3 and 16.2% +/- 3.4 vs. 6.6% +/- 2.8, p < 0.05) and by glybenclamide (a blocker of ATP-dependent potassium channels) (18.2% +/- 7.9 vs. 7.5% +/- 8.3 and 14.7% +/- 3.6 vs. 6.3% +/- 3.2, p < 0.05). ETb receptor activation produces selective pulmonary vasodilation during pulmonary hypertension in intact newborn lambs. The vasodilating properties are mediated in part by release of ENDO and by potassium channel activation.

Effects of portal hypertension on responsiveness of rat mesenteric artery and aorta.

1. We have examined the effects of pre-hepatic portal hypertension on the responsiveness of rat small mesenteric arteries and aorta. Rats were made portal hypertensive by creating a calibrated portal vein stenosis, or sham-operated. 2. In rat mesenteric arteries, there was no significant difference between portal hypertensive and sham-operated animals in the contractile potency of noradrenaline (NA), but the maximum contractile responses to NA, U46619 and KCl were significantly increased in vessels from portal hypertensive animals. This altered maximum contractile response was not due to alterations in smooth muscle mass. 3. In rat mesenteric arteries, there were no significant differences between portal hypertensive and sham-operated animals in endothelium-dependent relaxations to acetylcholine (ACh). The difference between portal hypertensive and sham-operated rats in the maximum response to U46619 was maintained following a combination of methylene blue (1 microM) and NG-monomethyl-L-arginine (100 microM), suggesting that any differences in endothelial function do not explain differences in the response to vasoconstrictors. 4. In rat aorta, there were no significant differences between portal hypertensive and sham-operated animals in the contractile response to NA or KCl or in the endothelium-dependent relaxations to ACh. 5. In pithed rats, there was no difference between portal hypertensive and sham-operated animals in the pressor potency of NA. 6. It is concluded that portal hypertension produces an increase in the contractile response to the vasoconstrictors NA, U46619 and KCl in rat mesenteric arteries but not in the aorta. This suggests that the diminished responsiveness to vasoconstrictors reported in portal hypertensive rats in vivo is not due to a diminished responsiveness at the level of the vascular smooth muscle.

U-46619-induced ischaemic electrocardiographic changes in rats: preventive effects of prostacyclin and nitroglycerin.

The anti-anginal effect of nitroglycerin and prostacyclin was examined using, as an index, the ischaemic electrocardiogram (ECG) change (ST elevation) induced by intracoronary arterial injection of 9,11-dideoxy-11 alpha,9 alpha-epoxymethano-PGF2 alpha (U-46619), a stable thromboxane A2 agonist, in anaesthetized rats. The ST elevation induced by U-46619 (5-20 micrograms kg-1, i.c.a.) was dose-dependent and reproducible. U-46619-induced ST elevation was markedly prevented by the pretreatment of intravenous administration of prostacyclin (0.01 micrograms kg-1), and to a lesser extent by nitroglycerin (0.3 mg kg-1). Simultaneously, platelet count decreased significantly in the coronary arterial blood which indicated that platelet aggregation was enhanced by U-46619. The decrease of platelet count in coronary arterial blood at the time of ST elevation was significantly suppressed by prostacyclin (0.1 microgram kg-1, i.v.), but not by nitroglycerin (0.3 mg kg-1, i.v.). These results suggest that the ST elevation induced by intracoronary arterial injection of U-46619 may be derived from spasm of coronary artery and platelet aggregation in the intracoronary artery in rats.

Platelet-induced vasomotion of isolated canine coronary artery in the presence of halothane or isoflurane.

To determine the effect of 1.5 MAC of two volatile anesthetics (halothane and isoflurane) on platelet-induced contraction of canine coronary artery, isolated, denuded coronary rings were suspended between two stirrups, placed into organ chambers filled with an oxygenated Krebs-Ringer solution, and connected to an isometric force transducer. Human platelets were obtained from healthy donors and introduced into the organ chambers in increasing amounts to reach 20,50 and, 70 x 10(9) platelets/L. The tension generated in both the control and anesthetic-treated rings was recorded; only halothane reduced the tension induced by platelet activation in the organ chambers. In some experiments, aliquots of Krebs-Ringer solution were taken to determine the amount of 5-HT and TB2 released by 70 x 10(9) human platelets in the presence and absence of the anesthetics. Only halothane reduced TA2 production by the activated platelets. Finally, the contractile response of isolated denuded canine coronary artery rings to U46619, a thromboxane analog, was measured in the presence and absence of the anesthetics. Neither halothane nor isoflurane attenuated the response. In another series of experiments, in vitro platelet aggregation was induced by epinephrine, collagen, ADP, or arachidonic acid in the presence or absence of 1.5 MAC isoflurane or halothane. Both anesthetics significantly reduced the aggregation.

Different modes of endothelin-1 action in pressor response in vivo and pulmonary parenchymal contraction in vitro in the guinea pig.

Intravenously administered endothelin-1 (ET-1) (2 x 10(-11)-6 x 10(-10) mol/kg) induced dose-dependent pressor responses in anesthetized guinea pigs. Pretreatment with indomethacin (5 mg/kg, i.v.) or with a thromboxane A2/prostaglandin endoperoxide receptor antagonist, ONO-3708 (0.5 and 1.0 mg/kg, i.v.) significantly attenuated the pressor responses. ET-1 (10(-11)-10(-7) M) dose-dependently contracted guinea pig pulmonary parenchymal strips in vitro. However, neither pretreatment with indomethacin (10(-5) M) nor one with ONO-3708 (10(-6) M and 10(-5) M) significantly affected the ET-1-induced guinea pig pulmonary parenchymal contraction in vitro. Moreover, pretreatment with a platelet activating factor receptor antagonist, CV-3988 (2 x 10(-5) M) did not significantly affect the contraction. Thus, in guinea pigs, the mechanism of ET-1-induced pressor response in vivo mediated via cyclooxygenase-generated-eicosanoid(s), possibly, thromboxane A2 is not identical to that of ET-1-induced contraction of pulmonary parenchymal strips in vitro.

U 46619 induces different blood pressure effects in male and female spontaneously hypertensive rats (SHR).

The effect of U 46619 (5 micrograms/kg i.v.) alone or in combination with acetylsalicylic acid (ASA) (100 mg/kg po.) on mean arterial blood pressure (MABP) was investigated in male and female spontaneously hypertensive rats (SHR). In male SHR, a significant increase of MABP was observed 1 min after administration of U 46619. Pretreatment of male SHR with ASA delayed the increase of MABP after intravenous injection of U 46619 compared to U 46619 alone. Whereas in control animals the elevated MABP returned to baseline values 5 min after intravenous application of U 46619, the MABP of ASA-pretreated male SHR remained significantly increased by about 30 mmHg. In contrast, the MABP of female SHR did not respond to U 46619 alone or to the combination of U 46619 and ASA. Sex differences were further shown by the vascular formation of thromboxane B2 (TXB2). Whereas in male SHR the vascular formation of TXB2 was increased by U 46619, the TXB2 formation of female SHR was decreased. The vascular formation of 6-keto-PGF1 alpha of male and female SHR was not influenced by U 46619 alone or a combination of U 46619 and ASA. In conclusion, our results demonstrate that the blood pressure of SHR respond differently to the TXA2 mimetic U 46619 in the two sexes. Furthermore, by modulating blood pressure response to TXA2, vasoactive prostanoids may be significantly involved in the maintenance of hypertension with male SHR.

Pharmacological relaxation of the saphenous vein during harvesting for coronary artery bypass grafting.

Spasm of the saphenous vein frequently occurs during harvesting from the leg and high-pressure distension is required to restore an adequate diameter for grafting. Forceful distention has been shown to damage the intima and media and may predispose to subsequent occlusion of the vein graft. Various pharmacologic vasodilators are capable of relaxing veins; in this study, we carried out a systematic investigation to determine the appropriate agents and concentrations for use during vein graft operations. In organ baths, human saphenous vein segments were contracted with potassium or a thromboxane mimetic, and vasodilator agents were then applied. Glyceryl trinitrate, 7.2 micrograms/mL, or papaverine hydrochloride, 11.9 micrograms/mL, caused 80% to 100% relaxation of contraction induced by potassium or thromboxane. Verapamil, 15.5 micrograms/mL, relaxed the potassium contraction by 100% and the thromboxane contraction by 75%. Comparison of the time course of action showed that glyceryl trinitrate had a rapid onset and a short duration of maximal action, whereas verapamil (like papaverine) had delayed onset and a long duration. A mixture of glyceryl trinitrate and verapamil combined rapid onset with long duration of action. We now use a mixture of glyceryl trinitrate and verapamil (pH 7.4) topically and intraluminally during harvesting of the saphenous vein to provide a relaxed conduit for coronary artery bypass grafting.

Tolerance to glyceryl trinitrate in isolated human internal mammary arteries.

The purpose of this study was to examine the vascular reactivity of segments of internal mammary artery removed from patients undergoing coronary artery bypass operations. Responses to relaxant and contractile agents were compared in arteries removed from patients who had or had not been treated with glyceryl trinitrate after admission to the hospital until operation. Segments of mammary artery were removed from 13 patients who underwent coronary artery bypass grafting. Endothelium-containing rings of artery, 3 to 5 mm long, were suspended in physiologic saline solution in 20 ml organ baths. Responses to the endothelium-dependent relaxant acetylcholine and the endothelium-independent relaxants glyceryl trinitrate and sodium nitroprusside were compared. In addition, contractile responses to phenylephrine and 9,11-dideoxy-9 alpha,11 alpha-methanoepoxy prostaglandin F2 alpha (U46619) were examined. Glyceryl trinitrate-induced relaxation was significantly impaired in mammary artery segments from patients treated with that nitrate before operation; the responses to acetylcholine and sodium nitroprusside were not affected. Previous treatment with glyceryl trinitrate also reduced the contractile responses to both phenylephrine and U46619. These studies indicate that treatment of patients with glyceryl trinitrate before operation induces significant tolerance to this agent in the mammary artery; however, there was no evidence of cross tolerance to sodium nitroprusside or the endothelium-dependent vasodilator acetylcholine. Glyceryl trinitrate may therefore not always be effective in dilating mammary artery grafts and sodium nitroprusside may be a more effective dilator of the internal mammary artery in patients who have been treated with glyceryl trinitrate before operation.

Characterization of thromboxane and prostacyclin effects on pulmonary vascular resistance.

Although thromboxane and prostacyclin (PGI2) have long been described as major controllers of pulmonary vascular resistance, little has been reported on the characteristics of the interactions between the two arachidonic acid products. The current study uses segmental vascular resistance and compliance measurements to evaluate the actions of thromboxane and PGI2 in isolated blood-perfused rat lung. The thromboxane analogue U-46619 increases pulmonary vascular resistance by increasing only small artery resistance and decreases pulmonary vascular compliance in the middle compartment. Among the vascular effects of U-46619 are a maximum increase in resistance (RmaxU-46619) of 60.3 +/- 15.6 cmH2O.l-1.min.100 g-1 and a concentration required for 50% of maximum increase (K0.5,U-46619) of 1.60 +/- 0.85 nM for small artery resistance, a minimum vascular compliance (CminU-46619) of -0.93 +/- 0.58 cmH2O, and a K0.5,U-46619 of 1.10 +/- 1.60 nM for middle compartment compliance. Similar results were obtained for total resistance and total compliance. The effects of PGI2 on thromboxane-induced resistance and compliance changes were evaluated using K0.5,PGI2, RmaxPGI2, and CmaxPGI2 at each dose of thromboxane. PGI2 was more effective in reversing the thromboxane constriction at higher concentrations of thromboxane. These data show that the absolute concentration of PGI2 and thromboxane and not a simple ratio of thromboxane to PGI2 determines vascular tone.

Effects of intracoronary administration of endothelin in anesthetized dogs: comparison with Bay k 8644 and U 46619.

The effects of synthetic endothelin on the coronary circulation were studied in pentobarbital-anesthetized dogs and compared with those of Bay k 8644, a dihydropyridine calcium channel agonist, and U 46619, a thromboxane analogue. Intracoronary bolus administration of endothelin reduced coronary blood flow and increased coronary arterial resistance. Similarly, intracoronary bolus administration of equipotent doses of Bay k 8644 or U 46619 significantly reduced coronary blood flow and increased coronary arterial resistance. The coronary vasoconstrictor effects of endothelin were long-lasting as compared with the transient actions of Bay k 8644 and U 46619. Intracoronary bolus injection of endothelin also reduced left ventricular (LV) dP/dt arterial pressure (MAP), and cardiac output (CO). In contrast, Bay k 8644 increased LVdP/dt but did not alter CO or MAP. Intracoronary bolus injection of U 46619 did not affect MAP, CO, or LVdP/dt. In a separate group of animals, intracoronary infusion of nitrendipine significantly increased coronary blood flow and reduced coronary arterial resistance. Other cardiovascular parameters measured were not significantly altered. In the presence of nitrendipine, the effects of intracoronary administration of endothelin and U 46619 on coronary blood flow, coronary arterial resistance, and LVdP/dt were only partially antagonized. On the other hand, the effects of Bay k 8644 were completely prevented in the presence of nitrendipine. These studies show that at doses which reduce coronary blood flow to the same extent, only endothelin produces myocardial depression in anesthetized dogs. The cardiovascular actions of endothelin were only partially antagonized by nitrendipine, suggesting that mechanisms other than calcium influx through voltage-operated channels are involved.

In vivo attenuation of endothelium-dependent pulmonary vasodilation by methylene blue.

In vitro evidence suggests that resting pulmonary vascular tone and endothelium-dependent pulmonary vasodilation are mediated by changes in vascular smooth muscle concentrations of guanosine 3',5'-cyclic monophosphate (cGMP). We investigated this hypothesis in vivo in 19 mechanically ventilated intact lambs by determining the hemodynamic effects of methylene blue (a guanylate cyclase inhibitor) and then by comparing the hemodynamic response to five vasodilators during pulmonary hypertension induced by the infusion of U-46619 (a thromboxane A2 mimic) or methylene blue. Methylene blue caused a significant time-dependent increase in pulmonary arterial pressure. During U-46619 infusions, acetylcholine, ATP-MgCl2, sodium nitroprusside, isoproterenol, and 8-bromo-cGMP decreased pulmonary arterial pressure. During methylene blue infusions, the decreases in pulmonary arterial pressure caused by acetylcholine and ATP-MgCl2 (endothelium-dependent vasodilators) and sodium nitroprusside (an endothelium-independent guanylate cyclase-dependent vasodilator) were attenuated by greater than 50%. The decreases in pulmonary arterial pressure caused by isoproterenol and 8-bromo-cGMP (endothelium-independent vasodilators) were unchanged. This study in intact lambs supports the in vitro evidence that changes in vascular smooth muscle cell concentrations of cGMP in part mediate resting pulmonary vascular tone and endothelium-dependent pulmonary vasodilation.