Adrenergic stimulation of vascular prostacyclin: role of alpha 1-receptors in smooth muscle cells

Sex differences in vascular reactivity of coronary artery bypass graft conduits

Females have increase in-hospital mortality and poorer outcomes following coronary artery bypass grafting (CABG). Biological differences in the reactivity of the graft conduits to circulating catecholamine may contribute to this sex difference. This study examined sex differences in the vasoconstrictor responses of internal mammary artery (IMA) and saphenous vein (SV) conduits to phenylephrine (PE) and endothelin-1 (ET-1). Functional IMA and SV were obtained from 78 male and 50 female patients undergoing CABG (67.7 ± 11 and 69 ± 10 years, respectively) and subjected to the following experimental conditions. (1) Concentration response curves for PE and ET-1 were generated in an intact IMA and SV and endothelium denuded IMA segments, (2) in the presence of the nitric oxide synthase inhibitor (L-NAME) or the cyclooxygenase inhibitor (indomethacin) in an endothelium-intact IMA and (3) the activity state (abundance and phosphorylation) of the α1-adrenergic receptor was investigated using Phos-tag™ western blot analysis. (1) Compared to male, female IMA and SV were hypersensitive to PE but not ET-1 (p < 0.05). The female IMA hypersensitivity response to PE was abolished following endothelial denudation, (2) persisted in the presence of L-NAME but was abolished in the presence of indomethacin and (3) there was no sex differences in the abundance and phosphorylation of the α1-adrenergic receptor in IMA. Female IMA and SV graft conduits are hypersensitive to α1-adrenergic stimuli. This endothelial cyclooxygenase pathway-mediated hypersensitivity may produce excessive IMA and SV graft constriction in females administered catecholamines and could contribute to their poorer CABG outcomes.

Prostacyclin and thromboxane production of rat and cat arterial tissue is altered independently by several vasoactive substances

The modulation of the production of prostacyclin and thromboxane from cat and cat aortic tissue slices by different vasoactive agents has been studied in order to reveal whether the release of these main two vasoactive prostanoids goes in parallel or may be controlled independently. Norepinephrine, isoproterenol, phentolamine, propranolol, angiotensin II, vasopressin, bradykinin, thrombin, verapamil, gallopamil, dopamine or methionin enkephalin were added to the incubation medium and 6-keto-PGF1 alpha (the stable metabolite of prostacyclin) and TxB2 (the stable metabolite of thromboxane) were determined in the supernatant by radioimmunoassay. The ratio of the release of prostacyclin and thromboxane was computed. Norepinephrine increased both prostacyclin and thromboxane release. Isoproterenol increased the ratio of prostacyclin and thromboxane released in cat aortic tissue slices. Phentolamine and propranolol had no effects. Angiotensin II induced a slight but statistically insignificant increase in the ratio of the two prostanoids released. Vasopressin increased thromboxane release only. Bradykinin stimulated the prostacyclin while thrombin stimulated the thromboxane release. Verapamil decreased both prostacyclin and thromboxane production. Gallopamil decreased prostacyclin release and increased thromboxane release from vessel wall slices in a certain concentration range causing a characteristic dose dependent minimum in the ratio of prostacyclin and thromboxane release. Dopamine separately increased prostacyclin release while enkephalin had no significant effect. The data obtained show that in vascular tissue some unidentified yet cytophysiological mechanisms might exist which specifically control the activities of the prostacyclin synthase and thromboxane synthase enzymes.

Altered endothelium-dependent alpha-adrenergic vascular reactivity following exposure to Pasteurella haemolytica antigens

Rats were injected with sterile saline (controls), 10(5) cfu of Pasteurella haemolytica (biotype A) obtained from a commercial vaccine, or a commercial Pasteurella leukotoxoid vaccine. Three days after vaccination, the animals were killed and the thoracic aorta was removed. In some experiments the vascular endothelium was mechanically removed. Each isolated aorta was placed in a tissue bath and the biophysical responses to methoxamine (alpha-1 agonist) were determined. In separate experiments the endothelial surface was examined by scanning electron microscopy. In endothelium-intact vessels both vaccines appeared to enhance the contractile response to methoxamine. On the other hand, in endothelial-denuded vessels, the methoxamine-mediated contractile response was enhanced in the P. haemolytica-treated group but not in animals vaccinated with leukotoxoid. Furthermore, scanning electron microscopy revealed deposition of a fibrin-like material on the endothelial surface of vaccinated animals. These results suggest that exposure to vaccine-derived P. haemolytica antigens alters the morphology and adrenergic responsiveness of vascular smooth muscle.

Role of endothelium on alpha-adrenoceptor responsiveness and prostacyclin release from the mesenteric arterial bed of the rat

1 Removal of endothelium from the isolated perfused mesenteric arterial bed (MAB) of the rat was associated with an increase in both basal release of prostacyclin and the pressor response to administered noradrenaline (NA). Under these conditions, the NA-stimulated release of prostacyclin was not altered. 2 In this preparation, prazosin, an alpha 1-adrenoceptor antagonist, caused a dose-dependent (10(-10)-10(-8) M) decrease in NA-stimulated prostacyclin production, whereas rauwolscine, an alpha 2-adrenoceptor antagonist (10(-8)-10(-7) M) had no significant effect. In addition, prazosin inhibited the NA-induced pressor responses (10(-10)-10(-8) M) while rauwolscine was only effective at a concentration of 10(-7) M. 3 L-NG mono-methyl-arginine (L-NMMA), an inhibitor of endothelial nitric oxide (NO) synthesis, was used to assess whether elimination of this substance was responsible for the increased basal release of prostacyclin and/or increased pressor responses observed after endothelium removal in the MAB. Concentrations of 3 x 10(-7)-3 x 10(-6) M of L-NMMA were without effect on either prostacyclin release or pressor responses to NA in the intact MAB. 4 These results indicate that in the endothelial denuded MAB, NA-induced release of prostacyclin is, mainly, alpha 1-adrenoceptor-mediated, in contrast to the intact MAB where it is alpha 2-adrenoceptor-dependent. It therefore appears that, in the MAB of the rat, the presence of endothelium obscures the alpha 1-mediated release of prostacyclin from vascular smooth muscle cells. In this preparation the endothelium may regulate both the release of prostacyclin and the contractile responses of the underlying smooth muscle via mechanisms independent of NO formation.

Inhibition of iloprost of the contractile effect of noradrenaline in mesenteric artery rings: evidence for a possible calcium-dependent mechanism

Iloprost caused a concentration-dependent decrease in the response to noradrenaline in the rabbit isolated endothelium denuded rings from superior mesenteric artery but not thoracic aorta. Similar inhibition was obtained by verapamil using identical concentrations. In Ca(2+)-free EGTA containing medium noradrenaline both at lower and higher concentrations elicited a reduced contractile response and further addition of Ca2+ (2.5 mM) to the medium produced a second contraction in both mesenteric artery and aortic rings which was significantly and equally inhibited by iloprost and verapamil using identical concentrations in mesenteric artery but not in aortic rings. Prior addition of iloprost to the medium did not protect the inhibitory effect of phenoxybenzamine against noradrenaline-induced contraction. These results were taken as an evidence for the possible Ca2+ entry reducing effect of iloprost in mesenteric artery but not thoracic aorta. These results were also taken as an indirect evidence supporting the hypothesis that increased synthesis of prostacyclin by noradrenaline in the vascular wall may inhibit the contractile effect of the agonist by a (-) feedback mechanism mediated by Ca2+ entry into the vascular smooth muscle.

Repeated sympathetic stimuli elicit the decline and disappearance of prostaglandin modulation and an increase of vascular resistance in humans

To investigate the role of prostaglandins I2 and E2 in modulating the vasoconstrictor response to sympathetic stimulation, repeated and proximate cold pressor tests were performed in 23 healthy young volunteers. Limb vascular resistance (blood flow measured by venous occlusion plethysmography), prostaglandin I2 (as 6-ketoprostaglandin F1 alpha) and prostaglandin E2 plasma levels (detected by radioimmunoassay), and plasma catecholamines (detected by high-performance liquid chromatography and electrochemical detection) were measured. A progressive increase in the duration of the vasoconstrictor response was observed with repetition of cold applications (p less than 0.001, by analysis of variance for trends). This increase was associated with a progressive decrease in cold-induced elevation of 6-ketoprostaglandin F1 alpha and prostaglandin E2 plasma levels until, after five stimulations, neither prostaglandin was detectable. The maximum detected concentration of norepinephrine did not significantly change, but its area under the curve in time showed a trend toward an increase. Epinephrine levels did not significantly change. The increase of vascular resistance was significantly correlated with the decrease of both prostaglandins (r = 0.93, p less than 0.05 for prostaglandin E2 and r = 0.89, p less than 0.05 for 6-ketoprostaglandin F1 alpha), whereas no significant correlations were found between variations of vascular resistance and catecholamines. Prostaglandin blockade induced by diclofenac sodium administration caused, from the first cold application, a pattern of the vasoconstrictor response and plasma prostaglandin and norepinephrine changes similar to that observed at the fifth cold application in untreated subjects, when prostaglandins are no longer detectable in plasma. We conclude that an increased vasoconstrictor response to sympathetic stimulation in humans may result from a diminished inhibitory influence of prostaglandins on adrenergic transmission.

Defective coronary prostaglandin modulation in anginal patients

In order to investigate whether coronary vasodilating prostaglandins (PGI2 and PGE2) have a role in the pathophysiology of myocardial ischemia, 26 patients with angina pectoris and 23 control subjects (nonischemic patients) were studied by assessing coronary hemodynamics and prostaglandin formation in relation to sympathetic stimulation. Following a cold pressor test (CPT), coronary prostaglandin output markedly increased (p less than 0.001) and coronary vascular resistance (CVR) decreased (p less than 0.001) in all control subjects. In contrast, in anginal patients prostaglandins in the coronary sinus were undetectable and after CPT prostaglandin output did not increase, whereas CVR paradoxically increased (p less than 0.001). In control subjects the inhibition of coronary prostaglandin formation (by ketoprofen [1 mg/kg intravenously] or by aspirin [15 mg/kg intravenously]) caused a paradoxical increase of CVR following CPT (p less than 0.001). In anginal patients the inhibition of prostaglandins further exaggerated the increase of CVR after CPT (p less than 0.001). These results indicate that coronary vasodilating prostaglandin PGI2 and PGE2 play a role in modulating coronary vascular response to sympathetic stimulation induced by CPT. Their defective production in anginal patients may be responsible for the paradoxical increase in CVR following sympathetic stimulation.

Contractile responses to hypoxia of isolated rings from the left branch of rabbit pulmonary artery

Contractile responses of left extrapulmonary artery segments with either intact or damaged endothelium were examined with changes in PO2 at constant pH. Hypoxia consistently reduced noradrenaline-induced contractions. This hypoxia-induced relaxation was sometimes followed by a contraction and a second relaxation. Hypoxia-induced relaxations were also obtained if precontraction was elicited with KCl (40 mM), but no triphasic response was observed. Relaxations faded away with time and only contractions were then observed. Relaxations were more considerable in rings from young animals and, although always present, decreased with the age of the animal. Endothelium damage reduced hypoxia-induced relaxations. Indomethacin, a potent blocker of cyclooxygenase, increased hypoxia-induced contraction and reduced relaxation in segments with intact endothelium. Without endothelium the indomethacin effect was less significant. It is concluded that the response to hypoxia of the pulmonary artery is similar to that of systemic vessels. Endothelium seems to play a modulatory role in the hypoxia-induced response in extrapulmonary artery. Prostaglandin metabolism seems to play a minor role in this modulation.

Possible calcium channel modulating activity of iloprost in rabbit isolated vascular segments

1. Iloprost produced a concentration-dependent decrease in the contractile responses to K+ and NA in CoeA and SMA strips without altering the responses in RA, MPA, CaA and ThA. 2. Nifedipine and verapamil also inhibited the contractile responses to NA and K+ in CoeA and SMA rings in a concentration dependent manner. 3. Removal of endothelium did not alter the inhibitory effects of iloprost and Ca2+ channel blockers in the investigated arterial segments. 4. In Ca2+ free and high K+ medium both iloprost and Ca2+ channel blockers potentially inhibited the contractile effect of external Ca2+. 5. Iloprost at lowest concentration which is not influence the contractile response of K+ can potentiate the inhibitory effect of nifedipine in CoeA and SMA rings without altering its inhibitory effect on NA responses. 6. These results suggest that iloprost may modulate Ca2+ entry into the cell probably by acting through the potential-operated Ca2+ channels in CoeA and SMA. These results also indicate that Ca2+ channels are heterogenously functioned in various vascular segments of rabbit.

Release of prostacyclin from the dog saphenous vein by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) stimulated the release of prostacyclin (PGI2) from strips of the dog saphenous vein. This stimulation was maintained after mechanical removal of the endothelium. The effect of 5-HT was mimicked by alpha-methyl-5-hydroxytryptamine (alpha-CH3-5-HT) but not by 5-carboxamidotryptamine (5-CT) and it was inhibited by ritanserin. It is concluded that, in the dog saphenous vein, the stimulation of PGI2 release by 5-HT involves 5-HT2 receptors distinct from the '5-HT1-like' receptors that mediate contraction of the vessel.