Modifications of canine vascular smooth muscle responses to dihydroergotamine by endogenous prostaglandin synthesis.

Changes in tension were monitored isometrically on spiral strips from canine saphenous veins and arteries. Dihydroergotamine (DHE) (1.7 X 10(-8) M) contracted vein strips about 30% more strongly than arterial strips when the effects were compared to a standard noradrenaline (NA) response. Phentolamine (3.6 X 10(-6) M) reduced the DHE-induced contraction in veins by about 30% and in arteries by about 10%. Indomethacin (2.8 X 10(-7) M) reduced DHE-induced contractions in veins by about 60% but enhanced the DHE effects in the arteries. When arachidonic acid (AA) was added to stimulated vascular strips it caused further contractions in veins but relaxation in arteries. Both effects were inhibited after indomethacin. In veins AA was significantly more effective in the presence of DHE as compared to controls, to NA- or to KCl-stimulated strips. In arteries AA showed the strongest relaxant activity in the presence of NA. Low doses of prostaglandin E2, however, relaxed both venous and arterial strips. The results suggest that contraction of canine vascular smooth muscle is associated with synthesis of a prostaglandin-like substance of substances having constrictor activity in veins but relaxant activity in arteries and that in venous smooth muscle cells the formation of this material is enhanced by dihydroergotamine.     

Effects of calcium channel blockers on the contractile response to dihydroergotamine in isolated human femoral veins

Summary Contractions induced in spirally cut human postmortem femoral veins by dihydroergotamine, noradrenaline, or 5-hydroxytryptamine (5-HT) were measured isometrically. The influence of calcium channel blockers (verapamil, diltiazem and nifedipine), lanthanum chloride and withdrawal of external calcium on the contractile response of veins to dihydroergotamine was investigated. The agonists noradrenaline and 5-HT were studied in comparison. Verapamil and nifedipine up to a concentration of 1 mol/l caused a shift to the right of the cumulative concentration-response curves for noradrenaline and 5-HT with a reduction of the maximum contraction. Diltiazem affected the contractile response to 5-HT slightly. The venocontractions induced by dihydroergotamine at a concentration of 0.03 mol/l were inhibited by the calcium channel blockers in a concentration-dependent manner. Verapamil was most effective, followed by nifedipine and diltiazem. In dihydroergotamine-precontracted veins nifedipine produced a more pronounced relaxation than verapamil. Lanthanum chloride (5 mmol/l) inhibited the dihydroergotamine-induced contraction by about 90%. Exposure of veins to a calcium-free medium for 30 min resulted in an inhibition of the dihydroergolamine-induced venocontraction by more than 90%. The addition of calcium ions to the medium caused an increase in the contraction with a maximum which closely resembled that of the control. The present studies demonstrate that the contractile response of human femoral veins to dihydroergotamine depends to a great extent on extracellular calcium. However, the calcium channel blockers verapamil, diltiazem and nifedipine, inhibit the agonist-induced venocontractions in part only. Send offprint requests to E. Glusa at the above address     

Alpha-adrenoceptors, 5-hydroxytryptamine receptors and the action of dihydroergotamine in human venous preparations obtained during saphenectomy procedures for varicose veins

Summary Changes in tension were monitored isometrically on spiral strips from human saphenous veins obtained during surgical removal of varicose veins. Concentration-response curves for noradrenaline and 5-hydroxytryptamine (5-HT) were established by cumulative administrations, curves for dihydroergotamine were constructed from the mean responses to single concentrations. The use of the antagonists prazosin, yohimbine and pizotifen provided evidence for the existence of both postjunctional ₁- and ₂-adrenoceptors and for the existence of 5-HT receptors. The venoconstrictor effects of dihydroergotamine were unchanged by prazosin. Yohimbine antagonized both dihydroergotamine and 5-HT at about 60 times higher concentrations than required against noradrenaline whereas pizotifen inhibited responses to both dihydroergotamine and 5-HT at about 100 times lower concentrations than those to noradrenaline.These new results are in contrast to conclusions drawn from animal studies and do not support the suggestion that in man the venoconstrictor activity of dihydroergotamine is mediated through stimulation of -adrenoceptors. The present results strongly suggest that in human saphenous veins the constrictor activity of dihydroergotamine is mediated at least in part through stimulation of 5-HT receptors.     

α-adrenoceptors in human peripheral lung

In this study, strips of human peripheral lung tissue were used to investigate the presence of a population of α-adrenoceptors. Lung strips contracted in response to stimulation with noradrenaline, adrenaline, methoxamine and phenylephrine. In the presence of propranolol, responses to noradrenaline were antagonized by phentolamine and a pA₂ of 7.29 was obtained. It is concluded that human peripheral lung tissue contains a pharmacologically distinct population of α-adrenergic receptors.     

In vitro demonstration of the existence of β-adrenoceptors in human saphenous and canine femoral veins

Summary Helical strips of human saphenous (normal and varicose tissues) and canine femoral veins relaxed in response to isoproterenol when there was pre-existing tone. This effect was antagonised by the -adrenoceptor blocking drugs pronethalol and pindolol demonstrating the presence of -adrenoceptors. Using the canine femoral vein, dose-response curves for isoproterenol were determined in the presence of phenoxybenzamine and a depolarising concentration of potassium. Under these conditions, both -adrenoceptor blocking agents caused a parallel shift to the right of the dose-response curves. Approximately one hundred times more pronethalol than pindolol was needed to produce a similar degree of blockade.     

Venoconstrictor effect of dihydroergotamine in superficial hand veins

Summary The effect of dihydroergotamine on superficial veins of the hand has been investigated in healthy volunteers. The compliance of the veins was assessed by measurement of their diameter at a standard congestion pressure. Direct local infusion into the vein under study of 0.4 µg and 2.0 µg dihydroergotamine reduced venous compliance by 15±5% and 30±3%, respectively. Local infusion of phentolamine 20 µg, an -adrenoceptor blocking drug, did not affect venous compliance, but markedly inhibited the venoconstrictor effect of dihydroergotamine 2 µg. It was concluded that the venoconstrictor effect of dihydroergotamine demonstrated in this experiment was mainly due to -adrenoceptor stimulation. Venous compliance also decreased after oral administration of dihydroergotamine 10 mg; after 150 min, venous diameter was reduced by 30±5%.     

An alpha-adrenoceptor inhibitory action of kynuramine

Kynuramine, an endogenously occurring amine, inhibited the vasoconstrictor responses to norepinephrine in the isolated perfused mesenteric arteries of rats and blocked the relaxation of rabbit intestinal smooth muscle by phenylephrine. Similarly, kynuramine reversed clonidine-induced inhibition of the cholinergic twitch response in the guinea pig ileum. These effects, seen with concentration ranging from 4 to 60 microgram/ml, are consistent with an alpha-adrenoceptor inhibitory action of kynuramine at both presynaptic and postsynaptic sites. Kynuramine itself, lacked intrinsic activity on alpha-adrenoceptors and showed a complete lack of affinity for beta-adrenoceptors in the rabbit intestine and guinea pig tracheal chain preparation. None of the effects of kynuramine could be attributed to the formation of 4-hydroxyquinoline, the deaminated metabolite of kynuramine. Additionally, no evidence was obtained for an action on muscarinic receptors or non-specific effects on vascular smooth muscle. However, a slight, transient stimulant action was evident on intestinal smooth muscle. It is concluded that kynuramine inhibits both presynaptic and postsynaptic alpha-adrenoceptors in vitro and it is conceivable that the compound might function as an endogenous inhibitor of alpha-adrenoceptors in vivo.     

Adenosine antagonizes the positive inotropic action mediated via beta-, but not alpha-adrenoceptors in the rabbit papillary muscle

In the isolated rabbit papillary muscle, adenosine (1-300 microM) alone scarcely affected the basal tension developed. The positive inotropic action of isoprenaline, mediated via beta-adrenoceptors, was inhibited by adenosine in a concentration-dependent manner. Atropine (0.3 microM) abolish the inhibitory action of carbachol on the isoprenaline-induced positive inotropic action but not affect the inhibitory asction of adenosine. Adenosine failed to inhibit the positive inotropic action exerted by phenylephrine via stimulation of alpha-adrenoceptors in the presence of pindolol (30 nM). The present results indicate that the positive inotropic action was mediated via alpha-adrenoceptors whose subecllular mechanism was not susceptible to the inhibitory action of adenosine as are beta-adrenoceptors.     

Pharmacokinetic investigation of oral and IV dihydroergotamine in healthy subjects

Summary A new radioimmunoassay (RIA) for the specific measurement of dihydroergotamine (DHE), sufficiently sensitive for the determination of low plasma concentrations, has been used to investigate the pharmacokinetics of unchanged DHE. In a randomized crossover trial, eight healthy male volunteers received single doses of DHE 5 mg, 10 mg and 20 mg orally and 0.1 mg and 0.5 mg intravenously. It was possible to determine plasma concentrations and urinary excretion of DHE over the following 48 h. A long terminal plasma elimination phase of unchanged DHE (half-life 15 h) was found. A similar terminal elimination half-life was also calculated from urine data. The multi-exponential decline in plasma DHE with a long terminal half-life suggests that distribution into a deep compartment contributes to the long-lasting effect of the drug. Plasma protein binding was 93%. Despite extensive tissue distribution (V_z=33 l/kg) and a high plasma clearance (CL_P=2l/min), dose-independent linear pharmacokinetics was observed. The present assay was at least 20-times more specific than the polyvalent RIA used previously and appears suitable to explore the pharmacokinetics of unchanged DHE in patients on low-dose therapy. The long terminal elimination half-life of DHE only reported previously in studies using ³H-labelled drug, and considered to be due to metabolites, was also true for the parent compound. This, in addition to the sustained pharmacological activity of the 8-hydroxy metabolite already shown, provides a further explanation for the long duration of action of DHE in animals and man.     

Biochemical and mechanical effects of phenylephrine on the heart.

Injections of phenylephrine into isolated perfused guinea pig hearts increased cyclic AMP and phosphorylase a. Reserpine pretreatment or propranolol decreased or abolished the phenylephrine-induced biochemical changes without affecting the contractile response. Phentolamine, on the other hand, shifted the phenylephrine dose-response curve for contractility to the right without affecting the other parameters. The biochemical effects of phenylephrine are apparently due to the release of noradrenaline while the contractile effect is mediated directly through stimulation of alpha-adrenoceptors.     

Dual effect of dihydroergotamine and dihydroergotoxin in isolated human femoral veins and arteries

In human femoral vein strips, dihydroergotamine and dihydroergotoxin caused a slowly proceeding, long-lasting increase in tone. Dihydroergotamine had markedly higher affinity than noradrenaline, it possessed nearly the same intrinsic activity as noradrenaline. Dihydroergotoxin was less effective than dihydroergotamine. In isolated femoral artery strips, dihydroergotamine showed lower affinity and lower intrinsic activity in terms of noradrenaline. Dihydroergotoxin had negligible vasoconstrictor effects in arteries. In veins, dihydroergotoxin had a stronger alpha-adrenolytic effect than in arteries. In isolated arteries, dihydroergotoxin was equieffective to dihydroergotamine whose alpha-adrenoceptor blocking effect did not significantly differ in veins and arteries. Our results show that dihydroergotamine at very low concentrations exerted predominantly venoconstrictor effects and, therefore, it is more suited to increase the venous tone than dihydroergotoxin which at low concentrations showed alpha-adrenoceptor blocking activity.     

Characterization of the adrenoceptors in coronary arteries of pigs

The effects of phenylephrine and norepinephrine and the influence of adrenoceptor blockers on them were investigated on isolated strips of pig coronary arteries of different diameter. Phenylephrine contracted large coronary arteries, an effect inhibited by phentolamine, but was ineffective on small coronary arteries. Norepinephrine either relaxed large coronary arteries or occasionally contracted them. Phentolamine potentiated the relaxing effect of norepinephrine and propranolol inhibited or even reversed it into a contraction.Small coronary arteries were without exception relaxed by norepinephrine. This effect was not influenced by phentolamine but was inhibited by propranolol. These findings prove the presence of α-adrenoceptors in pig coronary arteries with a large diameter although β-adrenoceptors predominate; α-adrenoceptors are not found in coronary arteries with a small diameter.The classification of β-adrenoceptors in the coronary musculature into one of the two β-types was determined by comparing the affinities of propranolol, practolol and H 35/25 on the β-adrenoceptors of isolated atrial preparation of guinea pigs and of strips of pig coronary arteries. While the affinities of propranolol were the same to both organs the affinity of practolol to the myocardium was 100 times higher than to the coronary arteries. On the other hand the affinity of H 35/25 to the coronary arteries was three times higher than to the myocardium.The selective effects of the β₁-adrenoceptor blocker practolol on the myocardium and the β₂-adrenoceptor blocker H 35/25 on the coronary arteries indicate that the β-adrenoceptors of the coronary arteries should be classified as β₂-adrenoceptors.     

Studies on the mode of action of methyridine

Methyridine [2-(β-methoxyethyl)pyridine] reached the blood and all regions of the alimentary canal following oral or subcutaneous administration of therapeutic doses of the drug. Concentrations in the lower gut were highest following subcutaneous administration, and there was a close association between blood and intestinal levels suggesting that the drug is excreted into the alimentary canal along its whole length. In vitro experiments with intact nematodes (Ostertagia spp. from sheep; Nematospiroides dubius and Heterakis spumosa from mice; and Nippostrongylus muris from rats) and with preparations of Ascaris sp. have demonstrated that methyridine passes through the cuticle and exerts an “irreversible” paralysing effect on the worms. This action is obtained within 1 hr at concentrations similar to peak levels shown to be present in the alimentary canal following the administration of therapeutic doses. Concentrations of the drug were found to be higher in the abomasum than in the intestine of sheep and yet the anthelmintic activity of methyridine is greatest against intestinal nematodes. No significant differences in the resistance of various nematode species to the drug were demonstrated, but the amount of methyridine entering ascarids was significantly greater at pH 8 than at pH 5 or 3. In addition, the in vitro anthelmintic activity of the compound was greatest in alkaline solutions, which probably accounts for the differences in anthelmintic activity noted in various parts of the alimentary canal. The paralysing effect of methyridine on nematodes is probably due to a neuromuscular block of the decamethonium type, and this can also be produced in the host by over-dosage. It is suggested that nematode tissues are more sensitive to this action than vertebrate tissues.     

Interaction of clonidine with pre- and post-synaptic adrenergic receptors of rat brain: effects on cyclic AMP-generating systems.

The locus and mechanism of interaction of clonidine with catecholamine-elicited accumulations of cyclic AMP has been investigated in brain slices from control and 6-hydroxydopamine-treated rats of the F-344 and Sprague-Daeley strains. The inhibitory effects of clonidine on the norepinephrine-stimulated accumulation of cyclic AMP and the potentiative effects of clonidine on the isoproterenol-stimulated accumulation of cyclic AMP are present to the same extent in cerebral cortical slices from control and 6-hydroxydopamine-treated Sprague-Dawley rats. Clonidine, at concentrations between 0.0001 and 100 muM has no intrinsic stimulatory activity on cortical cyclic AMP-generating systems from either control or 6-hydroxydopamine-treated rats. In F-344 rats phenoxybenzamine (100 muM) elicits a significant accumulation of cyclic AMP in cerebral cortical slices which can be abolished by pretreatment of rats with 6-hydroxydopamine, or by incubation of tissue slices with either clonidine or sotalol. The hyperresponsiveness to catecholamines usually observed following central administration of 6-hydroxydopamine failed to develop in the F-344 rat. Concentrations of phenoxybenzamine which have no significant stimulatory effects on cyclic AMP accumulation are capable of abolishing the potentiative effects of clonidine on the isoproterenol-stimulated formation of cyclic AMP. The beta-antagonist, sotalol, is a less effective antagonist of isoproterenol-stimulated accumulation of cyclic AMP in the presence of clonidine. Clonidine has no significant effect on the accumulation of cyclic AMP elicited by submaximal concentrations of isoproterenol in cerebellar slices. In toto, the data are consistent with the hypothesis that both the inhibitory effects on norepinephrine-stimulated accumulation of cyclic AMP and the stimulatory effects of clonidine on isoproterenol-elicited accumulation of cyclic AMP are exerted at postsynaptic alpha-adrenoceptors. No evidence was found for a presynaptic generation of cyclic AMP, although clonidine does reverse the stimulatory effects of phenoxybenzamine on cyclic AMP accumulation, presumably by interaction with a presynaptic site controlling norepinephrine release.     

Studies on the mode of action of ciguateric toxins

The effects of ciguatoxin, scaritoxin and maitotoxin, the main toxins involved in ciguatera fish poisoning, has been studied in pentobarbital anaesthetized cats. Intraveinous injections of increasing doses of these toxins (5 to 160 μg/kg of partially purified samples) evoked respiratory and cardiovascular disturbances: hyperventilation at low doses and respiratory depression leading to respiratory arrest at high doses; bradycardia and troubles of the atrioventricular conduction at low doses, arrhythmias and ventricular tachycardia with transient hypertension at sublethal doses, and falling arterial pressure leading to complete heart failure at high doses.The mode of action of ciguatoxin has been studied by testing the preventive efects of pharmacological compounds such as hexamethonium, atropine, propanolol and phentolamine and by proceeding to bilateral adrenalectomy. The results have indicated both central and peripheral effects. Cholinergic and also α - adrenergic actions were pointed out.