Comparative effects of acetylcholine on the reptilian and mammalian aortae.

Responses of the lizard and the rabbit thoracic aortae to acetylcholine were studied. Acetylcholine (ACh) (10 nM-100 microM), carbachol (10 nM-50 microM) and PGF2 alpha (100 nM-100 microM) produced concentration-dependent contractions of the lizard thoracic aorta (LTA) with rapid tachyphylaxis. On the other hand, acetylcholine and carbachol relaxed the rabbit thoracic aorta (RTA) dose-dependently. There was no tachyphylaxis. Tachyphylaxis in the LTA was prevented by indomethacin (1 microM) or by rubbing the endothelium. Endothelial removal abolished ACh-induced relaxation of the RTA. Relaxation of the RTA and contractions of LTA were blocked by atropine, pirenzepine, acetoxy-N-methyl-piperidine methbromide (4-DAMP) and diltiazem. Removal of the endothelium rendered RTA insensitive to ACh. However, ACh-induced contractions of the LTA were not so influenced by removal of endothelial cells. It is concluded that the lizard thoracic aorta releases a metabolite of arachidonic acid that mediates tachyphylaxis. Relaxation of the RTA after ACh is mediated via the release of endothelium-dependent relaxing factor.

Extracellular calcium dependence of contraction and endothelium-dependent relaxation varies along the length of the aorta and its branches.

Dependence of contraction and endothelium-dependent relaxation of segments of the rabbit aorta and the celiac, superior mesenteric and renal arteries on the presence of calcium in the physiological salt solution in which they were bathed have been studied in vitro. Arteries were tested before and 60 min after removal of Ca++ from the bath solution. Norepinephrine-induced contraction and acetylcholine and A23187-induced relaxation were calcium-dependent in the thoracic and upper abdominal aorta. Contractions were resistant to the effect of calcium removal in segments from the aorta below the origin of the renal artery and from the renal artery itself. The aorta between the superior mesenteric and the renal arteries was intermediate in its dependence. Relaxation of the aorta caudal to the origin of the celiac artery and the renal artery was also resistant. The insertion of an intact everted vessel into an artery whose intima had been inactivated by rubbing restored partially the relaxation to acetylcholine and A23187 in the presence of normal calcium. Vessels that did not relax to acetylcholine after Ca++ exclusion from the bathing solution relaxed when the renal artery was the donor. Donor arteries whose relaxation was sensitive to Ca++ removal did not increase the relaxation response of the renal artery after calcium removal. Diltiazem and D600 reduced the relaxation responses of acetylcholine in the thoracic aorta and enhanced those in the renal artery. It is concluded that there is a transition in the extracellular Ca++ dependence of contraction and endothelium-derived relaxation along the length of the abdominal aorta. Changes in the dependence of contraction occurred more gradually than relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in postjunctional alpha-adrenoceptors during postnatal growth in rabbit arteries.

The pharmacological characteristics of the postjunctional alpha-adrenoceptors of vascular smooth muscle were studied in ring segments of thoracic aorta, superior mesenteric, and central ear arteries of 4- and 8-week-old, and young adult (12- to 16-week-old) rabbits. Norepinephrine (alpha 1, alpha 2-agonist), phenylephrine (alpha 1-agonist), and UK 14,304-18 (alpha 1-agonist) caused a concentration-dependent contraction of all three arterial segments from 4- and 8-week-old animals. Norepinephrine and phenylephrine but not UK 14,304-18 contracted the young adult thoracic aorta and superior mesenteric artery, whereas all the agonists contracted the central ear artery. The effects of the alpha 1-adrenoceptor antagonists, prazosin and thymoxamine, and the alpha 2-adrenoceptor antagonists, rauwolscine and yohimbine, on these responses were studied. In the 4-week-old rabbits, responses to norepinephrine, phenylephrine, and UK 14,304-18 were reduced by all four antagonists with nominal pA2 values in the range of 7-8.5. The action of the antagonists was competitive. Between 4 and 8 weeks, there was a significant decrease in the pA2 values of rauwolscine against norepinephrine and UK 14,304-18 in the aorta and between rauwolscine and UK 14,304-18 on the superior mesenteric artery. The pA2 values of rauwolscine and yohimbine, but not prazosin and thymoxamine, were lower in the young adult compared with values from the 8-week-old rabbit. In the interpretation of these results, there are two possibilities.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization of histamine receptors in the rabbit renal artery.

Histamine receptors were characterized pharmacologically in the rabbit renal artery. Histamine concentration-dependent contractions were blocked by chlorpheniramine (pA2 = 8.25 +/- 0.16) and mepyramine (pA2 = 9.80 +/- 0.22). A low concentration of metiamide (0.5 microM) enhanced significantly the effect of histamine. Higher concentrations either had no effect or reduced the contractile response to histamine. Impromidine, dimaprit and, in the presence of chlorpheniramine or mepyramine, histamine (at the concentrations that produced contractions) relaxed the precontracted vessel. The relaxant effect of histamine was quantitatively greater in the presence of mepyramine than it was in the presence of chlorpheniramine. Endothelial removal did not influence the contractile response to histamine but enhanced the relaxation suggesting the presence of histamine receptors mediating contraction in the intima. Relaxation responses to histamine and dimaprit were blocked competitively by cimetidine (pA2 = 6.65 +/- 0.10 and 6.60 +/- 0.05 respectively). It is concluded that there are two types of histamine receptors in the renal artery of the rabbit; H1-receptor mediating contraction and H2-receptor mediating relaxation. Both types of receptors are present in the media: some excitatory receptors may be present in the intima.

Pharmacological analysis of the postjunctional alpha-adrenoceptors of the rat anococcygeus muscle and vas deferens.

The alpha-adrenoceptor sub-type of the rat vas deferens was characterized with noradrenaline (alpha-1, alpha-2-agonist), phenylephrine (alpha-1-agonist), prazosin (alpha-1-antagonist) and yohimbine (alpha-2-antagonist) and compared with the results obtained in the rat anococcygeus muscle. In the vas deferens, both prazosin and phentolamine were competitive antagonists of noradrenaline and phenylephrine whereas yohimbine was a non-competitive antagonist irrespective of the concentrations of these antagonists. On the other hand, in the anococcygeus muscle, at low concentrations, prazosin was non-competitive against noradrenaline whilst it was competitive against phenylephrine. Yohimbine behaved as a competitive antagonist of noradrenaline at both low and high concentrations whilst at low concentrations it was non-competitive against phenylephrine. These results suggest a predominance of alpha-1-adrenoceptors in the vas deferens located postjunctionally with a small population of alpha-2-subtype adrenoceptors, whilst the anococcygeus muscle seems to contain relatively more alpha 2-adrenoceptors postjunctionally than the vas deferens.

Tricyclic antidepressants antagonize prostaglandin (PG) E2-induced contractions of the guinea pig ileum and hypomotility in the mouse.

The interactions of PGE2 and 2 tricyclic antidepressants were tested both on the guinea pig ileum and motility in the mouse. PGE2-induced contractions of the guinea pig ileum were irreversibly blocked by amitriptyline and desipramine. Chronic administration of amitriptyline and desipramine blocked PGE2-induced hypomotility in the mouse.

Interactions of 5-hydroxytryptamine and tricyclic antidepressants in the rat stomach strip and the guinea-pig ileum.

The interactions of the tricyclic antidepressants, amitriptyline and desipramine with 5-hydroxytryptamine (5-HT) were studied in the guinea-pig ileum and in the rat stomach strip. The tricyclics blocked the contractile response of the guinea-pig ileum and severely depressed the maximum [amitriptyline greater than desipramine (DMI)], however, they potentiated (DMI greater than amitriptyline) 5-HT-induced contractions of the RSS. The potentiation induced with DMI seems to be a combination of uptake inhibition and some post-receptor sensitization whilst amitriptyline-induced potentiation may be due to the former. Higher concentrations of the tricyclics contract the RSS. These contractions were absent in reserpinized RSS. Reserpinization reduced the potentiation after tricyclics and DMI was unable to enhance maximal concentrations of 5-HT, i.e. the post-receptor effect of DMI seems to have been abolished. It is concluded that whilst the tricyclics may be irreversible antagonists of 5-HT in the guinea-pig ileum, they potentiate the effect of 5-HT in the RSS by uptake inhibition, and, in the case of DMI, some additional post-receptor effect.

Effects of frusemide, ethacrynic acid and chlorothiazide on the portal vein of normotensive and spontaneously hypertensive rats.

The effects of frusemide, ethacrynic acid and chlorothiazide were studied in the Wistar Kyoto (WKY) rats and spontaneously hypertensive rat (SHR) portal veins in vitro. Frusemide produced concentration-dependent contractions of the portal vein from both WKY control and SHR. The WKY were more sensitive (as judged by the EC50) whilst a greater maximal response was generated in the SHR. The contractile response to frusemide in WKY rats was attenuated reversibly by phenoxybenzamine whereas the latter was ineffective in the SHR. Ethacrynic acid produced reductions of the myogenic rhythmic contractions of the portal vein in a concentration-related manner in both WKY and SHR. The inhibitory action of ethacrynic acid was greater in the SHR than in the WKY control rats. The inhibitory action of ethacrynic acid was blocked by propranolol suggesting the involvement of beta adrenoceptors. Chlorothiazide, on its own, had no observable effect on the portal vein. However, it reduced the sensitivity of the vein to noradrenaline and K+. This reduction was greater in the SHR than in the WKY. It is concluded that some of the observed effects of these diuretics may contribute beneficially to their anti-hypertensive actions.

Presynaptic alpha-receptor control of adrenergic transmitter release in blood vessels.

The presynaptic alpha-adrenergic receptor control of transmitter release in vascular tissues is discussed. A model of adrenergic innervation of the vascular bed is proposed based on ultrastructural and histochemical evidence. Evidence is presented to support the concept of intermittent or periodic release of norepinephrine (NE) from the varicosity. Intermittency combined with a mechanism such as presynaptic control to ensure spatial distribution of release sites, along with a slow effector response and recovery, results in a smooth, generalized change in tone and an overall economy of transmitter. The effective concentration of NE around the presynaptic membrane is maintained for considerably less than 0.1 s. It is argued that the transient presence of transmitter in the synapse combined with intermittency of release does not favor accumulation of transmitter at the cleft at physiological frequencies or desensitization of presynaptic receptors. In addition, intermittency provides an explanation for why exogenous NE is more effective presynaptically in influencing release than endogenous NE. The importance of cleft width in presynaptic control of transmitter release, the possible complications caused by facilitation, and resolution of some apparent problems with the presynaptic hypothesis are also discussed.

Are there alpha-adrenoceptors in the young chick atria?

The effect of noradrenaline and isoprenaline were studied in the 15-day old chick atria. Noradrenaline increased both the rate and force of atrial contractions whilst isoprenaline only slightly increased force at very high concentrations. The increase in force after noradrenaline and isoprenaline was reduced by phentolamine, an alpha-adrenoceptor antagonist. The beta-antagonist, propranolol and the cardioselective beta 1-adrenoceptor antagonist, atenolol, did not reduce the positive inotropy. Noradrenaline-induced positive chronotropy was, however, resistant to both alpha- and beta-adrenoceptor blockade. It is concluded that the force of contraction of the chick atria may be alpha-adrenoceptor-mediated whilst the rate seems to be mediated by a mechanism sensitive to noradrenaline but insensitive to alpha- and beta-adrenoceptor blockade.

Fasting reduces the responses of the rat vas deferens to sympathomimetics.

The effect of fasting, L-DOPA pretreatment and high glucose medium on the response of the rat vas deferens to sympathomimetics was studied in vitro. The contractile responses of the rat isolated vas deferens were reduced by fasting, the effect being significant after 24-hr fast with noradrenaline and tyramine, after a 48-hr fast with dopamine, and after a 72-hr fast with methoxamine. Treatment with L-DOPA for 7 days before sacrifice abolished the effect of a 24-hr fast in reducing responses to noradrenaline and tyramine, and the response to methoxamine after a 72-hr fast was enhanced. Doubling the concentration of glucose in the solution bathing the isolated vas deferens from rats fasted for 24 hr increased the responses to noradrenaline and tyramine. It is concluded that fasting reduces the responses of the rat vas deferens to sympathomimetics whilst L-DOPA pretreatment and high glucose restore and/or enhance the responses.

Comparative effects of noradrenaline and serotonin in a central and a peripheral artery in vitro.

A comparative study was done on a central and a peripheral artery of the rabbit with noradrenaline, serotonin and K+ in vitro. The aorta was more sensitive to NA than to 5-HT whilst the cerebral artery responded better to 5-HT. In both vessels, K+ produced similar responses. The alpha-adrenoceptor antagonist, phentolamine, and the tryptaminergic receptor antagonists, methysergide and cyproheptadine reduced, competitively, the effects of the agonists. There was no significant difference in the degree of antagonism between the cerebral artery and the thoracic aorta. Potassium-induced contractions were also reversibly reduced by phentolamine but not by 5-HT antagonists. There was no cross-tolerance between 5-HT and noradrenaline and K+ in these arteries. These results indicate that there are differences in the sensitivities of these vessels to the vasoconstrictors. This is interesting in view of the fact that 5-HT has often been implicated in migraine, a central phenomenon.

Analysis of the antagonism by prazosin of noradrenaline and phenylephrine induced contractions of the rat anococcygeus muscle.

1 Blockade of alpha-adrenoreceptors was analyzed with prazosin in the rat anococcygeus muscle contracted with noradrenaline (NA) and phenylephrine (PHE). 2 Prazosin (1.1 x 10(-8) - 8.8 x 10(-7)M) competitively blocked phenylephrine-induced contractions (pA2 = 8.93 +/- 0.04, slope = 0.85 +/- 0.07) whilst its antagonism of NA was non-competitive (pA2 = 8.45 +/- 0.04, slope = 0.46 +/- 0.03). 3 At higher concentrations, (1.76 x 10(-6) - 5.28 x 10(-6)M) prazosin competitively antagonized NA-induced contractions (pA2 = 7.58 +/- 0.11, slope = 0.93 +/- 0.09). 4 Phentolamine also reduced the effect of NA competitively (pA2 = 8.11 +/- 0.09, slope = 0.82 +/- 0.26). 5 It is concluded that low concentrations of prazosin selectively blocked an alpha 1-adrenoreceptor component of NA-mediated response whilst higher concentrations of prazosin non-selectively blocked both alpha 1- and alpha 2-adrenoreceptors.

Agonist action of yohimbine on the perfused rabbit central ear artery.

The agonist action of yohimbine, an alpha-adrenoceptor antagonist, was studied on the perfused ear artery of the rabbit. Yohimbine, 5-hydroxytryptamine (5-HT) and noradrenaline produced concentration-dependent vasoconstriction of the central ear artery and gave EC50 values of 3.5 X 10(-7), 7.5 X 10(-9) and 3.0 X 10(-10) M, respectively. Phentolamine significantly reduced the effects of yohimbine, 5-HT and noradrenaline, and gave pA2 values of 8.40 (slope = 1.0), 8.60 (slope = 0.94) and 8.20 (slope = 1.04), respectively. Methysergide, a serotonergic antagonist, significantly reduced yohimbine- and 5-HT-induced vasoconstriction but failed to influence the effect of noradrenaline. Reserpine treatment induced hypersensitivity of the ear artery to yohimbine and noradrenaline but not to 5-HT. It is suggested that the agonist action of yohimbine on the ear artery may be mediated via activation of alpha-noradrenoceptors.

Pharmacological characterization of some receptors in the rat caecum.

A simple method is described using the rat isolated caecum to screen autonomic agents. The caecum contracts to cholinomimetics and 5-HT and relaxes to sympathomimetics. Pharmacological receptors characterized in the tissue were muscarinic, and adrenergic in nature. The adrenoceptors appear to be principally a mixture of B-1 and B-2. In addition, the tissue probably contains alpha-adrenoceptors and serotonergic receptors. It did not respond to histamine. The tissue offers an advantage over the guinea-pig ileum in that it readily relaxes to sympathomimetics. It is suggested that the caecum may be useful in practical pharmacology classes.

Further studies on the mechanism of action of methyldopa on the vas deferens of the guinea-pig.

Methyldopa (400 micrograms/ml) abolished neurally evoked contractions of the guinea-pig vas deferens after 1.5 hr incubation. The tissue became supersensitive to noradrenaline and acetylcholine but was now insensitive to potassium chloride. On the other hand, tissues treated with 6-hydroxydopamine (6-OHDA) were supersensitive to noradrenaline but not to acetylcholine. The results suggest that cholinergic mechanisms may also be affected by methyldopa.

Prejunctional inhibitory dopamine receptors in the rat isolated vas deferens.

The effects of apomorphine, clonidine, dopamine and noradrenaline as well as their antagonists were studied on the neuronally stimulated rat vas deferens in vitro. All agonists investigated significantly inhibited neuronally induced contractions of the vas deferens. The following IC50 values were obtained: clonidine 4.2 nM, dopamine 16.9 nM, apomorphine 58 nM and noradrenaline 455 nM. Phentolamine, yohimbine and pimozide significantly enhanced, whilst haloperidol inhibited the twitch responses. All the antaganists reduced, to a varying degree, the twitch-inhibitory effect of the agonists. The twitch inhibition produced by apomorphine was, however, relatively resistant to antagonism by yohimbine, whilst the inhibition produced by clonidine and noradrenaline was relatively resistant to antogonism by pimozide. The results suggest that different prejunctional receptors mediate the effects of dopamine and apomorphine on the one hand, and noradrenaline and clonidine on the other hand.