The involvement of Ca2+ and Mg2+ in the spontaneous and drug induced release of (3H)noradrenaline from mesenteric arteries

Effects of monoamines formed in the cecum of horses on equine digital blood vessels and platelets

OBJECTIVE

To determine in vitro vasoactive potency of monoamines formed in the cecum and found in the systemic circulation of horses.

SAMPLE POPULATION

Segments of digital blood vessels obtained from 6 healthy mixed-breed horses and ponies euthanatized at an abattoir and platelets isolated from 4 healthy ponies.

PROCEDURE

Paired rings of digital artery and vein from the same horse were examined, and isometric tension was recorded. Concentration-response curves for tryptamine (TRP), tyramine (TYR), phenylethylamine (PEA), isoamylamine (IAA), and isobutylamine (IBA) were obtained. Vasoconstrictor mechanisms were investigated for TRP and TYR by the use of antagonists. Washed platelets loaded with [3H]-5-hydroxytryptamine (5-HT) were incubated with monoamines; the amount of radioactivity displaced after 30 minutes was estimated.

RESULTS

TRP, TYR, and PEA were potent constrictors of arteries and veins, with TRP and TYR being more potent in veins than arteries. Constrictions induced by TYR were inhibited by benextramine (alpha-antagonist) and nisoxetine (neuronal-uptake blocker), whereas TRP responses were inhibited by ketanserin (5-HT receptor antagonist). All 5 amines displaced 5-HT from platelets with the order of potency being TYR > TRP > PEA > IAA > IBA.

CONCLUSIONS AND CLINICAL RELEVANCE

Amines from the equine cecum cause digital vasoconstriction. The most potent (TRP and TYR) cause selective venoconstriction. Tyrosine activates predominantly alpha-adrenoceptors through the release of neuronal norepinephrine, whereas TRP activates 5-HT receptors. All amines tested released 5-HT from platelets. Amines formed in the cecum and released into the systemic circulation warrant additional investigation as trigger factors for digital ischemia and subsequent laminitis.

Uptake of 5-hydroxytryptamine by equine digital vein endothelial cells: inhibition by amines found in the equine caecum

REASONS FOR PERFORMING STUDY

5-hydroxytryptamine (5-HT; serotonin) is a potent vasoconstrictor of equine digital blood vessels and has been implicated in the pathogenesis of acute laminitis.

OBJECTIVES

The aims of this study were firstly to examine whether cells of the digital blood vessel wall exhibited an active uptake mechanism for 5-HT and to characterise its efficiency; and secondly, to study the potential inhibitory effect on this process of other amines, produced in the equine caecum.

METHODS

Confluent monolayers of equine digital vein endothelial cells (EDVEC) and equine digital vein smooth muscle cells (EDVSMC) were incubated with [3H]5-HT (0.1-250 micromol/l) and the total and active uptake calculated. Equine pulmonary vein endothelial cells (EPVEC) were used as a positive control.

RESULTS

Both EDVEC and EDVSMC showed uptake of [3H]5-HT by nonfaci litated diffusion; however, only EDVEC showed evidence of saturable facilitated uptake mechanism, with a Km of 41.6 +/- 9.3 micromol/l, which was significantly higher than that of EPVEC (9.9 +/- 2.1 micromol/l). All 6 caecally-derived amines examined (tyramine, spermine, isoamylamine, tryptamine, phenylethylamine and isobutylamine) inhibited the total uptake of [3H]5-HT in a concentration-dependent manner, tyramine having the lowest IC50 (3.7 x 10(-6) mol/l).

CONCLUSIONS

These data suggest that facilitated uptake into the endothelium could play a role in moderating the vasoconstrictor response to 5-HT in the equine digital circulation.

POTENTIAL CLINICAL RELEVANCE

The vasoconstrictor action of 5-HT could be potentiated by gut-derived amines, providing a feasible link between GI disturbances and the pathophysiology of laminits.

Effects of tyramine on the human uterine artery in vitro

1. We have studied the effects of tyramine on the human uterine artery (HUA) in order to assess its site of action. 2. Tyramine (5 x 10(-5) to 10(-3) M) contracts the isolated human uterine artery. Tachyphylaxis appeared when concentration-response curves were repeated and the contraction was diminished by prazosin (10(-8), 10(-7) and 10(-6) M). The maximal contraction induced by tyramine (10(-3) M) was 25% of the maximal response to noradrenaline (10(-5) M). 3. After 2 hr of tyramine perfusion a decrease of the contractile response to KCl, 30 and 60 mM (15.0 and 12.9%) and noradrenaline 10(-6) M (83.2%) is shown. 4. However, when tyramine was previously added for 3 min to the bath, the response to KCl increased while the response to noradrenaline was lower. 5. A possible postsynaptic antagonistic effect for tyramine in the HUA is suggested in addition to its usual presynaptic effect.

The influence of amine metabolizing enzymes on the pharmacology of tyramine in the isolated perfused mesenteric arterial bed of the rat

1. The pressor response to the infusion of tyramine (Tyr) into the isolated perfused mesenteric arterial bed of the rat has been studied at both a low and a high dose (0.2 and 2.0 mumol) and the effect of monoamine oxidase-A (MAO-A) and semicarbazide-sensitive amine oxidase (SSAO) inhibition was examined. Very little MAO-B activity is found in homogenates of this tissue when Tyr is used as substrate. 2. Inhibition of SSAO by treating rats with 1 mg kg-1 (E)-2-(3',4'-dimethoxyphenyl)-3-fluoroally lamine (MDL 72145) 1 h before dissection, had no significant effect on the maximum pressure attained or the area under the curve (AUC) of the response to both low and high doses of Tyr. Inhibition of MAO-A, by inclusion of 10 microM clorgyline in the perfusing fluid, resulted in no significant potentiation at both low or high doses of Tyr. The inhibition of both these enzymes together substantially increased the AUC of the pressor response. 3. Cocaine (3 microM) significantly potentiated the responses to adrenaline (Ad). At this dose, cocaine significantly reduced the peak height and the AUC of the responses to both doses of Tyr. 4. Inhibition of extraneuronal uptake mechanisms with corticosterone (29 microM) did not potentiate the response to Ad and did not significantly alter the response to Tyr (low dose). 5. The effects of MDL 72145 and clorgyline on the directly acting amine, Ad, were studied. MDL 72145 caused a small but significant increase in the EC50 and in the maximum response to Ad, whilst clorgyline (10 microM) increased the EC50 value slightly and decreased the maximum response.

Calcium dependence of prejunctional inhibitory effects of adenosine and acetylcholine on adrenergic neurotransmission in canine saphenous veins

In canine blood vessels acetylcholine and adenosine inhibit the exocytotic release of norepinephrine during nerve stimulation. The present experiments were designed to determine the Ca2+ dependence of these prejunctional effects. Segments of canine saphenous veins were mounted for isometric tension recording in organ chambers filled with Krebs-Ringer or Tyrode solution. Altering the Ca2+ concentration of the solution did not affect the inhibitory response to acetylcholine during nerve stimulation; the prejunctional potency of adenosine was inversely related to the Ca2+ concentration of the bath content. the ionophore A23187 caused contractions which were inhibited by phentolamine, verapamil, and adenosine but were augmented by acetylcholine. Helical strips of dog saphenous veins were incubated in [3H]norepinephrine and mounted for superfusion and determination of [3H]norepinephrine in the superfusate. A23187 increased the overflow of [3H]norepinephrine. Acetylcholine augmented this efflux; by contrast adenosine decreased the release induced by the ionophore. The results demonstrate that the prejunctional effect of acetylcholine was not due to direct interference with the availability of Ca2+ for the electro-secretory process in adrenergic nerve terminals and suggest that adenosine interferes either with the coupling role of the activator ion or its extrusion from the neuroplasm.

The effects of acetylstrophanthidin and ouabain on the sympathetic adrenergic neuroeffector junction in canine vascular smooth muscle

We performed experiments to determine the effects of acetylstrophanthidin (ACS) and ouabain on the adrenergic neuroeffector junction in dog saphenous veins. In quiescent strips incubated with 3H-norepinephrine (3H-NE), the drugs caused contraction and a progressive increase in overflow of 3H-NE and O-methylated metabolites; 3,4-dihydroxyphenylglycol (DOPEG) decreased. Tissue uptake of 3H-NE was partially inhibited. After surgical sympathectomy, both contraction and 3H-NE overflow were markedly attenuated. Following chemical sympathectomy with 6-hydroxydopamine, ouabain contractions were 11% of control, whereas the contractions due to exogenous norepinephrine were exaggerated. The initial overflow of 3H-NE was unaffected by tetrodotoxin, but the later and larger overflow with prolonged exposure was depressed. The former occurred in the absence of Ca2+, but the latter was Ca2+ dependent. Inhibition of the neuronal amine carrier by cocaine or desipramine and blockade of the neuronal alpha-adrenoceptors with phentolamine or phenoxybenzamine attenuated the release of 3H-NE evoked by ACS and ouabain. During electrical stimulation, ACS augmented the overflow of 3H-NE. This was attenuated by cocaine, desipramine, and the alpha-adrenolytic drugs. ACS, like pargyline, augmented the overflow of 3H-NE evoked by tyramine and depressed that of DOPEG. These experiments suggest that acetylstrophanthidin and ouabain (1) cause contraction of vascular smooth muscle by displacement of norepinephrine from neuronal stores, (2) reduce neuronal monoamine oxidase activity, (3) facilitate and may trigger Ca2+-dependent exocytotic release of norepinephrine, (4) partially inhibit the neuronal amine carrier mechanism but do not interfere with extraneuronal disposition of norepinephrine, and, finally (5) may have unexplained interactions with prejunctional alpha-adrenoceptors.

Release of [3H]noradrenaline induced by 5-hydroxytryptamine from cat pial arteries

Pial arteries of cats were used to analyse the effects of 5-hydroxytryptamine (5-HT) on the release of [3H]noradrenaline. To achieve this the vessels were preincubated with [3H]noradrenaline and the effect of different concentrations of 5-HT (10(-6), 10(-5), 10(-4) M) on the release of tritium was studied. 5-HT elicited release of radioactivity in a dose-dependent manner. Removal of both superior cervical sympathetic ganglia 15 days before the experiment of pretreatment of the animals with reserpine (3 mg kg-1, total dose) produced a significant decrease in the outflow of tritium induced by 5-HT. In these arteries, the amount of radioactivity retained at the end of the experiment was much diminished. Cocaine (10(-6) M) caused a significant decrease in the tritium efflux induced by 5-HT (1"0(-5) M). These results show that 5-HT has an indirect adrenergic effect in the pial arteries of the cat only at high doses of 5-HT, and confirm that sympathetic innervation of these vessels mainly comes from the superior cervical ganglia.

Effects of magnesium on contractile responses induced by electrical transmural stimulation and noradrenaline in rabbit thoracic aorta

1 In rabbit isolated thoracic aortae, effects of magnesium ions on the contraction and (3)H-efflux in response to electrical transmural stimulation and on the contractile responses induced by noradrenaline and KCl were investigated.2 Addition of magnesium (1.2, 3.6, 12.0 and 24.0 mM) to the bathing solution inhibited the electrically induced contractions in a dose-related manner; the inhibition was complete with a concentration of 24.0 mM.3 The increase in (3)H-efflux induced by electrical transmural stimulation was inhibited by the addition of magnesium to the superfusing fluid, but a complete block was not obtained even in high concentrations of magnesium.4 Magnesium ions (1.2, 3.6, 12.0 and 24.0 mM) inhibited the contractile responses induced by low concentrations of noradrenaline (2 x 10(-8)M) and KCl (2 x 10(-2)M). However, the responses induced by higher concentrations of noradrenaline (5 x 10(-7) and 10(-5)M) and KCl (3 and 4 x 10(-2)M) were enhanced by low concentrations of magnesium.5 Magnesium ions affect both presynaptic and effector sites in rabbit thoracic aortae but in a different manner; magnesium manifests only an inhibitory effect on noradrenaline release from the adrenergic nerves, but dual effects on reactivity of vascular smooth muscle, depending on concentrations of magnesium and stimulants; it is suggested that the change in vascular reactivity is more important than the reduction in transmitter release when magnesium inhibits the response to nerve stimulation.

Effect of parathyroidectomy on cardiovascular reactivity in rats with mineralocorticoid-induced hypertension

Previous work has shown that parathyroidectomy protected Sprague-Dawley rats against mineralocorticoid hypertension. 2 In order to explain this protection, we studied vascular reactivity to noradrenaline and angiotensin II in several groups of rats with and without their parathyroid and thyroid glands. Work was performed in vagotomized, anaesthetized rats after ganglionic blockade with pentolinium, and atropine sulphate. 3 The reactivity to noradrenaline was significantly lower in parathyroidectomized rats, especially at the beginning of mineralocorticoid treatment. 4 Autotransplantation of parathyroid glands in thyroparathyroidectomized rats re-established normal cardiovascular reactivity and development of hypertension. 5 Cardiovascular reactivity to angiotensin II was not affected in parathyroidectomized rats and was lowered in thyroparathyroidectomized thyroxine-treated rats.