Prejunctional beta-adrenoceptors in human and canine saphenous veins

Human presynaptic receptors

Presynaptic receptors are sites at which transmitters, locally formed mediators or hormones inhibit or facilitate the release of a given transmitter from its axon terminals. The interest in the identification of presynaptic receptors has faded in recent years and it may therefore be justified to give an overview of their occurrence in the autonomic and central nervous system; this review will focus on presynaptic receptors in human tissues. Autoreceptors are presynaptic receptors at which a given transmitter restrains its further release, though in some instances may also increase its release. Inhibitory autoreceptors represent a typical example of a negative feedback; they are tonically activated by the respective endogenous transmitter and/or are constitutively active. Autoreceptors also play a role under pathophysiological conditions, e.g. by limiting the massive noradrenaline release occurring during congestive heart failure. They can be used for therapeutic purposes; e.g., the α₂-adrenoceptor antagonist mirtazapine is used as an antidepressant and the inverse histamine H₃ receptor agonist pitolisant has been marketed as a new drug for the treatment of narcolepsy in 2016. Heteroreceptors are presynaptic receptors at which transmitters from adjacent neurons, locally formed mediators (e.g. endocannabinoids) or hormones (e.g. adrenaline) can inhibit or facilitate transmitter release; they may be subject to an endogenous tone. The constipating effect of the sympathetic nervous system or of the antihypertensive drug clonidine is related to the activation of inhibitory α₂-adrenoceptors on postganglionic parasympathetic neurons. Part of the stimulating effect of adrenaline on the sympathetic nervous system during stress is related to its facilitatory effect on noradrenaline release via β₂-adrenoceptors.

Effect of skin temperature on cutaneous vasodilator response to the β-adrenergic agonist isoproterenol

The vascular response to local skin cooling is dependent in part on a cold-induced translocation of α2C-receptors and an increased α-adrenoreceptor function. To discover whether β-adrenergic function might contribute, we examined whether β-receptor sensitivity to the β-agonist isoproterenol was affected by local skin temperature. In seven healthy volunteers, skin blood flow was measured from the forearm by laser-Doppler flowmetry and blood pressure was measured by finger photoplethysmography. Data were expressed as cutaneous vascular conductance (CVC; laser-Doppler flux/mean arterial blood pressure). Pharmacological agents were administered via intradermal microdialysis. We prepared four skin sites: one site was maintained at a thermoneutral temperature of 34°C (32 ± 10%CVCmax) one site was heated to 39°C (38 ± 11%CVCmax); and two sites were cooled, one to 29°C (22 ± 7%CVCmax) and the other 24°C (16 ± 4%CVCmax). After 20 min at these temperatures to allow stabilization of skin blood flow, isoproterenol was perfused in concentrations of 10, 30, 100, and 300 μM. Each concentration was perfused for 15 min. Relative to the CVC responses to isoproterenol at the thermoneutral skin temperature (34°C) (+21 ± 10%max), low skin temperatures reduced (at 29°C) (+17 ± 6%max) or abolished (at 24°C) (+1 ± 5%max) the vasodilator response, and warm (39°C) skin temperatures enhanced the vasodilator response (+40 ± 9%max) to isoproterenol. These data indicate that β-adrenergic function was influenced by local skin temperature. This finding raises the possibility that a part of the vasoconstrictor response to direct skin cooling could include reduced background β-receptor mediated vasodilation.

Epinephrine and the genesis of hypertension

Several lines of evidence suggest a psychophysiological link between stress, adrenomedullary activation, and the genesis of hypertension. Experimental data support four important concepts: 1) epinephrine stimulates prejunctional beta 2-adrenergic receptors that facilitate norepinephrine release from sympathetic nerve endings; 2) epinephrine can be converted into a cotransmitter by neuronal uptake and on subsequent release augment the simultaneous discharge of norepinephrine; 3) exogenous epinephrine can induce sustained hypertension in rats; and 4) there is a period of critical sensitivity to endogenous epinephrine in a genetic model of rat hypertension. Plasma epinephrine concentrations are elevated in many young subjects with borderline or mild hypertension. The hypothesis that intermittent surges in epinephrine could initiate or promote the development of primary hypertension by amplifying peripheral neurotransmission, both directly (facilitative effect) and indirectly (cotransmitter action), is supported by reports that hemodynamic and noradrenergic responses to sympathetic activation can be augmented by increases in endogenous epinephrine or by its local or systemic (up to 30 ng/kg/min) infusion. Such responses have been documented in both normotensive and hypertensive subjects and can be blocked by propranolol. Although the weight of evidence (mostly indirect) indicates that epinephrine can augment norepinephrine release in humans, the epinephrine hypothesis, itself, remains unproven. Expression of hypertension by this mechanism may be restricted to a specific epinephrine-sensitive subset of individuals with a genetic predisposition to high blood pressure.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Lack of presynaptic modulation by isoprenaline of 3H-noradrenaline release from rabbit isolated ear artery

The aim of the present investigation was to examine whether or not presynaptic facilitatory beta-adrenoceptors are detectable on the postganglionic nerves in the rabbit isolated ear artery. Strips of rabbit central ear artery were incubated with 3H-noradrenaline (10(-7) mol/l; 30 min or 10(-6) mol/l; 60 min). Subsequently, they were washed repeatedly with physiological salt solution. The strips were subjected to electrical-field stimulation (S1-S8) and the resultant 3H-overflow was determined. When the ear artery was stimulated with 150 pulses (0.5 ms; 3 Hz; 225 mA), isoprenaline (10(-9)-10(-6) mol/l) either alone or in the presence of either rauwolscine (10(-6) mol/l) or phentolamine (10(-6) mol/l) did not alter the stimulation-evoked 3H-overflow. This was also the case in the presence of rauwolscine (10(-6) mol/l) plus either the selective phosphodiesterase inhibitor ICI 63 197 (3 x 10(-5) mol/l) or forskolin (10(-6) mol/l). When the ear artery was stimulated with 300 pulses (1 ms; 5 Hz; 225 mA), isoprenaline had no effect on the stimulation-evoked 3H-overflow. This was also the case when phentolamine (10(-6) mol/l) was present. Propranolol (10(-7)-10(-5) mol/l) did not alter the stimulation-evoked 3H-overflow. In some experiments, the stimulation current was reduced to 175 mA in order to obtain similar reference release (S3) values despite the presence of rauwolscine (150 pulses; 0.5 ms; 3 Hz). Even then, isoprenaline (10(-9)-10(-6) mol/l) did not change stimulation-evoked 3H-overflow. The results suggest that postganglionic sympathetic nerves in rabbit central ear artery do not possess presynaptic facilitatory beta-adrenoceptors.

Inhibition of noradrenaline release from the sympathetic nerves of the human saphenous vein via presynaptic 5-HT receptors similar to the 5-HT 1D subtype

The human saphenous vein preincubated with [3H]noradrenaline was used to determine the pharmacological properties of the release-inhibiting presynaptic serotonin (5-HT) receptor on the sympathetic nerves. The overflow of tritium evoked by transmural electrical stimulation (2 Hz) was concentration-dependently inhibited by drugs known to stimulate 5-HT receptors in the following rank order: oxymetazoline greater than or equal to 5-HT greater than 5-carboxamidotryptamine = 5-methoxytryptamine = sumatriptan greater than tryptamine greater than N,N(CH3)2-5-HT = yohimbine = 8-hydroxy-2-(di-n-propylamino)-tetraline. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT1B and 5-HT1D binding sites, but not with those for 5-HT1A or 5-HT1C binding sites. 5-Aminotryptamine, methysergide, ipsapirone, cyanopindolol, SDZ 21009 and metergoline dit not produce a significant inhibition. Metitepine and methysergide antagonized the inhibitory effect of 5-HT, whereas spiroxatrine, propranolol, ketanserin and ICS 205-930 did not. These data exclude the idea that the inhibitory presynaptic 5-HT receptor on the sympathetic nerves belongs to the 5-HT2 and 5-HT3 receptor class; the pattern of agonist potencies suggests that the receptor is very similar to the 5-HT1D receptor subtype.

Modulation of noradrenaline release by activation of presynaptic beta-adrenoceptors in the cardiovascular system

Peripheral sympathetic nerve terminals in many tissues, but not all, are endowed with beta-adrenoceptors. Activation of these result in an enhancement of noradrenaline release evoked by electrical nerve stimulation. These so-called presynaptic beta-adrenoceptors are possibly located on the outer surface of the varicosity of the noradrenergic nerves. A postsynaptic location, however, is also a possibility. The presynaptic beta-adrenoceptors appear to be of the beta 2-adrenoceptor subtype. However, specific classification is lacking. The stereospecificity of the beta-adrenoceptors is controversial. These receptors are not activated by noradrenaline released from sympathetic nerves. Adrenaline derived from the adrenal medulla may be the physiological activator. Either circulating adrenaline or adrenaline taken up by sympathetic nerve terminals and then released as a cotransmitter with noradrenaline activates the presynaptic beta-adrenoceptors. In the latter case, a "positive" feedback" loop may be formed.

Beta-adrenergic blockade decreases norepinephrine release in humans

Beta-Adrenergic blockade with propranolol (PRP) has been reported to cause an increase in plasma norepinephrine (NE) levels in humans, which suggests that a reflex increase in sympathetic nervous system (SNS) vasoconstrictor tone compensates for the hypotensive effect of beta-adrenergic blockade. However, plasma NE levels are an indirect measure of SNS activity. We have developed a two-compartment model of NE kinetics to estimate NE release into an extravascular compartment as a more comprehensive measure of systemic SNS activity. To determine whether beta-adrenergic blockade alters extravascular NE release, we studied nine healthy subjects during sequential infusions of saline and PRP. During PRP infusion, there was an increase in plasma NE levels [1.03 +/- 0.13 to 1.27 +/- 0.21 (SE) nM; P = 0.05], but the extravascular NE release rate decreased significantly (15.5 +/- 1.6 to 9.2 +/- 1.2 nmol.min-1.m-2, P = 0.0002). The plasma NE concentration increased despite the fall in extravascular NE release rate primarily because the clearance of NE from plasma declined (1.55 +/- 0.08 to 1.18 +/- 0.07 l.min-1.m-2, P = 0.0001); the NE spillover rate into plasma did not change (1.73 +/- 0.18 to 1.75 +/- 0.23 nmol.min-1.m-2, P = 0.89). We conclude that PRP decreases extravascular NE release in humans. Suppression of SNS activity may be an additional mechanism of action of nonselective beta-adrenergic antagonists in humans.

Desipramine blocks augmented neurogenic vasoconstrictor responses to epinephrine

The forearm vasoconstrictor response to lower body negative pressure (LBNP), a reflex stimulus to norepinephrine release, can be augmented by a prior brachial artery infusion of epinephrine. We wished to determine whether this sustained aftereffect of epinephrine could be replicated by systemic infusion and, if so, whether it could be prevented by prior uptake-1 blockade with desipramine. Eight normal men (mean age 30 years) were studied on two separate study days at least 1 week apart, 2.5 hours after taking, at random, either desipramine (125 mg p.o.) or placebo. Forearm vascular resistance was measured at rest and at the end of 6 minutes of LBNP at -40 mm Hg. This was done both before and 30 minutes after a 60-minute infusion of epinephrine (1.5 micrograms/min i.v.). From similar baselines, the forearm vasoconstrictor response to LBNP was significantly augmented 30 minutes after epinephrine on the placebo day (+17 +/- 4 vs. +12 +/- 3 resistance units, mean +/- SEM, p less than 0.01) but not on the desipramine day (+14 +/- 2 vs. +16 +/- 3 resistance units). The heart rate response to LBNP was also greater after epinephrine on the placebo day (+20 +/- 3 vs. +16 +/- 2 beats/min, p less than 0.05). Mean arterial pressure was higher after epinephrine infusion on the placebo (p less than 0.01) but not on the desipramine day.(ABSTRACT TRUNCATED AT 250 WORDS)

Subtype-selective up-regulation of human saphenous vein beta 2-adrenoceptors by chronic beta-adrenoceptor antagonist treatment

To study beta-adrenoceptor antagonist-induced changes in human vascular beta-adrenoceptors, we determined the effects of chronic treatment with different beta-adrenoceptor antagonists without intrinsic sympathomimetic activity (ISA) on beta 2-adrenoceptor density (assessed by (-)-[125I]-iodopindolol binding) in human saphenous vein membranes obtained from patients undergoing coronary artery bypass grafting. In patients chronically treated with the non-selective beta-adrenoceptor antagonists propranolol or sotalol, the density of saphenous vein beta 2-adrenoceptors was significantly higher than in control (i.e. patients not treated with beta-adrenoceptor antagonists), whereas in patients chronically treated with the selective beta 1-adrenoceptor antagonists metoprolol or bisoprolol it was not different from control. It is concluded that beta-adrenoceptor antagonists without ISA increase human saphenous vein beta 2-adrenoceptors in a subtype-selective fashion.

Vasoactive responses of a human cystic artery: adrenoceptor characterization

1. The pharmacology of various agonists and antagonists was studied in the human isolated cystic artery. 2. The estimated pA2s for the alpha 1-adrenoceptor antagonist prazosin against the alpha-adrenoceptor agonists phenylephrine, alpha-methylnoradrenaline and noradrenaline were not significantly different. Similar results were seen for estimated pA2s of the alpha 2-adrenoceptor antagonist yohimbine against these same agonists. Equivalent responses to exogenous noradrenaline and to transmural electrical stimulation were blocked to the same degree by an antagonist with alpha 1-adrenoceptor blocking properties (prazosin). Responses to transmural electrical stimulation, however, tended to be more resistant than equivalent responses to exogenous noradrenaline to blockade by antagonists with alpha 2-adrenoceptor blocking properties (phentolamine, yohimbine). 3. Relaxation to isoprenaline was observed in partially contracted arterial strips using isoprenaline concentrations of up to 10(-6) M, but cumulative addition of higher concentrations of isoprenaline sometimes then evoked a contraction from the relaxation nadir. The relaxation effect of isoprenaline was antagonized by propranolol (10(-5) M). 4. These findings suggest the human cystic artery has almost exclusively alpha 1-adrenoceptors postjunctionally, although prejunctional alpha 2-adrenoceptors may be present; and, it also has some postjunctional beta-adrenoceptors which mediate relaxation.

Epinephrine facilitates neurogenic vasoconstriction in humans

Numerous studies have suggested that epinephrine may facilitate neural release of NE. There have been no studies in humans that demonstrate the functional significance of this action. To determine whether epinephrine facilitates neurogenic vasoconstriction in humans, we contrasted forearm vasoconstrictor responses to a reflex stimulus (lower body negative pressure [LBNP]) and to intraarterial NE before, during, and 30 min after infusion of epinephrine (50 ng/min) or isoproterenol (10 or 25 ng/min) into a brachial artery. These doses had no systemic effects. We reasoned that if prejunctional stimulation of beta receptors by epinephrine and isoproterenol had functional significance, the vasoconstrictor response to LBNP would be potentiated in comparison to the response to NE (postjunctional mechanism). Studies were done on 23 normal male volunteers. Forearm blood flow was measured with a strain gauge plethysmograph and intraarterial pressure was recorded. The ratio of vasoconstrictor responses to LBNP/NE was used as an index of neural release of the neurotransmitter NE. This ratio increased during infusions of both epinephrine and isoproterenol. 30 min after epinephrine the vasoconstrictor response to LBNP (n = 15) was augmented from +9.9 +/- 2.2 (SE) resistance units (RU) before epinephrine to +16.4 +/- 3.2 RU (P less than 0.05); whereas the response to NE (n = 8) tended to decrease from +8.8 +/- 3.1 RU before to +4.2 +/- 1.2 RU after epinephrine (P greater than 0.05). In contrast, 30 min after isoproterenol the vasoconstrictor responses to LBNP and NE were the same as before isoproterenol. The augmented ratio of responses to LBNP/NE after epinephrine and not after isoproterenol supports the concept that epinephrine, but not isoproterenol, is taken up by the adrenergic terminal, is released subsequently during reflex stimulation, and augments the release of the neurotransmitter NE. These experiments provide the first hemodynamic evidence in humans that epinephrine and isoproterenol facilitate neurogenic vasoconstriction. The sustained effect of epinephrine in contrast to isoproterenol suggests that the late facilitation by epinephrine is related to its neural uptake and subsequent release.

Effect of hypercholesterolemia on vascular reactivity in the rabbit. I. Endothelium-dependent and endothelium-independent contractions and relaxations in isolated arteries of control and hypercholesterolemic rabbits

We studied the effects of hypercholesterolemia on vascular responsiveness in different arteries isolated from rabbits: control groups of rabbits and groups receiving the atherogenic diet consisted of eight animals each. In the arteries, 16 weeks of cholesterol-rich (0.3%) diet evoked intimal lesions which were more pronounced than those noted after 8 weeks of hypercholesterolemia; the aortic arch was affected significantly more by the lesions than the abdominal aorta and the pulmonary artery. Segments of the arteries were mounted in organ chambers for isometric tension recording or for measurement of the endothelium-derived relaxant factor. Contractions caused by acetylcholine and prostaglandin F2 alpha were not altered by the hypercholesterolemia; those evoked by serotonin were moderately augmented only in the aortic arch of hypercholesterolemic rabbits. As the degree of intimal lesion formation increased, the contractions to norepinephrine and clonidine were progressively inhibited. The endothelium-independent relaxations to nitroglycerin were inhibited in only the most severely affected arteries; the endothelium-dependent relaxations to acetylcholine and adenosine triphosphate were progressively inhibited as the degree of fatty streak formation augmented. Thus, in the aortic arch, the relaxations to 3 X 10(-6) M acetylcholine, expressed as percent of the initial contraction, decreased from 86.7 +/- 3.3% in control tissues to 16.3 +/- 8.6% in the 16-week hypercholesterolemic vessels; in the abdominal aortas these relaxations averaged 93.5 +/- 2.2% in control vessels and 72.0 +/- 6.9% in the hypercholesterolemic tissues. The acetylcholine-induced release of endothelium-derived relaxant factor from the abdominal aorta was not significantly affected by the hypercholesterolemia. We conclude from these studies that in arteries obtained from hypercholesterolemic rabbits: the contractions caused by serotonergic mechanisms tend to be augmented, while those to alpha-adrenergic activation are decreased, the endothelium-independent relaxations are modified only in the more severely affected arteries, and the endothelium-dependent relaxations are progressively inhibited as the degree of fatty streak formation augments, probably because a step subsequent to the release of endothelium-derived relaxant factor is altered.

Circulatory response to beta-adrenergic blockade at rest and during exercise

The beta adrenoceptors involved in the regulation of the cardiovascular system include the beta 1 subtype in the heart, coronary arteries and juxtaglomerular cells of kidney, and the beta 2 subtype in skeletal muscle resistance vessels and on terminals of sympathetic nerves. The beta 1 receptors are activated primarily by norepinephrine released from the sympathetic nerves, the beta 2 by circulating epinephrine from the adrenal medulla. The function of these receptors is to adjust the circulation to meet the stresses imposed by gravitational forces including those that occur in changing from supine to standing position, in muscular exercise and during emotional stress. In normal subjects, other systems can compensate if the beta receptors are prevented from functioning. Thus, during beta-adrenergic blockade it is only when the cardiovascular system is taxed severely that deficiencies in its performance become apparent. In patients with cardiovascular diseases, other effects of beta blockers, not yet understood, may also be important.