Evidence for lack of innervation of beta-2 adrenoceptors in the blood vessels of the gracilis muscle of the dog

The effects of adrenalectomy in blood flow adjustments produced by sciatic nerve stimulation in rats

Sciatic nerve stimulation (SNS) produces hypertension, tachycardia, vasodilatation in the stimulated limb and vasoconstriction in the limb contralateral to the stimulation site. These autonomic adjustments represent an equivalent of the cardiovascular responses described as features of defense-alerting reactions. However, while the muscle vasodilatation observed in rats during stimulation of hypothalamic sites involved in defense reactions has been referred to as a general, non-selective increase in blood flow, previous studies conducted on cats have demonstrated that high-intensity cutaneus stimulation produces vasodilatation in the stimulated limb and vasoconstriction in the other limbs. In anesthetized rats, the effects of SNS in the muscle vascular territory of the contralateral limb, as well as the participation of circulating catecholamines in these adjustments continued to be unknown. Therefore, in the present study we investigated the blood flow adjustments elicited by SNS in the stimulated and in the contralateral hind limbs before and after bilateral adrenalectomy. In urethane-anesthetized, paralyzed, and artificially ventilated rats, SNS (800-1000 microA, 1 ms, 100 Hz, 10 s) produced hypertension, tachycardia, vasodilatation in the stimulated hindlimb and vasoconstriction in the contralateral hindlimb. After bilateral adrenalectomy the pressor responses to SNS were abolished, the vasodilatation in the stimulated limb was greater and the vasoconstriction in the contralateral limb was replaced by vasodilatation. These results suggest that the vasodilatation in the stimulated limb does not depend on circulating catecholamines.

Effects of salbutamol and BRL 37344 on diastolic arterial blood pressure, plasma glucose and plasma lactate in rabbits

The aim of this study was to investigate in rabbits the diastolic arterial blood pressure, plasma glucose and plasma lactate responses to salbutamol (a selective beta-2 adrenoceptor agonist) and BRL 37344 (a selective beta-3 adrenoceptor agonist) in comparison with CGP 12177 (a potent beta-1 and beta-2 adrenoceptor antagonist which also acts as a partial beta-3 agonist), isoprenaline (a non-selective beta-1, beta-2 and beta-3 adrenoceptor agonist) and adrenaline (a non-selective beta and alpha adrenoceptor agonist). All drugs were iv infused at the same dose: 0.3 microgram/kg/min (30 min). In sodium pentobarbitone (40 mg/kg)-anasthetized animals none of these compounds altered diastolic arterial blood pressure. BRL 37344 (0.1, 0.3, 1 microgram/kg/min) did not modify this parameter either. In conscious 24-h fasted rabbits, only adrenaline was able to increase plasma glucose levels. By contrast, under the same experimental conditions, salbutamol, isoprenaline and adrenaline, but not BRL 37344 or CGP 12177, induced a significant increase in plasma lactate levels. Finally, the salbutamol-mediated plasma lactate response was inhibited in the presence of clonidine (2 micrograms/kg/min, an alpha-2 adrenoceptor agonist), a drug considered to have opposite effects (stimulatory and inhibitory) on the adenylate cyclase system. In conclusion, these data suggest that only beta-2 adrenoceptor stimulation is able to increase plasma lactate levels, a response which is inhibited by alpha-2 adrenoceptor stimulation.

Beta 1- and beta 2-adrenoceptor-mediated relaxation in human internal mammary artery and saphenous vein: unchanged beta- and alpha-adrenoceptor responsiveness after chronic beta 1-adrenoceptor blockade

1. We have recently reported that patients taking beta 1-adrenoceptor-selective antagonists exhibit marked sensitization of beta 2-adrenoceptor responses but unaltered beta 1-adrenoceptor responses in the heart, both in vitro and in vivo. We therefore investigated beta 1- and beta 2-adrenoceptor-mediated relaxant responses in rings of human internal mammary artery and saphenous vein without endothelium, taken from beta 1-blocked and non-beta-blocked patients undergoing coronary artery bypass graft surgery, for comparison. We also examined alpha 1-adrenoceptor-mediated contraction in these vessels, to determine whether beta 1-blockade had any cross-regulatory effect. 2. Following alpha-blockade with 10 microM phenoxybenzamine, both noradrenaline adrenaline produced concentration-dependent relaxations in both blood vessels, their effects being mediated predominantly through beta 2-adrenoceptors; a lesser beta 1-adrenoceptor component to relaxation was also found in internal mammary artery and a minor beta 1-adrenoceptor component was present in saphenous vein. No differences were found in beta 1- or in beta 2-adrenoceptor-mediated vasorelaxation between beta 1-blocked and non-beta-blocked patients. 3. Methoxamine produced concentration-dependent contractions in both blood vessels, and the potency and efficacy were not significantly different between vessels from beta 1-blocked and from non-beta-blocked patients. 4. These findings indicate that, in these tissues, which possess a relatively minor beta 1-adrenoceptor component in contrast to myocardial tissue, chronic beta 1-blocker treatment does not alter either beta 1- or beta 2-adrenoceptor responses. Likewise, in such tissues, alpha 1-adrenoceptor responses are unaffected by prior beta 1-blockade.

Feedforward control of coronary blood flow via coronary beta-receptor stimulation

It is usually assumed that the increase in coronary blood flow observed with norepinephrine occurs through local metabolic vasodilation secondary to cardiac beta-receptor activation. However, direct feedforward beta-receptor-mediated coronary vasodilation is also a possibility. In dogs with alpha-receptor blockade, the left circumflex artery was perfused at constant pressure. The vasodilator effect of intracoronary norepinephrine injections was determined during prolonged diastoles to avoid the chronotropic and intropic effects of norepinephrine. Norepinephrine caused a dose-dependent increase in coronary blood flow that was attenuated by both the selective beta 1-antagonist practolol and the selective beta 2-antagonist ICI 118,551. These data indicate that norepinephrine activates beta 1- and beta 2-receptors in coronary resistance vessels to cause vasodilation independent of inotropic and chronotropic effects. The physiological significance of coronary beta-receptor-mediated vasodilation was investigated in the beating heart. The coronary blood flow response and coronary venous oxygen tension response were compared when myocardial oxygen consumption was increased over the same range by one of three positive inotropic interventions: (1) norepinephrine, (2) paired-pulse stimulation, or (3) norepinephrine after alpha-blockade. During norepinephrine infusion (intervention 1), coronary venous oxygen tension decreased, indicating that the match between myocardial oxygen consumption and oxygen delivery is not maintained when coronary blood flow is controlled by alpha- and beta-receptors in addition to local metabolic factors. Paired-pulse stimulation (intervention 2) also resulted in a decrease in coronary venous oxygen tension, demonstrating that the balance between oxygen consumption and delivery is not maintained when blood flow is controlled by local metabolic factors alone. However, when coronary beta-receptor-mediated vasodilation was unmasked by alpha-blockade, norepinephrine infusion (intervention 3) produced no change in coronary venous oxygen tension. Therefore, coronary beta-receptor vasodilation helps maintain the balance between flow and metabolism in a feedforward manner in the beating heart.

Analysis of pulmonary vascular responses in cats to sympathetic nerve stimulation under elevated tone conditions. Evidence that neuronally released norepinephrine acts on alpha 1-, alpha 2-, and beta 2-adrenoceptors

The influence of an increase in vascular tone on responses to sympathetic nerve stimulation and the receptors mediating these responses were investigated in the pulmonary vascular bed of the cat. Under conditions of controlled blood flow and constant left atrial pressure, stimulation of the sympathetic nerves to the lung elicited a biphasic response characterized by an initial increase in lobar arterial pressure followed closely by a decrease. The response to nerve stimulation was reproducible with respect to time and was not altered when a delay coil was added to the perfusion circuit, indicating that the response was directly mediated. The increase in pressure was reduced by prazosin and by yohimbine, whereas the decrease in pressure was blocked by propranolol or ICI 118551. These data suggest that the pressor component of the response is mediated by alpha 1- and postjunctional alpha 2-adrenoceptors, whereas the depressor response is mediated by beta 2-receptors. The pressor response was enhanced by propranolol or ICI 118551, whereas the depressor response was enhanced by prazosin or yohimbine, suggesting that the response to nerve stimulation represents the net effect of the actions of neuronally released norepinephrine on alpha- and beta-receptors. The pressor response to nerve stimulation was enhanced when tone was elevated with a prostaglandin endoperoxide analogue and when beta-receptors were blocked. The effects of an increase in tone and a passive increase in pressure on responses to sympathetic nerve stimulation were different.(ABSTRACT TRUNCATED AT 250 WORDS)

The guinea-pig trachea O-methylating system is more effective in modulating beta 2- than beta 1-adrenoceptor-mediated responses to isoprenaline

Assuming that responses of the guinea-pig trachea to isoprenaline in the presence of atenolol (10 mumol L-1) are exclusively, or at least predominantly, beta 2-adrenoceptor mediated and that responses to isoprenaline in the presence of ICI 118,551 (erythro-DL-1(7-methylindan-4-yloxyl)-3-isopropylaminobut an-2-ol) (1 nmol L-1) are exclusively, or at least predominantly beta 1-adrenoceptor mediated, the influence of inhibition of COMT by U-0521 (dehydroxy-2-methyl propiophenone) (50 mumol L-1) has been compared in both conditions. U-0521 enhanced beta 2-adrenoceptor mediated responses to isoprenaline 3.3-fold, while those mediated by beta 1-adrenoceptors were enhanced only 2.2-fold. It is concluded that in guinea-pig trachea COMT activity is functionally more effective in modulating responses which are mediated by beta 2-adrenoceptors than responses mediated by beta 1-adrenoceptors.

Prejunctional beta 2-adrenoreceptor blockade reduces nerve stimulation evoked release of endogenous noradrenaline in skeletal muscle in situ

Prejunctional beta-adrenoceptor-mediated modulation of endogenous noradrenaline (NA) overflow elicited by sympathetic nerve stimulation was studied in blood-perfused canine gracilis muscle in situ. An attempt was made to subclassify these beta-adrenoceptors by comparing the effects of beta 1-selective (metoprolol) and non-selective (propranolol) beta-adrenoceptor blockade. Animals were pre-treated with desipramine and phenoxybenzamine in order to counteract possible influences of neuronal uptake and stimulation-evoked changes in vascular resistance on the diffusion of NA into the blood stream. Metoprolol did not decrease stimulation-evoked NA overflow, as compared with control experiments (-10 and -8%, respectively). However, propranolol reduced stimulation-evoked NA overflow by 30% in metoprolol pre-treated animals (P less than 0.05 vs. control experiments). Both antagonists elevated basal perfusion pressure, suggesting that vascular post-junctional beta 1- as well as beta 2-adrenoceptors are present. Propranolol increased stimulation-evoked vasoconstriction in metoprolol pre-treated animals, indicating that neuronally released NA may activate postjunctional beta 2-adrenoceptors under these experimental conditions. In conclusion, our findings suggest that NA release can be enhanced by activation of prejunctional beta 2-adrenoceptors in vivo.

Activation of cardiac beta 2 adrenoceptors in the human heart

Comparative pharmacologic studies have indicated that the cardiac beta 2 adrenoceptors of vertebrate species are "adrenaline" receptors; i.e., the distribution of beta 2 receptors in the heart seems to be related to the amounts of adrenaline in the sympathetic nerves and in the circulation, and the beta 2 receptors seem to be stimulated mainly by adrenaline. In the human right atrium the order of potency for the agonists and the blocking agents indicate a relatively high proportion of active beta 2 receptors. These findings are in agreement with radioligand binding studies demonstrating up to 50% beta 2 receptors in myocardial membrane preparations. The pharmacologic studies thus add support to the assumption that these beta 2 receptors are functionally active and not merely experimental oddities. It is hypothesized that in normal situations the beta 2-receptor effects are additive to the beta 1 effects. However, during acute stress situations the large amounts of released adrenaline are assumed to increase markedly both inotropy and chronotropy in the heart via beta 2 receptors. It is postulated that only unselective beta blockers can abolish all beta-receptor effects in the heart during stress reactions with profound catecholamine stimulation.

Regulation of beta-adrenoceptor density and function in rat vas deferens

beta-Adrenoceptor density and responsiveness were examined in rat vas deferens following surgical and pharmacological treatments. Receptor density was measured by Scatchard analysis of saturation isotherms of specific [125I]pindolol ([125I]PIN) binding in membrane homogenates. Functional responsiveness was measured by isoprenaline-induced inhibition of field stimulated (60 V, 1 ms, 0.1 Hz) or 40 mM K+-induced contractions. Four days following surgical denervation of vas deferens there was no change in the density of [125I]PIN binding sites, suggesting that these sites are not located on prejunctional neurons. Neither 7 day bilateral adrenalectomy, 21 day denervation, nor 7 days treatment with 10 mg/kg per day desmethylimipramine caused changes in either the potency of isoprenaline in inhibiting contraction or the density of [125I]PIN binding sites compared to controls. Infusion of 3 mg/kg per day isoprenaline for 8 days significantly reduced the potency of isoprenaline in inhibiting field stimulated contractions, reduced the maximum degree of inhibition, and reduced the density of [125I]PIN binding sites. These results suggest that beta-adrenoceptor density and responsiveness in rat vas deferens are not affected by removal of adrenal hormones or neuronal stimulation, but that receptor density and responsiveness can be decreased by increasing the concentration of beta-adrenoceptor agonists at the receptor. Therefore, beta-adrenoceptors in rat vas deferens probably receive little tonic stimulation under normal circumstances.

Influence of acetylcholine, peptides, and other vasodilators on endogenous noradrenaline overflow and vasoconstriction in canine blood perfused gracilis muscle

The effects of acetylcholine, substance P and vasoactive intestinal polypeptide (VIP) on the endogenous noradrenaline (NA) overflow were compared to those of two other vasodilators, nitroglycerin and felodipine, neither of which are thought to influence NA neurotransmission in blood perfused skeletal muscle. The lack of prejunctional effects of nitroglycerin was verified in vitro. The sympathetic nerve stimulation (SNS) evoked NA overflow was reduced by 37 +/- 9% by a dose of acetylcholine which reduced the perfusion pressure at rest by 44 +/- 6%. Conversely, atropine tended to enhance SNS evoked NA overflow. Acetylcholine reduced the vasoconstrictor responses to SNS when compared to the responses observed with an equipotent vasodilatory dose of, e.g. nitroglycerin. The SNS evoked NA overflow was not influenced by a moderate mechanical blood flow reduction or by pronounced reductions of vascular resistance induced by either substance P, VIP, nitroglycerin or felodipine, supporting the idea that the transport of NA from nerve terminal to blood is not importantly influenced by moderate decreases in blood flow or vascular tone. Prejunctional muscarinic inhibition of NA release in skeletal muscle was verified under in vivo conditions, but the other substances tested did not influence sympathetic neurotransmission. Endogenous NA overflow appears to mirror NA release in vivo also when diffusion is influenced by changes in blood flow or vascular tone in this experimental model.

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.

Relationship between the overflow of endogenous and radiolabelled noradrenaline from canine blood perfused gracilis muscle

The effects of sympathetic nerve stimulation (SNS) on the overflow of endogenous noradrenaline (NA) and on vasoconstrictor responses were studied in blood perfused canine gracilis muscle in situ. A conventional tracer technique with 3H-labelled NA (3H-NA) was used in parallel. At rest there was a net extraction of endogenous NA and adrenaline across the tissue. The SNS evoked overflow of endogenous NA was frequency-dependent and logarithmically correlated to the vasoconstrictor responses. The neuronal uptake inhibitor desipramine doubled the SNS induced overflow of endogenous NA without enhancing the vasoconstrictor responses. A further fourfold increase in NA overflow was caused by a dose of the alpha-blocker phenoxybenzamine which reduced the vasoconstrictor responses by 50-75%. Less than 10% of the spontaneous 3H efflux was recovered as unmetabolized 3H-NA, whereas virtually all 3H overflow evoked by SNS was 3H-NA. The fractional release of NA or 3H-NA per nerve impulse increased with increasing frequencies of SNS under all conditions studied. Although there was a preferential release of the newly stored radiolabelled transmitter, results concerning endogenous NA and 3H-NA overflow were qualitatively similar, also under conditions with marked changes in transmitter overflow. Endogenous NA gave a more reproducible index of transmitter overflow than did 3H-NA and, in particular, total 3H. The overflow of endogenous NA closely reflects SNS evoked neuronal release of NA in blood perfused skeletal muscle and seems more suitable than conventional radiotracer techniques for studies of NA release under in vivo conditions.

Reserpine-induced supersensitivity occurs for beta-adrenoceptor-mediated responses of heart and trachea but not of the uterus and lung

This study was undertaken to determine whether reserpine-induced supersensitivity occurs in tissues containing beta1-adrenoceptors and in those with beta 2-adrenoceptors. Guinea-pigs and rats were pretreated with reserpine for either 3 days (5 mg kg-1 i.p. at 72 h, 3 mg kg-1 at 48 h and 3 mg kg-1 at 24 h before use) or 7 days (1 mg kg-1 daily). The sensitivities of left and right atria, papillary muscles, tracheal spirals, lung strips and uteri to isoprenaline were compared with those from untreated animals. The positive inotropic responses of left atria and papillary muscles and chronotropic responses of right atria from reserpine-pretreated animals were supersensitive to isoprenaline, the concentration-response curves being to the left. The relaxation response of the carbachol-contracted trachea also exhibited supersensitivity, but to a lesser extent. However, no supersensitivity occurred for the relaxation of carbachol-contracted lungs, K+-depolarized guinea-pig uteri or electrically stimulated rat uteri. As a pharmacological index of the presence of releasable noradrenergic stores, tyramine was added cumulatively to each tissue. Only cardiac and tracheal preparations yielded substantial responses, indicating the presence of sympathetic innervation. A relaxation of the rat uterus by tyramine was not attributable to releasable noradrenaline stores. The supersensitivity of the heart and trachea could therefore be associated with their sympathetic innervation and with the fact that their responses are mediated via beta 1-adrenoceptors; the trachea containing a small proportion of beta 1-adrenoceptors. The responses of the lung and uterus, however, are beta 2-adrenoceptor-mediated and failed to exhibit supersensitivity. Since the supersensitivity is a consequence of the neuronal depleting action of reserpine, these results are compatible with the concept that beta 1-adrenoceptors are associated with sympathetic innervation whereas beta 2-adrenoceptors are not.

Physiological and pharmacological aspects of adrenergic receptor classification

The questions raised are: what is the physiological or pharmacological basis for the differentiation into beta 1- and beta 2-, and alpha 1- and alpha 2-adrenergic receptors?; and do the neurotransmitter norepinephrine and the hormone epinephrine differ in their receptors? On the basis of a preference of beta 2- and alpha 2-adrenergic receptors for epinephrine, the hormone, and of beta 1-and alpha 1-receptors for norepinephrine, the neurotransmitter, it was postulated that the alpha 2- and beta 2-receptors are predominantly epinephrinergic in nature and located extrajunctionally and presynaptically whereas the alpha 1- and beta 1-receptors are predominantly norepinephrinergic in nature and located postsynaptically in the sympathetic terminal junction. The alpha 2- and beta 2-character of the presynaptic receptors matches that of the corresponding extrajunctional receptors. This indicates that a circulating catecholamine, namely epinephrine, is involved in the regulation of adrenergic transmitter release.

Comparative aspects of adrenergic receptors in the hearts of lower vertebrates

The cardiac adrenoceptors of lower vertebrates were characterized in atrial preparations. Adrenaline (A) potentiated the force and frequency of contraction in the spontaneously beating atria of the frog, trout and flounder and in electrically paced atrial strips from the shark. The inotropic responses of A were most pronounced at the lower temperatures for the frog and trout, while A enhanced frequency to a greater extent at higher temperatures in the frog atria. Atrial alpha-receptors activated by A at 8 degrees C could not be detected in any of the species under study. The apparent affinities for the inotropic and chronotropic responses of agonist in the frog (15 degrees C) and trout (8 degrees C) atria were: Iso greater than Sal greater than or equal to A greater than NA. A cocaine-sensitive uptake for A and NA was apparent in these atria, consistent with sympathetic innervation. The affinities for the catecholamines in the flounder and shark atria were not increased by cocaine, in accordance with absence of sympathetic innervation of the atria in these species. These atria were also insensitive to corticosterone. The affinities for A and NA were on the other hand higher in the sympathetically non-innervated atria of the flounder than in the innervated atria of the frog and trout. The apparent orders of relative affinities for agonists were Iso greater than A = NA greater than Sal for the flounder, and of the relative potencies Iso = A greater than NA greater than Sal for the shark atrium. The results are consistent with the hypothesis that catecholamines enhance cardiac performance in lower vertebrates chiefly via "adrenaline" receptors which resemble the beta 2-type of mammalian adrenoceptors in many respects. Unlike that in mammals, cardiac adrenaline receptors in the frog and trout are activated by the sympathetic neurotransmitter ("innervated" receptors). On the other hand, the adrenaline receptors of the flounder and shark are responding to the circulating catecholamines ("humoral" receptors). However, the flounder atrium, with equal affinities for A and NA, appears as an exception to the rule by having a mixed population of humoral beta 1- and beta 2-adrenoceptors, indicating a role for circulating NA in cardiac regulation in this species.

Prejunctional and postjunctional actions of endogenous norepinephrine at the sympathetic neuroeffector junction in canine coronary arteries

The effects of endogenous and of exogenous norepinephrine were studied in isolated rings of canine left circumflex coronary artery and its first ventricular branch. Norepinephrine was released from adrenergic nerve endings by transmural electrical stimulation and by tyramine. In rings contracted with prostaglandin F2 alpha, transmural electrical stimulation resulted in frequency-dependent relaxations which were blocked by propranolol or tetrodotoxin; tyramine and exogenous norepinephrine caused concentration-dependent relaxations which were blocked by propranolol. The tyramine-induced relaxations also were inhibited by cocaine. The left circumflex artery was less sensitive than its branch to beta-adrenergic activation; this difference was significant even between rings of the two vessels immediately adjacent to the branching point and was abolished by phentolamine. In the presence of propranolol, transmural electrical stimulation, tyramine and phenylephrine, produced contractions of the left circumflex artery, but not the branch; these contractions were prevented by phentolamine. Phentolamine, but not prazosin, augmented the beta-adrenergic response of left circumflex artery to low frequency stimulation; in arteries preincubated with 3H-norepinephrine, this was accompanied by an increased overflow of tritiated neurotransmitter. The prejunctional effect of phentolamine was also evident in branch coronary arteries which exhibit no postjunctional alpha-adrenergic responses. With high frequency stimulation, both alpha-adrenergic antagonists equally augmented the relaxation of left circumflex artery; the efflux of tritiated norepinephrine was not different from untreated arteries. These experiments demonstrate, in isolated coronary arteries, that the primary adrenergic response to released endogenous norepinephrine is beta-adrenergic relaxation. The prejunctional effects of nonspecific alpha-adrenergic antagonists preclude their use in determining the importance of postjunctional coronary alpha-adrenergic receptor activation caused by sympathetic nerve stimulation.

Beta-adrenergic vasodilator mechanism in the finger

The digital vasospastic phenomena, which are induced by beta-adrenergic-blocking agents, suggest a beta-adrenergic finger vasodilator mechanism. We measured fingertip total blood flow with venous occlusion plethysmography and studied nutritional blood flow with Na131I clearance. During fingertip vasoconstriction caused by branchial artery infusion of norepinephrine or angiotensin, intra-arterial isoproterenol caused a sustained increase in fingertip total blood flow. Furthermore, propranolol significantly potentiated the vasoconstriction caused by intra-arterial norepinephrine and attenuated the vasodilator action of isoproterenol. No change in branchial artery blood pressure occurred to explain the changes in fingertip blood flow. Isoproterenol did not change nutritional flow, implying beta-adrenergic vasodilation solely of the fingertip arteriovenous shunts. When fingertip vasoconstriction was induced by reflex sympathetic nerve action during environmental cooling and mental stress, or by norepinephrine release from sympathetic nerves caused by intra-arterial tyramine infusion, isoproterenol and propranolol had no effect on fingertip total blood flow. This effect is probably specific for the beta-receptor agonist, since intra-arterial histamine caused a large increase in finger blood flow during environmental cooling. We conclude that there is a beta-adrenergic vasodilator mechanism in human digital arteriovenous shunts that may be humorally activated, but which has no apparent functional role in modulating sympathetic vasoconstriction. Our results suggest a spatial dissociation of the effector sites for vasoactive humoral agents and sympathetic vasoconstrictor nerves.

Relative location of alpha- and beta-adrenoceptors to sites of release of sympathetic transmitter in the rabbit facial vein

We studied alpha- and beta-adrenoceptor-mediated responses of rabbit facial vein rings to adrenergic stimulation to determine the location of the two types of receptors in relation to the sympathetic nerve terminals. Transmural electrical nerve stimulation (TNS) at low frequency elicited large beta-receptor-mediated relaxation responses in rings pretreated with phentolamine (6 x 10(-7)M). These responses were significantly greater than the corresponding alpha-receptor-mediated contractions in rings pretreated with propranolol (10(-6)M). Blockade of neuronal uptake with desmethylimipramine (DMI, 10(-7)M) increased significantly the neurogenic relaxation but had little effect on neurogenic contractility. DMI pretreatment caused a shift to the left (x 5.6) in the relaxant dose-response curve to exogenous l-norepinephrine (NE). The NE contractile dose-response curve was also shifted to the left after DMI pretreatment but by a significantly smaller amount (x 3.1). Neurogenic activation of beta-receptors evoked almost maximal relaxations in facial vein rings (85% at 2 Hz), whereas the maximum neurogenic contraction was approximately half the maximum contraction with exogenous NE (40% at 6 Hz). These results imply that the beta-adrenoceptors in the rabbit facial vein are located in close proximity to sites of adrenergic transmitter release and neuronal reuptake, whereas the alpha-receptors are more distant.

Pulmonary vasodilator responses to catecholamines and sympathetic nerve stimulation in the cat. Evidence that vascular beta-2 adrenoreceptors are innervated

We investigated the effects of catecholamines and sympathetic nerve stimulation in the feline pulmonary vascular bed under conditions of controlled pulmonary blood flow. Norepinephrine and nerve stimulation caused dose- and stimulus frequency-dependent increases in pulmonary vascular resistance. However, when pulmonary vascular tone was enhanced and alpha receptors blocked, norepinephrine and nerve stimulation caused dose- and frequency-dependent decreases in pulmonary vascular resistance. The decreases in pulmonary vascular resistance were blocked with propranolol and were of greater magnitude than were constrictor responses observed under basal conditions. Vasodilator responses to nerve stimulation were not modified by atropine. Epinephrine and isoproterenol had marked vasodilator activity in the pulmonary vascular bed when pulmonary vascular tone was elevated. When alpha receptors were blocked, isoproterenol and epinephrine had similar vasodilator activity, and when beta receptors were blocked, epinephrine and norepinephrine had marked vasoconstrictor activity. Selective beta-1 receptors antagonists had little effect on vasodilator responses to isoproterenol, whereas responses to this substance were blocked by propranolol. These results suggest that presence of alpha-and beta-2 adrenoreceptors in the feline pulmonary vascular bed and that both types of adrenergic receptors are innervated by the sympathetic nervous system.

Mini-review. The postsynaptic alpha 2-adrenoreceptor

The discovery of postsynaptic alpha 2-adrenoreceptors with the drug specificities of presynaptic alpha 2-adrenoreceptors has contributed to a refinement of the classification of alpha-adrenoreceptors. postsynaptic alpha 2-adrenoreceptors have been identified by pharmacological means and with the aid of direct radioligand-receptor binding studies. The evidence for the existence of this class of alpha 2-adrenoreceptors in the brain and in vascular smooth muscle is particularly strong. Central postsynaptic alpha 2-adrenoreceptors play a major part in the hypotensive action of centrally acting antihypertensive drugs such as clonidine and alpha-methyl-DOPA. Vascular smooth muscle cells contain postsynaptic alpha 2-adrenoreceptors which mediate vasoconstriction, like the more classical alpha 1-adrenoreceptors. The simultaneous occurrence of contractile alpha 1- and alpha 2-adrenoreceptors in vascular smooth muscle offers a simple model for the characterization of alpha-adrenoreceptor agonists and antagonists. At present, highly selective agonists of alpha 2-adrenoreceptors have been found. These new compounds may be useful for the classification of other alpha-adrenoreceptor populations. It has been suggested that the vascular postsynaptic alpha 2-adrenoreceptor might be located at extrasynaptic sites. Accordingly, adrenaline released by the adrenal medulla would be the endogenous stimulant. Finally, observations have been made in vivo indicating that a transmembrane influx of calcium ions is necessary for linking the drug-induced activation of these alpha 2-adrenoreceptors to vasoconstriction.