Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Mediation of the hypotensive action of systemic clonidine in the rat by alpha 2-adrenoceptors

1. During the past few years it has been shown that the sympatholytic effect resulting from localized microinjection of clonidine and other imidazolines into the rostral ventrolateral medulla (RVL) results from activation of 'imidazoline' receptors (I1 receptors) rather than from an alpha 2-adrenoceptor-mediated effect. 2. The relative contributions of these two receptor systems to the hypotensive action of systemically administered clonidine have not been studied. Clonidine has affinity for both I1 and alpha 2-adrenoceptors; guanabenz represents a useful pharmacological tool, since it activates only the alpha 2-adrenoceptor. 3. Antagonists acting at both I1 and alpha 2-adrenoceptors (idazoxan) and at only alpha 2-adrenoceptors (SK&F 86466; 6-chloro-3-methyl-2,3,4,5-tetrahydro-3-benzazepine) are available. Idazoxan (1 mg kg-1, i.v.) and SK&F 86466 (3 mg kg-1, i.v.) produced an equivalent degree of blockade of the pressor response to guanabenz or clonidine in the pithed rat, a response mediated by the alpha 2-adrenoceptor. 4. Guanabenz (30 micrograms kg-1, i.v.) and clonidine (10 micrograms kg-1, i.v.) lowered blood pressure in the chloralose-anaesthetized spontaneously hypertensive rat by 48 +/- 4.6 mmHg and 44 +/- 5.4 mmHg, respectively; this response, for either agonist, was blocked by both idazoxan and SK&F 86466. 5. These data show that the hypotensive effect of intravenously administered clonidine results from activation of central alpha 2-adrenoceptors, with no significant contribution from an I1-mediated effect. Thus clonidine can lower blood pressure by different receptor mechanisms, dependent on the route of administration.

The alpha 2-adrenoceptors of the human retinoblastoma cell line (Y79) may represent an additional example of the alpha 2C-adrenoceptor

1. In agreement with the literature, correlation of the ability of a series of agonists and antagonists to displace [3H]-rauwolscine binding shows the alpha 2-adrenoceptors of HT29 cells, NG108-15 cells, OK cells and homogenates of rat sublingual gland to represent four distinct subtypes. 2. [3H]-rauwolscine also bound with high affinity (KD = 0.30 +/- 0.10 mM) to a human retinoblastoma cell line (Y79). Specific binding represents 73% of total binding, and a Bmax of 38 +/- 1 fmol mg-1 protein was determined. 3. Correlation of antagonist affinities against [3H]-rauwolscine with corresponding values in the other four tissue sources showed the Y79 cells to resemble most closely the OK cells, the prototype example of an alpha 2C-adrenoceptor, with a correlation coefficient of 0.90 and a regression slope of 1.01 being obtained for 10 antagonists in these two systems. 4. Comparison of KD values for [3H]-rauwolscine also showed a similarity between the OK cells (0.19 +/- 0.07 nM) and Y79 cells. 5. These data suggest that the human retinoblastoma cell line may represent an additional example of the alpha 2C-adrenoceptor subtype.

Cardiovascular actions of a new selective postjunctional alpha-adrenoceptor antagonist, SK&F 104856, in normotensive and hypertensive dogs

1. SK&F 104856 (2-vinyl-7-chloro-3,4,5,6-tetrahydro-4- methylthieno[4,3,2ef][3]benzazepine) is a novel postjunctional alpha 1- and alpha 2-adrenoceptor antagonist. 2. SK&F 104856 as well as prazosin and SK&F 86466 reduced blood pressure in the anaesthetized normotensive dog. 3. SK&F 86466 and rauwolscine but not SK&F 104856 or prazosin, produced a marked increase in myocardial contractility which corresponds with their ability to block prejunctional alpha 2-adrenoceptors. 4. Intravenous or oral administration of SK&F 104856 resulted in dose-dependent antihypertensive responses in 1-kidney, 1-clip (1-K, 1-C) Goldblatt hypertensive dogs with baseline blood pressure of approximately 140 mmHg. At 0.1 and 1 mg kg-1, i.v., mean arterial blood pressure fell by 11 +/- 5 and 23 +/- 5 mmHg, respectively. At 3 and 10 mg kg-1, p.o., blood pressure fell by 9 +/- 3 and 22 +/- 5 mmHg, respectively. At 10 mg kg-1, p.o., the antihypertensive effect of SK&F 104856 was still evident at 4 h. 5. The data indicate that SK&F 104856 shows selectivity in vivo for postjunctional versus prejunctional alpha-adrenoceptors and is a potent and long-acting antihypertensive agent in 1-K, 1-C Goldblatt hypertensive dogs.

Ability of SK&F 104078 and SK&F 104856 to identify alpha-2 adrenoceptor subtypes in NCB20 cells and guinea pig lung

Alpha-2 adrenoceptors were characterized in three tissue culture cell lines and in membrane homogenates of guinea pig lung via the ability of a series of alpha adrenoceptor antagonists to inhibit the binding of [3H]clonidine, [3H]UK-14,304 and [3H]rauwolscine. The cells studied included those known to possess receptors of alpha-2A (HT29) and alpha-2B (NG108-15) subtypes as well as the previously uncharacterized NCB20 cells. Correlation of the ability of the antagonists to inhibit [3H]clonidine or [3H]UK-14,304 binding did not identify alpha-2 adrenoceptor subtypes. On the other hand, correlation of antagonist affinities against [3H]rauwolscine binding showed HT29 cells and guinea pig lung to have similar characteristics (r = 0.911) as did NG108-15 and NCB20 cells (r = 0.985). These data suggest subtle differences in the binding of [3H]agonists and [3H]antagonists to the alpha-2 adrenoceptor, resulting in the failure of [3H]clonidine and [3H]UK-14,304 to recognize differences between alpha-2A and alpha-2B receptor subtypes. 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4,5-tetrahydro-3- benzazepine (SK&F 104078) did not differentiate between the alpha-2A and alpha-2B receptor subtypes. However, 2-vinyl-7-chloro-3,4,5,6-tetrahydro-4-methylthieno[(4,3,2ef] [3]benzazepine (SK&F 104856), which has similar selectivity in functional in vitro models, was about 35-fold more potent in displacing [3H]rauwolscine binding to the alpha-2B site. These data provide additional evidence that the functional subclassification of alpha-2 adrenoceptors based on sensitivity to SK&F 104078 and SK&F 104856 subdivides this receptor in a different manner than does the alpha-2A/alpha-2B subclassification scheme.

The water diuretic effect of the alpha-2 adrenoceptor agonist, AGN 190851, is species-dependent

The effect of the novel alpha-2 adrenoceptor agonist, AGN 190851, was evaluated for its diuretic action in the rat, dog and cynomolgus monkey and its ability to inhibit vasopressin-stimulated cyclic AMP accumulation in rat and dog cortical collecting tubules in vitro. The data indicate that in the rat, AGN 190851 resulted in a dose-dependent water diuresis, which was accompanied by an increase in blood pressure and osmolar clearance. In addition, AGN 190851 resulted in a dose-dependent inhibition of vasopressin-stimulated cyclic AMP accumulation in rat cortical collecting tubules in vitro. In contrast, AGN 190851 was unable to cause either a water diuresis in conscious dogs or inhibit vasopressin-stimulated adenylate cyclase activity in canine tissue in vitro. In the lightly anesthetized cynomolgus monkey, AGN 190851 also failed to alter renal function significantly. Administration of the vasopressin receptor antagonist, SK&F 105494, to either dogs or cynomolgus monkeys demonstrated that antagonism of the vasopressin V2 receptor could result in a brisk water diuresis in both species. The data demonstrate that alpha-2 adrenoceptors can functionally antagonize vasopressin antidiuretic activity in the rat, but not in the dog or cynomolgus monkey.

Additional evidence for functional subclassification of alpha-2 adrenoceptors based on a new selective antagonist, SK&F 104856

SK&F 104856 (2-vinyl-7-chloro-3,4,5,6-tetrahydro-4-methyl-thieno[4,3,2ef][3] benzazepine) shows a similar selectivity profile to the previously reported alpha adrenoceptor antagonist, SK&F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4,5-tetrahydro-3- benzazepine), having the ability to block alpha-1 and postjunctional alpha-2 adrenoceptors, although having little or no activity at most prejunctional alpha-2 adrenoceptors. SK&F 104856 is more potent than SK&F 104078, and lacks the 5-hydroxytryptamine receptor antagonist activity associated with the earlier compound. The postjunctional vs. prejunctional selectivity of SK&F 104856 at alpha-2 adrenoceptors in the same tissue preparation was demonstrated in both canine and human saphenous vein. Concentrations substantially higher than those required to block postjunctional alpha-2 adrenoceptor-agonist induced vasoconstriction had no effect on the ability of norepinephrine, acting on prejunctional alpha-2 adrenoceptors, to inhibit stimulation evoked transmitter overflow in the human tissue, and only a small effect in the canine vein. As observed with SK&F 104078, SK&F 104856 has some prejunctional alpha-2 adrenoceptor antagonist activity in the rat vas deferens, although the receptor dissociation constant is nearly 50-fold higher than that at the postjunctional alpha-2 adrenoceptor in the canine saphenous vein. The results obtained with SK&F 104856 provide additional evidence to support the premise that alpha-2 adrenoceptors can be functionally differentiated. Because SK&F 104856 can selectively antagonize certain alpha-2 adrenoceptor-mediated responses, this agent may be a useful tool to evaluate the functional roles of the multiple alpha-2 adrenoceptor subtypes that have been identified in biochemical and molecular studies.

Lack of a pharmacological distinction between alpha-1 adrenoceptors mediating intracellular calcium-dependent and independent contractions to sympathetic nerve stimulation in the perfused rat caudal artery

The contractile response of the rat caudal artery to field stimulation occurred in two phases; an initial rapidly developing contraction (phasic) and a subsequent more slowly developing contraction (tonic). Phenoxybenzamine, an irreversible antagonist, and prazosin, a competitive antagonist at the alpha-1 adrenoceptor, nonselectively inhibited both contractile phases. Ryanodine, an inhibitor of intracellular calcium release, selectively inhibited the phasic component, whereas nifedipine, a membrane calcium channel blocker, selectively inhibited the tonic component. These data demonstrate that both contractile phases are mediated by alpha-1 adrenoceptors and the phases are distinguished by the calcium source mobilized; intracellular stores of calcium are mobilized during the phasic component and extracellular calcium is mobilized during the tonic component. Inasmuch as alpha-1 adrenoceptors on nonvascular smooth muscle may be divided into alpha-1A and alpha-1B subtypes based on ability to open membrane calcium channels or ability to stimulate intracellular calcium release, respectively, we characterized the alpha-1 adrenoceptor mediating the contraction to determine whether a specific subtype was coupled to each phase. WB4101, a marginally alpha-1A adrenoceptor selective antagonist, 5-methyl-urapidil, a highly alpha-1A adrenoceptor selective antagonist and chloroethylclonidine, a highly alpha-1B adrenoceptor selective antagonist, all inhibited both contractile phases. These findings indicate that endogenous norepinephrine contracts the rat caudal artery via alpha-1 adrenoceptors that mobilize both the release of intracellular calcium and the influx of calcium through membrane channels.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of alpha adrenoceptor subtypes in sympathetic control of the acral-cutaneous microcirculation

In pithed and anesthetized rats, laser-Doppler flowmetry was used to evaluate the role of alpha-1 and alpha-2 adrenoceptors in mediating sympathetic responses in acral regions of the cutaneous circulation. The intravenous administration of the selective alpha-1 agonist, phenylephrine, was a more potent vasopressor agent than BH-T 933 (a selective alpha-2 adrenoceptor agonist) in pithed rats. However, BH-T 933 was more potent and more efficacious than phenylephrine in reducing cutaneous microvascular perfusion (CP). BH-T 933 also caused a greater increase in cutaneous microvascular resistance. Neural and humoral sympathetic effects on CP were characterized with selective alpha-1 and alpha-2 adrenoceptor antagonists (prazosin and rauwolscine, respectively). It was found that frequency-related reductions in CP elicited by sciatic nerve stimulation were antagonized by prazosin, but not by rauwolscine. In fact, rauwolscine enhanced neurally evoked reductions in CP at the highest stimulation frequencies. However, both prazosin and rauwolscine antagonized reductions in CP elicited by electrical stimulation of the thoracolumbar outflow (sympathoadrenal activation). Ganglionic stimulation (intravenous 1,1-dimethyl-4-phenylpiperazinium) also caused a profound, transient reduction in CP that was abolished by rauwolscine, but was not significantly altered by prazosin. In contrast, 1,1-dimethyl-4-phenylpiperazinium-induced increases in mean arterial pressure were reduced by prazosin, but not by rauwolscine. In ketamine-anesthetized rats, rauwolscine caused a dose-related increase in CP without altering mean arterial pressure, whereas prazosin lowered mean arterial pressure but did not alter CP. We conclude that acral regions of the cutaneous vasculature are more sensitive to alpha-2 vis-a-vis alpha-1 adrenoceptor-mediated vasoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathetic regulation of cutaneous circulation in the rat

The purpose of the present study was to characterize, in vivo, the function and origin of peripheral alpha-adrenergic mechanisms in acral regions of the cutaneous microvasculature. Laser-Doppler flowmetry was used to continuously monitor changes in local cutaneous microvascular perfusion (CP) measured at the plantar surface of the terminal phalange in the rat. In ketamine-anesthetized rats, the intravenous administration of phentolamine (a nonselective alpha-adrenoceptor antagonist) elicited a dose-dependent (0.01-1.0 mg/kg, i.v.) increase in CP, a decrease in mean arterial pressure (MAP) and a marked reduction in cutaneous vascular resistance (CVR). These results demonstrate a high degree of endogenous alpha-adrenergic tone in the cutaneous microvasculature of the rat. In pithed rats, cutaneous vasoconstriction could be evoked by ganglionic stimulation with DMPP or by electrical stimulation of the peripheral cut end of the sciatic nerve trunk. DMPP and sciatic nerve responses were not altered by propranolol or atropine, but were inhibited by phentolamine. In addition, the effects of DMPP on CP were abolished by bilateral adrenal demedullation, but were unaltered by hindlimb denervation. These results suggest that the predominant vasoconstrictor tone in the cutaneous vasculature is mediated by a humoral action of circulating catecholamines at postjunctional alpha-adrenoceptors. The adrenal medulla appears to be the origin of this humoral tone. Postganglionic sympathetic nerves play an insignificant role in mediating cutaneous vasoconstriction elicited by ganglionic stimulation.

Evidence for heterogeneity of prejunctional alpha-2-adrenoceptors

The interactions between SK&F 104078 and several selective alpha 2-adrenoceptor agonists at pre- and postjunctional alpha 2-adrenoceptors were investigated in order to assess the previously reported selectivity of SK&F 104078 for postjunctional alpha 2-adrenoceptors and to determine whether or not SK&F 104078 could uncover subtypes of alpha 2-adrenoceptors located prejunctionally as has also been suggested. The alpha 2-adrenoceptor agonists, UK 14,304, xylazine, B-HT 933, B-HT 920, clonidine and M-7, produced concentration-dependent prejunctional alpha 2-adrenoceptor-mediated inhibition of neurogenic responses in the guinea pig atrium and rat vas deferens and produced postjunctional alpha 2-adrenoceptor-mediated contraction of the canine saphenous vein. The alpha 2-adrenoceptor antagonist, rauwolscine, blocked all the agonists at both pre- and postjunctional alpha 2-adrenoceptors without demonstrating preference for any agonist or any synaptic location of alpha 2-adrenoceptors. In marked contrast, SK&F 104078 produced equivalent antagonism of all agonists in the canine saphenous vein but had no significant effect against the same agonists in the guinea pig atrium, suggesting a high degree of selectivity for postjunctional alpha 2-adrenoceptors in these test systems, consistent with our previous observations. In the rat vas deferens, however, SK&F 104078 significantly antagonized the prejunctional alpha 2-adrenoceptor-mediated effects of clonidine and M-7 but did not block the responses to UK 14,304, xylazine, B-HT 933 and B-HT 920. These results indicate that the prejunctional alpha 2-adrenoceptor antagonist effects of SK&F 104078 are tissue and agonist dependent, and that there may be at least two subtypes of prejunctional alpha 2-adrenoceptors that can be discriminated with SK&F 104078 but not with rauwolscine. Both subtypes of prejunctional alpha 2-adrenoceptors may be present in the rat vas deferens, while only the SK&F-104078-insensitive subtype is present in the guinea pig atrium.

Metabolic regulation by alpha 1- and alpha 2-adrenoceptors

The role of alpha-adrenoceptors in the mediation of autonomic function, particularly in the control of the cardiovascular system, is widely known. However, alpha-adrenoceptors are also important in the regulation of a variety of metabolic processes that occur in the body either through direct action or by stimulation of the release of other mediators that control metabolic function. Thus, alpha 2-adrenoceptor activation by circulating or neuronally released catecholamines inhibits the release of insulin from pancreatic islet beta-cells and, by inhibiting this response, alpha 2-adrenoceptor antagonists have been shown to have an antihyperglycemic effect. The alpha-adrenoceptor-mediated regulation of the release of pituitary hormones is indirect, with alpha-adrenoceptors being located on peptidergic neurons in the hypothalamus that secrete releasing hormones into the hypophysial portal system to regulate the secretion of hormones from the anterior pituitary gland. Thus, the increase in cortisol secretion from the adrenal glands following a meal is produced, at least in part, by an alpha 1-adrenoceptor-mediated increase in vasopressin and CRF-41 secretion from neurons on the hypothalamus that stimulate the release of adrenocorticotrophic hormone secretion from the pituitary gland, which subsequently stimulates the synthesis and release of cortisol from the adrenal medulla. In addition to metabolic regulation by alpha 1- and alpha 2-adrenoceptors within the endocrine system, alpha-adrenoceptors are also a component of the system that regulates certain aspects of metabolism within autonomic effector cells, such as the control of smooth muscle cell division and growth during periods of continued alpha-adrenoceptor activation as a result of activation of second messenger systems.

Studies on the mechanism of neuropeptide Y induced potentiation of neurogenic vasoconstriction in the isolated rabbit ear artery

Neuropeptide Y (NPY) has been shown to potentiate the response of the isolated rabbit ear artery to field stimulation, provided that the vascular endothelium is intact. This report describes efforts to elucidate the mechanism by which NPY produces this effect. The response of the perfused rabbit ear artery to brief intermittent field stimulation was significantly enhanced by NPY (100 nmol/L). The administration of indomethacin (10 mumol/L) to inhibit cyclooxygenase did not affect the ability of NPY to potentiate neurogenic vasoconstriction. Blockade of calcium channels with nifedipine (1 mumol/L) produced a partial attenuation of the NPY effect. Our studies therefore suggest that an arachidonic acid metabolite is not involved in NPY induced potentiation of vascular neuroeffector transmission in the rabbit ear artery. On the other hand, the translocation of extracellular calcium may play a role in this process.

Distinction of NPY receptors in vitro and in vivo. I. NPY-(18-36) discriminates NPY receptor subtypes in vitro

We studied the possibility of multiple neuropeptide Y (NPY) receptor subtypes. NPY-stimulated Ca2+ mobilization in human erythroleukemia (HEL) cells was used to screen a number of NPY analogues. The potencies of three of these analogues [peptide YY (PYY), [D-Tyr-36]NPY, and NPY-(18-36)] were compared with that of NPY in the following model systems: Ca2+ mobilization and inhibition of adenosine 3',5'-cyclic monophosphate accumulation in HEL cells, potentiation of vasoconstriction in the isolated rabbit ear artery, reduction of cutaneous microvascular perfusion in the rat digit, and inhibition of [3H]serotonin release in rat brain. In each of the five models, PYY was a full agonist that exhibited a similar or slightly higher potency than NPY, whereas [D-Tyr-36]NPY and NPY-(18-36) were partial agonists with lower potencies: NPY-(18-36) had a lower potency and efficacy than [D-Tyr-36]NPY in HEL cells and the rabbit ear artery, but was more effective than [D-Tyr-36]NPY for constricting cutaneous microvasculature and inhibiting serotonin release. Because of its weak partial agonism, we also tested NPY-(18-36) as an antagonist of NPY-stimulated Ca2+ mobilization in HEL cells. NPY-(18-36) shifted the NPY concentration-response curve to the right with a KB affinity value of 297 nM. In summary, [D-Tyr-36]NPY and NPY-(18-36) are partial agonists, the relative potency of which varies between systems. These data demonstrate the presence of multiple NPY receptor subtypes. We propose a modified classification scheme of NPY receptor subtypes.

Cardiovascular effects of SK&F 104078, a novel alpha-adrenoceptor antagonist, in normotensive and hypertensive rats

SK&F 104078 is a novel alpha-adrenoceptor antagonist derived from the 3-benzazepine alpha 2-adrenoceptor antagonist SK&F 86466. SK&F 104078 will block both alpha 1- and vascular postjunctional alpha 2-adrenoceptors but does not block most prejunctional alpha 2-adrenoceptors. Intravenous (i.v.) administration of SK&F 104078 decreased blood pressure (BP) in both spontaneously hypertensive and DOCA-salt hypertensive rats. SK&F 104078 potentiated the hypotensive response to tilt in anesthetized spontaneously hypertensive rats (SHR). Although SK&F 104078 had no effect on BP in normotensive rats, the tilt-induced decrease in BP in these animals was also potentiated. In this regard, SK&F 104078 resembled the selective alpha 1-adrenoceptor antagonist prazosin, rather than the alpha 2-adrenoceptor antagonists rauwolscine or SK&F 86466. Oral administration of SK&F 104078 had no significant effect on BP in SHR unless extremely high doses were administered. This was consistent with low plasma concentrations of SK&F 104078 observed after oral administration. After i.v. administration, the clearance of SK&F 104078 from plasma was 123 ml/min/kg, the steady-state volume of distribution was 17 L/kg, and the fraction excreted unchanged in urine was less than 1%. The low oral bioavailability of SK&F 104078 did not appear to be due to high first-pass oxidative metabolism, since pretreatment of SHR with the suicide substrate inhibitor of cytochrome P-450, 1-aminobenzotriazole (ABT), did not result in increased oral efficacy. SK&F 101253, a close structural analogue of SK&F 104078, was an effective antihypertensive when administered orally. Comparison of the stability of SK&F 101253 and SK&F 104078 in acid media showed SK&F 104078, but not SK&F 101253, to be rapidly degraded.(ABSTRACT TRUNCATED AT 250 WORDS)

The pharmacology of carvedilol

Carvedilol is a potent antihypertensive agent with a dual mechanism of action. At relatively low concentrations it is a competitive beta-adrenoceptor antagonist and a vasodilator, whereas at higher concentrations it is also a calcium channel antagonist. The antihypertensive activity of carvedilol is characterized by a decrease in peripheral vascular resistance, resulting from the vasodilator activity of the compound, with no reflex tachycardia, as a result of beta-adrenoceptor blockade. The antihypertensive activity of carvedilol is associated with an apparent "renal sparing" effect in that the reduction in mean arterial blood pressure does not compromise renal blood flow or urinary sodium excretion. Studies on the mechanism of action of carvedilol indicate that the compound is a potent competitive antagonist of beta 1- and beta 2-adrenoceptors with a dissociation constant (KB) of 0.9 nM at both beta-adrenoceptor subtypes. Carvedilol is also a potent alpha 1-adrenoceptor antagonist (KB = 11 nM), which accounts for most, if not all, of the vasodilating response produced by the compound. At concentrations above 1 microM, carvedilol is a calcium channel antagonist. This activity can be demonstrated in vivo at doses that represent the higher end of the antihypertensive dose-response curve. Although the calcium-channel blocking activity of carvedilol may not contribute to the antihypertensive activity of the compound, it may play a prominent role in certain peripheral vascular beds, such as the cutaneous circulation, where marked increases in blood flow are observed. The data indicate that carvedilol is an antihypertensive agent that is both a beta-adrenoceptor antagonist and a vasodilator.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of alpha-1 and alpha-2 adrenoceptors in the sympathetic control of the proximal urethra

In the pithed rat, postjunctional alpha-1 and alpha-2 adrenoceptors mediate increases in proximal urethral perfusion pressure (UPP). The present study examined the role of alpha-1 and alpha-2 adrenoceptors in sympathetic control of the isolated in situ proximal urethra of the rat. An endogenous alpha adrenergic constriction was demonstrated in the proximal urethra by eliciting dose-related and phentolamine-sensitive increases in UPP after the systemic administration of tyramine. Transient dose-related and hexamethonium-sensitive increases in UPP were also elicited by ganglionic stimulation with 1,1-dimethyl-4-phenylpiperazinium (DMPP). Based on the relative sensitivity of the DMPP response to standard autonomic blockers, it was determined that constriction is the predominant autonomic response in the proximal urethra and this response is mediated by alpha adrenoceptor mechanisms. Prazosin, a selective alpha-1 adrenoceptor antagonist, substantially reduced (72%) the steady-state increases in UPP elicited by discrete low frequency electrical stimulation of the spinal hypogastric outflow. In contrast, selective alpha-2 adrenoceptor blockade with rauwolscine significantly increased (28%) the UPP response to hypogastric stimulation. Prazosin also abolished the increase in urethral tone elicited by tyramine, whereas rauwolscine had no effect on the tyramine. A different response profile was observed for prazosin and rauwolscine when the steady-state increase in UPP was evoked by simultaneously stimulating the midthoracic and hypogastric spinal outflow. Under these conditions of sympathoadrenal activation, UPP was reduced by both prazosin and rauwolscine (63 and 50%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)