Subclassification of alpha-adrenergic receptors by direct binding studies

Modification of the beta- and alpha2-adrenergic sensitivity of rat submandibular glands by environmental stimuli and stress

In man, the rate of resting salivary secretion can be influenced by environmental stimuli related to light dark cycles or by noxious stimuli (stressors) of psychological origin. The sympathetic branch of the autonomic nervous system and the adrenal medulla play an important part in homeostatic responses. Previous observations have shown that chronic exposure of rats to constant light promotes degranulation of parotid acini and desensitization of submandibular beta-adrenergic receptors. Now the submandibular secretory response elicited by beta- and alpha2-adrenergic agonists was studied in rats chronically exposed to environmental conditions that modified the activities of sympathetic efferents to the pineal, salivary and adrenal glands. Adult male rats were exposed to constant light (LL) or constant darkness (DD) for 20 days, or to stress (2 h daily immobilization) for 14 days. Control animals were kept under the usual lighting conditions and without immobilization. Dose response curves to isoproterenol (i.v), before and after administration (i.v.) of a dose (20 microg/kg) of clonidine were obtained. Beta-adrenergic desensitization was observed in all the experimental groups, while alpha2-adrenergic desensitization was only observed in the stress and LL groups. The results suggest that circulating catecholamines could mediate light and stress effects on submandibular beta-adrenergic secretory responses. Extrasynaptic alpha2-adrenoceptors might modulate the submandibular secretory response when predictable environmental stimuli (daily light phase) or unpredictable stressors raise the concentrations of circulating catecholamines.

Role of different subtypes of adrenoceptors in pressor responses to catecholamines released from sympathetic nerve endings

The role of alpha 1- and alpha 2-adrenoceptors in the vascular effects of catecholamines, either released locally from sympathetic nerve endings (e.g., in vascular smooth muscle) or derived from the adrenal medulla or administered intravenously, was studied using selective antagonists of these adrenoceptors. The ganglionic stimulant dimethylphenyl-piperazinium-iodide (DMPP) exerted dual actions on blood pressure: a rapid and short-term pressor reaction (phase I) resulting from catecholamine release elicited by ganglion stimulation, followed by a more sustained blood pressure elevation (phase II) resulting from the circulating catecholamines released from the adrenal medulla. The selective alpha 2-adrenoceptor, but a not subtype selective, antagonist 7,8-(methylenedioxi)-14-alpha-alloberbane HCl (CH-38083) (50-100 micrograms/kg, IV) significantly (p < 0.05) inhibited the pressor effects of epinephrine and norepinephrine given intravenously and phase II of the DMPP-induced pressor reaction. Idazoxan exerted similar effects, but at higher doses (400-600 micrograms/kg, IV). WB-4101 (50-100 micrograms/kg, IV) and BRL-44408 (2-3 mg/kg, IV), two selective alpha 2A-adrenoceptor antagonists, had the same activity as CH-38083, except did not inhibit the pressor effect of intravenously administered norepinephrine. The alpha 2B-adrenoceptor selective antagonist, ARC-239 (150 micrograms/kg, IV) did not influence phase II of DMPP-induced pressor reaction. Prazosin (200 micrograms/kg, IV), an antagonist of alpha 1 and alpha 2B-adrenoceptors, reduced blood pressure, the pressor response to intravenously administered epinephrine, and phase I of the DMPP-induced pressor effect. In addition, it completely inhibited the pressor responses to DMPP remaining after administration of CH-38083. These results suggest that the postsynaptically located alpha 1- and alpha 2(A and B)-adrenoceptors are involved in pressor response to norepinephrine and epinephrine, and are sensitive and accessible to catecholamines released locally from the axon terminals, and from the circulation to a different extent. These results may have great therapeutical importance in hypertension, for which the involvement of both a high level of circulating and locally released catecholamines may be indicative of the usefullness of a combination (alpha 1- and alpha 2-adrenoceptors- and Ca-channel-blocking agents) therapy.

Effects of imidazoline antagonists of alpha 2-adrenoceptors on endogenous adrenaline-induced inhibition of insulin release

We studied the effects of adrenoceptor antagonists and imidazoline derivatives on endogenous adrenaline-induced inhibition of insulin release in anesthetized rats. The intracerebroventricular injection of neostigmine increased plasma levels of catecholamines and glucose but not insulin. Pretreatment with an i.p. injection with phentolamine caused a dose-dependent increase in insulin secretion. When atropine was coadministered with phentolamine, the phentolamine-induced increase in insulin secretion was inhibited. Neither phentolamine nor atropine affected plasma levels of catecholamine. Yohimbine and idazoxan, which are alpha 2-adrenoceptor antagonists, and tolazoline, a non-selective alpha-adrenoceptor antagonist, also reversed adrenaline-induced inhibition of insulin secretion. Phenoxybenzamine, prazosin, propranolol, and antazoline, an imidazoline without alpha 2-adrenoceptor activity, did not affect insulin levels. When agents were preinjected i.p. in rats that were given saline into the third cerebral ventricle, phentolamine and antazoline, but not yohimbine and idazoxan, increased plasma levels of insulin. The results suggest that the inhibition of insulin release induced by adrenaline was reversed by antagonism of alpha 2-adrenoceptors. Phentolamine and antazoline, both of which are imidazoline derivatives, induced insulin secretion independently of the adrenoceptors only under the resting conditions.

Antagonist drugs and bone vascular smooth muscle

An ex vivo canine tibia model was used to quantitate the specific adrenergic subtype contribution in bone vasculature. Tibiae were obtained from mongrel dogs, the nutrient artery was catheterized, and the bone was placed in an ex vivo perfusion apparatus at constant flow. Perfusion was accomplished using oxygenated Krebs-Ringer solution. A norepinephrine dose-response curve was obtained by using incremental single bolus doses. Each bone was perfused with a vasoactive drug at a standard physiologic dosage. After 30 min of perfusion, a second norepinephrine dose-response curve was generated. The degree of attenuation of the norepinephrine dose-response curve, as determined by the total area under the curve, was interpreted as the relaxation effect of the drug on the smooth muscle of the vascular bed. Prazosin (alpha 1-receptor antagonist), rauwolszin (alpha 2-receptor antagonist), propranolol (beta-receptor antagonist), and diltiazem (calcium-entry inhibitor) were evaluated. Our data suggest that alpha 1 and alpha 2 adrenergic receptor antagonism results in a quantitatively similar attenuation of norepinephrine-induced vascular smooth muscle contraction. Calcium-entry antagonism produced less, but significant, attenuation of smooth muscle contractility. Beta-adrenergic receptor blockade yielded only a slight, although consistent, reduction in reactivity. Simple perfusion with Krebs-Ringer solution had no effect.

Role of adrenoceptors in vitro and in vivo in the effects of lithium on blood glucose levels and insulin secretion in the rat

1. Pretreatment of rats with the non-selective alpha-adrenoceptor antagonist dihydroergotamine counteracts the inhibition of glucose-induced insulin secretion caused by lithium both in vitro and in vivo. The present study was therefore carried out to specify further which type of adrenoceptor is involved in lithium-induced hyperglycaemia and inhibition of insulin secretion. 2. The lithium-induced effects were reversibly blocked by pretreatment of rats with the alpha 2-adrenoceptor antagonist yohimbine or a combination of yohimbine and the non-selective beta-receptor antagonist propranolol, whereas the alpha 1-receptor antagonist prazosin and propranolol alone were ineffective in blocking these effects. 3. These findings suggest that the effects of lithium on plasma glucose and insulin levels are mediated mainly by the stimulation of alpha 2-adrenoceptors.

Persistence of sympathetic-mediated forearm vasoconstriction after alpha-blockade in hypertensive patients

Sympathetic vasoconstriction not mediated by alpha-adrenoceptors has been identified in vitro and in animals but not in humans. We evaluated the effect of alpha-adrenoceptor blockade on either endogenous vascular sympathetic activation (obtained through the application of a nonhypotensive lower-body negative pressure, -10 mm Hg for 5 minutes) or selective postsynaptic alpha-adrenoceptor stimulation by exogenous norepinephrine (0.005 micrograms/100 ml forearm tissue/min for 3 minutes) in the presence of beta-blockade by propranolol (10 micrograms/100 ml forearm tissue/min for 15 minutes). Drugs were infused into the brachial artery at systemically ineffective rates while continuously monitoring forearm blood flow (by venous plethysmography), intra-arterial mean arterial pressure, and heart rate in patients with essential hypertension. The irreversible antagonist phenoxybenzamine was used at a rate of 20 micrograms/100 ml forearm tissue/min for 1 hour, which antagonized the local responses to norepinephrine in a range of 0.005-0.05 micrograms/100 ml forearm tissue/min. During saline administration, either lower-body negative pressure or exogenous norepinephrine decreased forearm blood flow comparably. However, after phenoxybenzamine administration, forearm vasoconstriction to norepinephrine was abolished while a residual response to lower-body negative pressure remained in each patient. To exclude insufficient alpha-adrenoceptor blockade, the same experimental protocol was repeated by doubling phenoxybenzamine concentrations. No difference from the data obtained with the lower level of antagonist was found. Further studies were performed to confirm the sympathetic origin of the residual vasoconstriction. Bretylium tosylate, a neurotransmitter blocker, infused into the brachial artery (50 micrograms/100 ml forearm tissue/min for 90 minutes) abolished the effect of endogenous sympathetic activation but did not alter the effect of exogenous norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and function of alpha-adrenergic receptors

Alpha 1- and alpha 2-adrenergic receptors are the initial recognition sites on a wide variety of catecholamine-responsive target cells. This article addresses several major questions related to subtypes, structure, signal transduction mechanisms, and regulation of alpha 1- and alpha 2-adrenergic receptors. The application of biochemical and cell and molecular biologic techniques has provided many new insights regarding alpha-adrenergic receptors. Two (and perhaps three) distinct alpha 2-adrenergic receptor subtypes have been identified, and subtypes may exist for alpha 1-adrenergic receptors as well. These multiple subtypes imply much greater diversity among alpha-adrenergic receptors than among beta-adrenergic receptors. Alpha-adrenergic receptors are membrane glycoproteins with several common structural features (including seven membrane-spanning domains with extracellular amino terminus and intracellular carboxyl terminus) that are shared with other types of membrane receptors linked to guanine nucleotide-binding regulatory (G) proteins. These G proteins appear to link alpha-adrenergic receptors to multiple effector systems, including enzymes such as adenylate cyclase and phospholipases, and ion channels. The receptors themselves are dynamic entities, the number of which is regulated as a consequence of a poorly understood life cycle. Although unproven, it seems likely that several important clinical disorders represent alterations in alpha-adrenergic receptors themselves or in the G proteins or effector systems to which these receptors couple. New tools for studying receptor structure and function should help clarify the numerous, inadequately understood issues regarding alpha-adrenergic receptors and their possible alteration in disease states.

Evidence for multiple [3H]prazosin binding sites in canine brain membranes

Two classes of alpha 1 adrenoceptors were identified in canine brain and liver using conventional radioligand binding methods. Scatchard plots of specific [3H]prazosin binding to brain and liver membranes prepared from 100-150-day-old Doberman pinscher dogs were consistently curvilinear and best fit a two-site binding model (frontal cortex, Kd1 = 57.7 +/- 10.0 pM, Bmax1 = 64.6 +/- 17.1 fmol/mg protein, Kd2 = 1.5 +/- 0.5 nM, Bmax2 = 159 +/- 37.6 fmol/mg protein; liver, Kd1 = 82.6 +/- 36 pM, Bmax1 = 7.0 +/- 5.1 fmol/mg protein, Kd2 = 0.8 +/- 0.2 nM, Bmax2 = 62.1 +/- 8.7 fmol/mg protein). Kinetically derived affinity constants from association and dissociation experiments agreed with those obtained by Scatchard analyses of equilibrium binding data. Binding sites were saturable, heat labile, bound ligand reversibly, and appeared to be appropriately distributed in relation to endogenous catecholamine. [3H]Prazosin also bound with high affinity to two classes of binding site in porcine and bovine brain membrane but [3H]prazosin binding in monkey and rat brain was best described by a single-site binding model. Affinities obtained were in between values obtained for high and low affinity Kds in the other species. Competitions for [3H]prazosin binding sites in canine frontal cortex were conducted with the following antagonists: WB-4101, corynanthine, phentolamine, benoxathian, phenoxybenzamine, chlorethylclonidine, thymoxamine, prazosin, yohimbine and agonists: methoxamine, (-)-norepinephrine, and clonidine. All ligands but prazosin, norepinephrine and clonidine competed for specific [3H]prazosin binding in a statistically significant biphasic manner. Benoxathian and WB-4101 displayed the highest affinities (benoxathian: Ki1 = 0.26 nM, WB-4101: Ki1 = 0.20 nM) and selectivity (high affinity/low affinity: benoxathian = 1640, WB-4101 = 13204) for the high affinity [3H]prazosin binding site; chlorethylclonidine had highest affinity (Ki2 = 91 nM) and selectivity (low affinity/high affinity = 405) for the lower affinity [3H]prazosin binding site. As defined, the two sites were similar to the alpha 1a and alpha 1b recently described in the rat and rabbit. A noticeable difference was that the subtypes described in dog brain had a 30-fold difference in affinity for prazosin.

From epinephrine to cyclic AMP

Binding of catecholamines to the beta-adrenergic receptor results in the activation of adenylate cyclase and the intracellular formation of adenosine 3',5'-monophosphate (cAMP). In the past 20 years the events that lead from hormone binding at the cell surface receptor site to the synthesis of cAMP at the inner layer of the membrane have been intensively studied. Signal transduction in this system involves the sequential interaction of the beta-adrenergic receptor with the guanine nucleotide-binding protein (Gs) and the adenylate cyclase catalyst (C). The mechanism of signal transduction from the receptor through Gs to C, as well as the role of the adenylate cyclase inhibitory G protein Gi, is discussed.

Differentiation of alpha-adrenergic receptors using pharmacological evaluation and molecular modeling of selective adrenergic agents

Subtypes of alpha adrenergic receptors were studied using selective adrenergic agonists. A-53693, A-54741, and related compounds were evaluated for their affinity for alpha receptor subtypes using radioligand binding techniques. Efficacy and potency were also evaluated using in vitro bioassays of alpha-1 receptors in rabbit aorta smooth muscle and alpha-2 receptors in the phenoxybenzamine-pretreated canine saphenous vein. Active and inactive compounds were then submitted for computer-assisted molecular modeling evaluation to ascertain the structural requirements for optimal potency and selectivity. Rigid catecholamines such as A-53693 display a high degree of selectivity for alpha-2 compared to alpha-1 receptors, probably because of the unique regions of space at the ligand binding site occupied by active compounds. Imidazolines such as A-54741 also interact with extremely high affinity and potency for alpha-2 receptors, and to a lesser extent at alpha-1 receptors. The spatial domains occupied by phenethylamines and imidazolines differ, each having unique regions of permissable space at alpha receptors. Compounds such as A-53693 and A-54741 are extremely useful probes of the molecular interactions of alpha agonistic compounds which will help in the design of even more selective drugs for alpha adrenergic receptors.

Adrenocorticotropic hormone stimulation of lacrimal peroxidase secretion

The effect of adrenocorticotropic hormone (ACTH) on secretion of lacrimal gland peroxidase was studied using an in vitro perifusion technique. The peptide stimulated a dose-dependent (1 nM to 100 nM) release of peroxidase, with the maximum level of secretion induced by 20 nM ACTH. Secretion in the presence of submaximal ACTH was potentiated with either 100 microM iso-butylmethylxanthine or 0.3 microM carbachol. In contrast, the combination of ACTH and phenylephrine was additive. Time-dependence studies demonstrated that the stimulation of peroxidase release by ACTH, as with other cyclic adenosine monophosphate mediated secretagogues, showed a latency in reaching the maximum rate which was not evident with either cholinergic or alpha-adrenergic stimulation. Furthermore, where potentiation of the response to ACTH occurred, the time course was distinctly altered from that obtained with either ACTH or the potentiating agonist alone. The data suggest that lacrimal gland function is regulated by a multiple system of neurotransmitters and (or) neuromodulators that involves the activation of peptidergic as well as cholinergic and alpha-adrenergic receptors.

Noradrenergic effects on rat visual cortex: single-cell microiontophoretic studies of alpha-2 adrenergic receptors

The catecholamine noradrenaline has been proposed to modulate the excitability of cortical neurons, and such a regulation may be mediated by specific adrenergic receptors. We characterized, using electrophysiological recordings, the types of responses of single cells in the rat visual cortex (areas 17 and 18) to the iontophoretic application of adrenergic agents. For the majority of spontaneous and visually-driven cells sampled, noradrenaline decreased the firing frequency, and in some cases of visually-driven cells could increase the signal/noise ratio. These effects were also documented after the application of the alpha-2 adrenergic agonists clonidine and oxymetazoline, and could be reduced or blocked by a previous ejection of the specific alpha-2 antagonist idazoxan. The present study supports a role for alpha-2 adrenoceptors in the modulation of sensory inputs to the visual cortex.

Alpha-1 and alpha-2 adrenoceptor binding in cerebral cortex: competition studies with [3H]prazosin and [3H]idazoxan

The tritiated adrenergic antagonists prazosin ([3H]PRZ) and idazoxan ([3H]IDA, or RX-781094) bind specifically and with high affinity in membrane preparations from cerebral cortex to alpha-1- and alpha-2-adrenoceptors respectively. Saturation experiments, performed to determine the density of receptors (Bmax; maximum binding capacity) and the dissociation constant (Kd 25 degrees C), were analyzed by the methods of Eadie and Hofstee, iterative modelling, and the procedure of Hill. The pharmacologic properties and specificity of the labelling was verified by displacement experiments using alpha-adrenergic antagonists and agonists. The antagonist drugs showed the following order of potency to displace [3H]prazosin: prazosin much greater than phentolamine much greater than corynanthine greater than pyrextramine much greater than yohimbine much greater than piperoxan greater than benextramine greater than idazoxan; for the agonists: clonidine much greater than (-)-noradrenaline much greater than (-)-adrenaline much greater than phenylephrine, while other drugs, such as (-)-propranolol, dopamine, (-)-isoproterenol and serotonin only competed with the alpha-1-ligand at concentrations above 20 microM. The alpha 2-sites labelled by [3H]idazoxan were characterized by the antagonist displacement sequence idazoxan much greater than phentolamine greater than yohimbine = greater than piperoxan much greater than pyrextramine much greater than benextramine much greater than prazosin much greater than corynanthine. The agonists order of potency to compete with [3H]idazoxan was clonidine much greater than phenylephrine = greater than (-)-adrenaline greater than (-)-noradrenaline, and for other related drugs it was (-)-propranolol much greater than dopamine much greater than serotonin greater than (-)-isoproterenol. These competition experiments clearly showed two pharmacologically distinct sites, but question the relative specificity of some of the adrenergic drugs.

Effect of clonidine on secretion of fluid and ions by the parotid and submandibular glands of the rat

Secretion in response to this alpha 2-adrenergic agonist was evaluated in the presence and absence of several adrenergic antagonists, reserpine and sympathectomy (Sx). In both glands, the response was qualitatively but not quantitatively similar to that induced by the alpha 1-adrenergic agonist, phenylephrine, in the presence of propranolol. With clonidine, the volume of submandibular saliva was much higher but the Ca concentration was 3-4 times lower than that of the parotid; both salivas had low Na but high K concentrations. Clonidine-induced secretion was almost completely blocked by the alpha 1-adrenergic antagonist, prazosin and a mixed alpha-adrenergic antagonist, phentolamine, and markedly reduced by the alpha 2-adrenergic antagonist, yohimbine, but unaffected by the beta-adrenergic antagonist, propranolol. Reserpine reduced the parotid, but enhanced the submandibular secretory response to clonidine. Results in Sx glands were similar. Thus, in the rat glands clonidine may activate alpha 1-rather than alpha 2-adrenoceptors, which appear to play a part similar to alpha 1-adrenoceptors only after reserpine or Sx.

Autonomic effects on protein secretion by rat submandibular salivary glands

1. Following treatment with cholinergic and beta-adrenergic drugs, the beta-type protein, associated with cAMP, was secreted regardless of the doses used. 2. Following treatment with alpha 1-adrenergic drugs, both the beta-type and alpha-type proteins were secreted depending on the doses used and the alpha-type protein was completely converted to the beta-type with alpha-blockers. 3. Following treatment with alpha 2-adrenergic drugs, the gamma-type protein, associated with cGMP, was secreted independent of the doses used.

Effects of alcohol on alpha-adrenergic receptor regulation of calcium ATPase in liver plasma membranes

The effects of alpha 1- and alpha 2-adrenergic agonists, viz., phenylephrine and clonidine, respectively, were studied on rat liver plasma membrane Ca++-ATPase. Phenylephrine produced a 23% inhibition of enzyme activity at 5 microM. Prazosin, an alpha 1 antagonist, completely prevented the effect of phenylephrine. Clonidine produced a comparable inhibition of Ca++-ATPase, but was not reversed by the antagonist yohimbine, suggesting a lack of functionally significant alpha 2 receptors as previously reported. The results support the role of high-affinity Ca++-ATPase in liver plasma membranes in the control of cytosolic free Ca++ levels through regulation by alpha 1-adrenergic receptors. In vitro and acute ethanol exposure produced inhibition of plasma membrane Ca++-ATPase. In addition, ethanol treatment significantly reversed the inhibitory effect of phenylephrine on Ca++-ATPase. Chronic ethanol exposure for four weeks increased Ca++-ATPase activity over control and increased enzyme activity in the presence of phenylephrine. These results demonstrate that ethanol alters the alpha-adrenergic receptor interaction with Ca++-ATPase resulting in reduced receptor regulation of cytosolic Ca++ levels. These changes may prevent the liver from maintaining Ca++ levels for second messenger functions, such as glycolysis and gluconeogenesis.

Alpha-adrenoceptors and monoamine contents in the cerebral cortex of the rodent Jaculus orientalis: effects of acute cold exposure

The tritiated adrenergic antagonists prazosin ([3H]PRZ) and idazoxan ([3H]IDA, or RX-781094) bind specifically and with high affinity to alpha 1- and alpha 2-adrenoceptors respectively, and were used to measure adrenoceptors in membrane preparations obtained from the cerebral cortex of Jaculus orientalis. Membrane preparations were also obtained from a group of cold exposed animals, to determine whether these adrenoceptors could be modified by a thermic stress. The density of receptors (Bmax; maximum binding capacity) and the dissociation constant (Kd 25 degrees C) were estimated by iterative modelling, and by using the procedure of Hill. After acute cold exposure (16 hr, 5 degrees C) there was a decrease in the affinity of the alpha 1-adrenoceptors, as judged by the Kd 25 degrees C for [3H]PRZ, with no changes in the Bmax. The alpha 2-sites did not show any significant changes, as revealed by [3H]IDA binding. Pretreatment of the membrane preparations from control animals with the disulfide and sulfhydryl reactives DL-dithiothreitol, 5,5'-dithiobis-(2-nitrobenzoic acid) and N-ethylmaleimide decreased specific [3H]PRZ and [3H]IDA binding, with minor changes in non-specific counts, indicating that the fixation of these ligands was to the receptor proteins. The endogenous cortical monoamine contents were also determined in the frontal cerebral cortex of these same animals, using high performance liquid chromatography with electrochemical detection. The catecholamine levels and their major metabolites were found to be stable in the cortex after the acute thermic stress, but there was a marked reduction in serotonin with a normal content in 5-hydroxyindole-3-acetic acid.

Effects of isoproterenol pretreatment on phosphatidylinositol turnover in rat parotid gland

The effects of isoproterenol pretreatment on phosphatidylinositol turnover in rat parotid slices were studied to elucidate the relationship between beta- and alpha 1-adrenoceptors. 32P-Labeling of phosphatidylinositol in parotid slices was increased by an alpha 1- and alpha 2-agonist (epinephrine and norepinephrine) and alpha 1-agonists (methoxamine and phenylephrine), but not by an alpha 2-agonist (clonidine) and a beta-agonist (isoproterenol). Prazosin inhibited the increase in phosphatidylinositol turnover elicited by epinephrine, but propranolol did not. These results indicate that the stimulation of phosphatidylinositol turnover elicited by adrenergic agonists is mediated by activation of alpha 1-adrenoceptors in the parotid glands. Isoproterenol pretreatment of the parotid slices caused a significant increase in 32P-labeling of phosphatidylinositol and a decrease in that of phosphatidic acid. The epinephrine- or phenylephrine-induced increase in 32P-labeling of phosphatidylinositol were further enhanced by the isoproterenol pretreatment of the slices. In the isoproterenol-treated membranes of the parotid glands, [3H]prazosin binding to alpha 1-receptors increased, but [3H]dihydroalprenolol binding to beta-receptors did not. These findings indicate that the acceleration of phosphatidylinositol turnover induced by the isoproterenol pretreatment may be associated with an increase in alpha 1-adrenoceptor binding sites which might have appeared as a result of the isoproterenol pretreatment of the parotid slices.

Presynaptic alpha-adrenoceptor mediated inhibition of the neurogenic cholinergic contraction in the isolated intestinal bulb of the carp (Cyprinus carpio)

The effects of norepinephrine, epinephrine and clonidine on neurogenic cholinergic contraction were examined in the presence of a beta-adrenoceptor blocking agent, carteolol (5 X 10(-6) M), in the isolated intestinal bulb of the carp. Norepinephrine, epinephrine (10(-9)-10(-6) M) and clonidine (10(-8)-10(-5) M) inhibited the contraction induced by low frequency (2 or 5 Hz) transmural stimulation (TMS) without inhibiting the contraction induced by acetylcholine (ACh, 6 X 10(-8)-4 X 10(-7) M). Methoxamine (10(-4) M) and phenylephrine (10(-4) M) showed no such inhibitory effect on the TMS-induced contraction. The inhibitory effects of catecholamines and clonidine were decreased by phentolamine (5.4 X 10(-6) M) and yohimbine (10(-7)-10(-6) M) but not by prazosin (7 X 10(-7)-10(-6) M). Nicotine (10(-6)-10(-4) M) and serotonin (3 X 10(-8)-3 X 10(-6) M) caused contraction of the intestinal bulb indirectly by releasing endogenous ACh. This contraction was inhibited by norepinephrine, epinephrine and clonidine in a concentration-dependent manner. The present results suggest that catecholamines and clonidine inhibit cholinergic transmission via the activation of a presynaptic alpha-adrenoceptor (presumably of alpha-2 type) located on the cholinergic nerve terminals innervating the smooth muscle of the intestinal bulb of the carp.

Characterization of alpha-adrenoceptors in myometrium of preparturient rats

We describe three methods for the quantitative analysis of the alpha-adrenoceptor subtypes in preparturient rat myometrial membrane fractions. A non-subtype-selective antagonist radioligand. [3H]dihydroergocryptine ([3H]DHE), was used to label all of the alpha-receptors. [3H]DHE bound to both alpha 1- and alpha 2-receptors with indistinguishable affinity. Computer modelling of competition curves of unlabeled selective antagonists or agonists was then required in order to determine reliably alpha 1 and alpha 2 affinities and proportions: the alpha 1-receptors represent 45% and the alpha 2-receptors 55% of the entire alpha-receptor population in rat uterus. The second approach involved the administration of phenoxybenzamine (POB) that irreversibly blocks the alpha 1-adrenoceptors. Myometrial membranes obtained from rats 1 h after the administration of varying amounts of POB showed a dose-dependent reduction in specific [3H]DHE binding. This reduction was accompanied by a progressive increase of the value of the dissociation constant. Our data indicate that a dose of 1 mg of POB left the alpha 2-receptors intact while entirely blocking the alpha 1-receptors in rat myometrium. The third approach utilized the selective radioligand antagonists [3H]prazosin ([3H]PRAZ) and [3H]rauwolscine ([3H]RAUW). The results obtained with these radioligands confirmed our observations on the alpha-adrenoceptor subtypes in experiments with [3H]DHE. The results obtained with the 3 methods are in good agreement. Each approach appears valid and applicable to the characterization of alpha 1- and alpha 2-adrenoceptor subtypes in rat uterus, but the method using [3H]PRAZ and [3H]RAUW demonstrates more directly the presence of the two receptor subtypes.

A redox cycling model for the action of beta-adrenoceptor agonists

A cyclic redox mechanism for the action of beta-adrenoceptor agonists is proposed. It has the following features: a) beta-adrenoceptor agonists act by 'reductive activation' of the beta-adrenoceptor (R); b) the redox state of R is reciprocally coupled to the redox state of the guanine nucleotide binding protein (G); c) binding of GTP to G reverses the agonist-induced alteration of the redox states of R and G; d) according to a specific version of the model the activation process involves a disulfide-thiol interchange reaction which leads to a GTP-revertible cross-linking of R and G by a disulfide bond. The way in which desensitization events may interfere with the proposed redox cycle is discussed.

Purification and biochemical characterization of alpha 2-adrenergic receptor from the rat adrenocortical carcinoma

The alpha 2-adrenergic receptor was purified from rat adrenocortical carcinoma 494 by an affinity chromatographic step using a novel para-aminoclonidine-sepharose resin followed by a gel-permeation high performance liquid chromatographic step. The iodinated receptor protein was homogeneous as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and by high performance liquid chromatography. Both SDS-PAGE and high performance liquid chromatographic studies revealed that Mr of the protein was 64,000, suggesting the monomeric nature of the receptor protein. The purified protein showed the typical binding characteristics of alpha 2-adrenergic receptor.

Alpha-adrenergic antagonists: correlation of the effect on intraocular pressure and on alpha 2-adrenergic receptor binding specificity in the rabbit eye

Six alpha-adrenergic antagonists, which have a range of selectivities for alpha 1- and alpha 2-adrenoreceptor subtypes, were compared with respect to their ability to reduce intraocular pressure (IOP) after topical application to the rabbit eye, and their affinity and selectivity for alpha 2-adrenoreceptors, as determined by binding to membranes prepared from rabbit iris-ciliary body. A routine assay for alpha 2-adrenoreceptors using [3H]-rauwolscine was developed for this purpose. ICB contained 200-300 fmol (mg protein)-1 alpha 2-adrenoreceptors which represents approximately two-thirds of the total number of alpha-adrenoreceptor sites present in this tissue. All six antagonists bound at alpha 2-adrenergic receptor sites in an apparently simple competitive manner. The Kd for three of the drugs was about 10 nM (rauwolscine, yohimbine, WB-4101) and the Kd for the other three was greater than 3500 nM (prazosin, corynanthine, thymoxamine). However, all six antagonists were effective ocular hypotensive agents when given topically in a 50 microliter dose of 1% (w/v) concentration. The ability of alpha-adrenergic antagonists to lower IOP in the rabbit did not correlate with a single alpha-receptor subtype and appears to involve at least two separate mechanisms, one mediated by alpha 2-adrenergic receptors and one mediated by alpha 1-adrenergic receptors.

Beta-adrenergic activity and conformation of the antihypertensive specific alpha 2-agonist drug, guanabenz

In recent research a new series of specific drugs, one of which is guanabenz (GBZ, 2,6(dichlorobenzyliden)-aminoguanidine) has been introduced into the clinical treatment of centrally mediated hypertension. Guanabenz (GBZ) is considered to be among the most specific alpha 2-adrenergic agonists, acting similarly to clonidine by decreasing the sympathetic outflow from the brain to the peripheral circulatory system. In the present report we show that GBZ displays a significant affinity for beta-adrenoceptors. In displacement studies of the iodinated beta-antagonist [125I]cyanopindolol (CYP) from turkey erythrocyte membranes, the dissociation constant of GBZ was 3.8 microM. Inhibition of the (-) epinephrine induced adenylate cyclase activity by GBZ is competitive, with an apparent dissociation constant of 30 microM. A similar value was obtained by studies of GBZ's effect on the (-) epinephrine-induced [3H]cAMP accumulation in intact turkey erythrocytes. In view of its unexpected affinity for beta-adrenoceptors, we examined the three-dimensional structure of crystalline GBZ. In these studies substantial differences between clonidine and GBZ were observed, despite their strong structural resemblance. These dissimilarities (angle of rotation phi = 39.7 degrees as compared to 76 degrees in clonidine, and the rotational restriction of clonidine as compared to the greater mobility in rotation of GBZ) could explain the difference of specificity between these two compounds.

Anatomy of brain alpha 1-adrenergic receptors: in vitro autoradiography with [125I]-heat

Much useful information on the localization of alpha 1-adrenergic binding sites has been gained by using tritiated radioligands for in vitro autoradiography. However, the iodinated alpha 1-adrenergic antagonist HEAT [( 2-beta (4-hydroxyphenyl)-ethylaminomethyl)-tetralone], BE 2254), a radioligand with high affinity and specificity, provides autoradiographs with a higher signal to noise ratio. This has allowed us to describe the anatomy of these binding sites in much greater detail than previously possible. Regions showing the highest levels of binding include external plexiform layer of the olfactory bulb, layers Va and Vc of frontoparietal cortex, lateral and central amygdaloid nuclei, thalamus, and inferior olive. Other regions were generally less intensely labeled, with the least evidence of labeling in white matter, such as corpus callosum. Some regions (e.g., hippocampus) had only moderate labeling, but the binding appeared in a discrete pattern that reflected the functional organization of the structure. Although the [125I]-HEAT binding sites were distributed in a pattern similar to that previously reported for [3H]-WB 4101 and [3H]-prazosin, the anatomical detail seen with the iodinated ligand is greater. As a result, an association of alpha 1-adrenergic antagonist binding sites with specific layers in the cortex and with some catecholamine-containing nuclei in the brainstem, such as the locus coeruleus, have been seen for the first time.

In vitro characterization of the alpha-adrenoceptors in human prostate

The alpha-adrenoceptors of the human prostate gland were characterized in vitro by the use of antagonists selective for alpha 1- and alpha 2-adrenoceptor subtypes. The contractile response induced by norepinephrine in this tissue could be antagonized by prazosin, a selective alpha 1-antagonist, with a receptor dissociation constant (KB) of 4.0 +/- 0.9 nM. The selective alpha 2-antagonists rauwolscine and SK&F 86466 were less potent antagonists of this response, with KB values of 1020 +/- 400 nM and 2400 +/- 800 nM, respectively. The irreversible alpha-antagonists benextramine and phenoxybenzamine, both of which preferentially inactivate the alpha 1-subtype, produced marked depression of norepinephrine-induced contraction. These data would suggest that the alpha-receptors on prostatic smooth muscle are predominantly of the alpha 1-subtype.

The effects of oxymetazoline on secretion of protein and some electrolytes by rat submandibular and parotid glands

Oxymetazoline is a potent secretagogue for the salivary glands of rats. In the parotid gland, it activates preferentially alpha-adrenoceptors. As for the submandibular glands, it activates alpha-adrenoceptors at relatively low doses but at higher doses it allows secretion of new types of proteins.

Functional characterization of the pre- and postjunctional alpha-adrenoceptors in the in situ perfused rat mesenteric vascular bed

Pressor responses to periarterial nerve stimulation in the in situ perfused mesentery were potentiated by rauwolscine while responses to norepinephrine and phenylephrine were unaffected. Pressor responses to nerve stimulation, norepinephrine and phenylephrine were abolished by prazosin. No responses could be elicited in the mesentery by tramazoline. Thus, pressor responses to nerve stimulation and norepinephrine in this vascular bed are mediated by alpha 1-adrenoceptors while prejunctional alpha 2-adrenoceptors exert a negative feedback influence on vascular neurotransmission.

The alpha 2-adrenergic system of the platelet in cystic fibrosis

The ability of norepinephrine to inhibit prostaglandin E1 (PGE1)-stimulated accumulation of adenosine 3':5' cyclic monophosphate (cyclic AMP) in intact washed platelets was determined in 12 patients with cystic fibrosis, 6 parents of patients with cystic fibrosis, and a total of 21 healthy age-matched controls. Patients with cystic fibrosis and their parents did not differ from their age-matched controls in basal or PGE1-stimulated levels of cyclic AMP, nor in the dose dependent inhibition of cAMP accumulation by norepinephrine. Moreover, binding sites for [3H]-dihydroergocryptine were present in normal numbers and had normal ligand affinity in platelet membranes from patients with cystic fibrosis. In all measures tested, the alpha 2-adrenergic system in the platelet was normal in cystic fibrosis.

Inhibition of α2-adrenergic agonists on AVP-induced cAMP accumulation in isolated collecting tubule of the rat kidney

A microradioimmunoassay for cAMP was developed in order to analyse the effects of alpha-adrenergic agonists on vasopressin (AVP)-induced cAMP cell accumulation in single pieces of microdissected medullary (MCT) and cortical (CCT) rat collecting tubules. Under the experimental conditions chosen (4 min of incubation in the presence of a phosphodiesterase inhibitor), no cAMP could be detected either in the bathing solution or in non-stimulating samples of tubule. In MCT, 10(-6) M AVP stimulated cAMP generation up to 128.3 +/- 9.0 (SEM) fmoles per mm of tubule per 4 min, N = 11. The response was dose-dependent with a KA value below 10(-10) M AVP. The addition of norepinephrine (NE) (10(-5) M in the presence of propranolol) suppressed the larger part of the response to AVP (from 92% with 2 X 10(-11) M AVP to 76% with 10(-6) M AVP); the addition of 10(-7) M NE still reduced by 59% the MCT response to 10(-10) M AVP (26.2 +/- 5.9 vs. 64.0 +/- 6.4 fmoles/mm, N = 3). In CCT, 10(-5) M NE reduced by 84% the cAMP generation induced by 10(-10) M AVP (8.8 +/- 2.0 vs. 54.2 +/- 3.5 fmoles/mm, N = 3). This inhibitory action of NE against the AVP effect in CCT was mimicked by 10(-7) M clonidine; in MCT it was suppressed by phentolamine and yohimbine, but not by prazosin, suggesting that alpha 2-adrenoreceptors are involved. On the other hand, the addition of the alpha-agonists to the incubation solution produced no inhibition of the cAMP cell accumulations induced by glucagon, calcitonin and isoproterenol in CCT, or glucagon in MCT, an observation demonstrating that alpha 2-adrenergic agonists selectively inhibit vasopressin-dependent cAMP generation by these nephron segments.

Antagonism of the antidiarrhoeal effect of clonidine and the lethal effect of noradrenaline in rats: a reliable procedure to evaluate the in-vivo alpha 1- and alpha 2-blocking activity of drugs?

Eight compounds with alpha-adrenergic blocking activity were tested for their ability to antagonize the antidiarrhoeal effect of clonidine (clonidine test) and the lethal effect of noradrenaline (noradrenaline test). Six of the compounds studied are alpha-adrenergic blocking agents with known alpha 2/alpha 1 selectivity. Two compounds, ketanserin (R 41 468) and butanserin (R 53 393), are 5-hydroxytryptamine S2-antagonists. The ED50-values (mg kg-1) obtained in the clonidine test were: phentolamine (0.34), RX781094 (0.34), yohimbine (0.51), piperoxan (9.36), butanserin (greater than 5.0), prazosin (greater than 10.0), phenoxybenzamine (greater than 40.0), and ketanserin (greater than 80.0). In the noradrenaline test the ED50's (mg kg-1) were: butanserin (0.014), prazosin (0.032), phentolamine (0.59), phenoxybenzamine (1.02), ketanserin (4.69), RX781094 (12.4), piperoxan (21.5), and yohimbine (25.0). The selectivity alpha 2/alpha 1-ratios (ED50 clonidine/ED50 noradrenaline were: yohimbine (0.020), RX781094 (0.027), piperoxan (0.44), phentolamine (0.58), ketanserin (greater than 39), prazosin (greater than 312), and butanserin (greater than 357). These results show that yohimbine and RX781094 are equipotent and relatively selective alpha 2-antagonists; piperoxan and phentolamine block both alpha 1- and alpha 2-receptors at closely related doses; ketanserin, prazosin and butanserin are selective blockers of alpha 1-receptors, ketanserin being very weak, prazosin and butanserin being very potent compounds in this respect. The potent and selective alpha 1-blocking activity of butanserin, combined to its 5-HT S2-antagonism makes butanserin a very interesting experimental drug in view of earlier reported data concerning the amplifying effects between 5-hydroxytryptaminergic and noradrenergic vascular mechanisms.

In vitro and in vivo antiarrhythmic effects of prazosin in the rat

The possible antiarrhythmic action of prazosin was studied on two experimental models: a focus of ectopic automaticity in the rat isolated right ventricle and arrhythmias induced by i.v. CaCl2 in anaesthetized animals. In vitro, prazosin at 10(-5)M abolished the ectopic automaticity focus in 83% of cases (10/12). The effects of prazosin were not modified by reserpine pretreatment. In vivo, prazosin modified the range of CaCl2 arrhythmias, reduced the mean control sinus rate (by 21.7% at 1 mg/kg), reduced incidence and mortality by ventricular fibrillation and lengthened survival time of animals that eventually died. The results indicate that antagonism of postsynaptic alpha 1-adrenoreceptors may partly explain the efficacy of prazosin against some experimental cardiac arrhythmias.

Blockade of the release of LH induced by pharmacological suppression of adrenergic inflow into the locus coeruleus

Previous studies have indicated that stimulation of the locus coeruleus (LC) produced inhibition of the release of LH induced by stimulation of the medial preoptic area (mPOA) in ovariectomized, estrogen-primed rats. A similar response following the application of drugs into the LC was taken as an index of activation of the LC neurons. The injection of the phenylethanolamine N-methyltransferase (PNMT) blocking-agent, 2,3- dichloromethylbenzylamine ( DCMB , 0.5 micrograms) into the LC, 2 and 1 h before starting the mPOA stimulation greatly attenuated the induced release of LH. Injection of saline had no effect. The reduced release of LH in DCMB -treated rats was restored to normal by injecting clonidine (0.5 micrograms) into the LC immediately before the mPOA stimulus started, but not by injecting phenylephrine (0.5 micrograms) or saline (0.5 microliter). The release of LH induced by mPOA stimulation was also blocked in rats in which the alpha 2-antagonist, piperoxane (0.5 micrograms) was injected into the LC but not in those injected with the alpha 1-antagonist, phenoxybenzamine (0.5 micrograms) or the beta-antagonist, propranolol (0.5 micrograms). It is concluded that adrenergic afferents into the LC tonically inhibited the activity of LC neurons and that this effect is mediated by alpha 2-adrenoceptors. Interference with the synthesis of epinephrine or blockade of alpha 2-adrenoceptors resulted in activation of LC neurons and thereby in inhibition of LH release.

The hepatic alpha 1-adrenergic receptor

Since the relatively recent advent of radioligand binding techniques, it has been possible to directly identify and characterize hepatic adrenergic receptors as well as study their physiological regulation. While it is now clear that alpha 1-adrenergic receptors constitute the major population of hepatic adrenergic receptors and are primarily responsible for the actions of catecholamines in liver, relatively little is known about the molecular mechanisms underlying alpha 1-responses. Recent results suggest that guanine nucleotides may be implicated in the transmission of the hormonal signal from the hepatic alpha 1-receptor to its effectors in a manner analogous to that described for adenylate cyclase-linked receptors. The lack of an easily measurable proximal membrane response for the alpha 1-receptor has been a severe handicap in our understanding of the mechanism of transmission of the hormonal signal. It is likely that until such a response is defined, alpha 1-adrenergic research will continue to lag behind research on the beta-adrenergic receptor.

Binding and uptake of [3H]adrenaline by perfused rat liver

The binding and uptake of [3H]adrenaline by the intact perfused rat liver was investigated. We showed that the administration of [3H]adrenaline to liver resulted in the rapid uptake of the radioligand, and that such uptake was independent of any Ca2+ redistributions induced by the hormone. At low adrenaline concentrations (less than 50 nM) uptake was inhibited by prazosin, whereas at higher hormone concentrations a significant proportion of total [3H]adrenaline uptake could not be inhibited by this antagonist. [3H]Adrenaline uptake could be directly correlated with adrenaline-induced responses such as an increased rate of respiration and glycogenolysis. The partial inhibition (approx. 25%) of [3H]adrenaline uptake by antagonists was sufficient for the total inhibition of hormone-induced responses. The effect of various pharmacological agents on [3H]adrenaline uptake was investigated, and the contribution of tissue-related factors to alpha-adrenergic agonist-antagonist interactions in vivo is discussed.

Induction of taurocholate release from isolated rat hepatocytes in suspension by alpha-adrenergic agents and vasopressin: implications for control of bile salt secretion

Hepatocytes incubated with 25 muM [3H] taurocholate rapidly deplete the extracellular medium of [3H] taurocholate and achieve a steady-state level of intracellular bile salt within 15 min. Exposure of cells at steady state with extracellular taurocholate to the catecholamines norepinephrine or epinephrine results in release of 3H from the cells into the incubation medium; the 3H released represents almost exclusively unmetabolized [3H] taurocholate. The hierarchy of effectiveness of the catecholamines, norepinephrine congruent to epinephrine greater than phenylephrine much greater than isoproterenol, is indicative of an alpha-adrenergic mechanism. Induction of [3H] taurocholate release by norepinephrine is inhibited by the alpha-antagonists phenoxybenzamine and phentolamine and by chlorpromazine, but is not affected by the beta-antagonist propranolol, further supporting an alpha-adrenergic basis for this phenomenon. Arginine vasopressin, at concentrations of 1 X 10(-9) M and greater, also induces bile salt release. Classical alpha- and beta-antagonists have minimal effects on vasopressin induced bile salt release. While the peptide hormones angiotensin and oxytocin are, alone, relatively ineffective inducers of bile salt release, oxytocin potentiates the induction of bile salt release by vasopressin, suggesting complex interactions with membrane receptor function. Further studies assessing the interaction of sympathetic neurotransmitters and peptide hormones with bile salt transport and release in the hepatocyte may provide insight into the regulation of hepatic secretory function in the intact animal.

In vitro selectivity of lisuride and other ergot derivatives for alpha 1- and alpha 2-adrenoceptors

The alpha-adrenoceptor agonist and antagonist actions of lisuride and other ergot derivatives were assessed in spleen strips (alpha 1-adrenoceptor) and transmurally stimulated ileal preparations (alpha 2-adrenoceptor) from the guinea-pig. The compounds tested were devoid of alpha-adrenoceptor agonistic activity, however many were selective alpha-adrenoceptor antagonists. Lisuride was alpha 2-selective, having a pA2 value of 9.70 in guinea-pig ileal and 7.47 in splenic preparations. Conversely, bromocriptine was alpha 1-selective with a pA2 of 7.46 in spleen strips; it had no measurable effect in the ileum.

The effects of alpha-methylnoradrenaline on protein and electrolyte secretion by rat submandibular and parotid glands

alpha-Methylnoradrenaline (alpha-mNA) is a potent secretagogue for the parotid and submandibular glands of rats. With regard to the parotid glands, alpha-mNA activates mainly beta-adrenoceptors. In the submandibular glands, alpha-mNA activates alpha-adrenoceptors at higher doses whereas at relatively lower doses it activates beta-adrenoceptors. alpha-mNA may not stimulate the specific alpha 2-adrenoceptors of the salivary glands of rats.

Multiple binding sites of [3H]clonidine on hepatic plasma membranes

The binding of [3H]clonidine on mouse liver plasma membrane was a rapid, saturable and reversible process. It was characterized by two types of population: high affinity receptors with KD of 6.76 +/- 1.02 nM and Bmax of 106.15 +/- 24.05 fmol/mg protein, and low affinity receptors with KD of 63.66 +/- 12.85 nM and Bmax of 818.06 +/- 128.49 fmol/mg protein. Displacement of [3H]clonidine from its binding sites by various ligands indicated that alpha 1--as well as alpha 2--adrenoceptors were involved in the high affinity system. The respective participation of these two types of receptors was discussed.

[3H]RX 781094: a new antagonist ligand labels alpha 2-adrenoceptors in the rat brain cortex

[3H]RX 781094 [(imidazolinyl-2)-2 benzodioxane-1,4 [3H]chlorhydrate], a specific alpha 2-adrenoceptor antagonist radioligand, has been used to characterize alpha 2-adrenoceptors in rat cortical membranes. [3H]RX 781094 binding is reversible, saturable and stereospecific. It labels with high affinity a single population of non-interacting sites. The KD value was 3.9 +/- 0.4 nM and the Bmax 189.0 +/- 12.4 fmol/mg protein. Competition curves with different alpha-adrenoceptor agonists and antagonists showed that the binding sites labelled with [3H]RX 781094 had the pharmacological characteristics of alpha 2-adrenoceptors. Pretreatment with reserpine (2.5 mg/kg s.c., 24 h before the experiment) did not affect the KD or Bmax values of [3H]RX 781094 binding. Chemical destruction of noradrenergic pathways by systemic injection of DSP4 or intracerebral injection of 6-hydroxydopamine did not modify the KD or the Bmax of [3H]RX 781094 binding. It is concluded that the major proportion of alpha 2-adrenoceptors labelled with [3H]RX 781094 are not localised to noradrenergic nerve terminals.

Peroxidase secretion from rat lacrimal gland cells in vitro. I. Alpha-adrenergic stimulation in the absence of alpha-adrenoceptors

The identification of alpha-adrenergic receptors and subdivision into alpha 1- or alpha 2-subtypes were studied by measuring the specific binding of the radioligands [3H]-prazosin as well as [3H]-clonidine to membranes prepared from homogenized rat lacrimal glands. The absence of high-affinity binding for [3H]-prazosin as well as for [3H]-clonidine indicates that rat lacrimal glands do not possess a substantial amount of alpha 1- and alpha 2-adrenoceptors. The binding data correspond with the characterization by pharmacological means. Monolayers of lacrimocytes were incubated with various selective alpha 1-, alpha 2-, beta-adrenergic agonists and antagonists or other substances, and peroxidase discharge was measured over a period of 1 hr. Among various substances only L-norepinephrine, L-phenylephrine, tyramine and ionophore A 23187 were stimulants of peroxidase secretion, whereas the adrenergic-stimulated secretory response was only suppressed by phentolamine.

Study of two alkylating derivatives: the p-isothiocyanato- and the p-methylisothiocyanato-clonidine

Pharmacological experiments with isolated rat aorta and radioligand binding studies in rat cerebral membranes were performed with the p-isothiocyanato (p-NCS) and p-methylisothiocyanato (p-CH2-NCS) derivatives of clonidine in order to assess their selectivity for alpha 1- and alpha 2-adrenoceptors, and to characterize their ability to alkylate alpha-adrenoceptors. Preincubation of rat aortic strips with both derivatives produced non-parallel rightward shifts in the dose-response curves of noradrenaline and significantly depressed the maximum response in a manner characteristic of irreversible receptor antagonists. The p-CH2-NCS derivative was slightly more potent than the p-NCS derivative. Further analysis of the data indicated that treatment of rat aorta with a 30 microM concentration of the p-CH2-NCS derivative alkylated all but 2.4 percent of the alpha-adrenoceptors, whereas a 100 microM concentration of the p-NCS derivative was required to produce a similar degree of alpha-adrenoceptor alkylation. Radioligand binding studies indicate an apparent 2 fold alpha2-adrenoceptor selectivity for the p-NCS derivative. In contrast, the p-CH2-NCS derivative displayed 7 fold selectivity for alpha 1-adrenoceptors. Interestingly, both alkylating derivatives of clonidine produced dose-dependent contractile responses in rat aorta with pD2 values of 6.30 and 5.56 for the p-NCS and p-CH2-NCS derivatives, respectively, relative to a pD2 of 7.67 for clonidine. The order of potency of the two alkylating derivatives of clonidine for producing contraction of rat aorta is the opposite of that for antagonizing the contractile effects of noradrenaline. The results suggest that the p-NCS and p-CH2-NCS derivatives of clonidine non-competitively antagonize noradrenaline by irreversibly alkylating alpha-adrenoceptors.

The selectivity of some ergot derivatives for alpha 1 and alpha 2-adrenoceptors of rat cerebral cortex

The affinities of several ergot derivatives for rat cerebral cortex alpha 1- and alpha 2-adrenoceptors were assessed using radioligand binding techniques. In most cases both [3H]prazosin (labelling alpha 1-adrenoceptors) and [3H]rauwolscine (labelling alpha 2-adrenoceptors) binding were displaced by the ergot derivatives tested, but with markedly different potency. Generally compounds displayed selectivity toward the alpha 2-adrenoceptor, particularly CQ 32-084, which had a Ki value against [3H]rauwolscine binding of 35 nM, but had little effect on [3H]prazosin binding at concentrations in excess of 5 microM. Lisuride was the most potent displacer of [3H]rauwolscine binding with a Ki value of 0.54 nM. Only bromocriptine was relatively alpha 1-selective with a Ki against [3H]prazosin binding of 18 nM and against [3H]rauwolscine binding of 120 nM. The results indicate that the ergot derivatives tested display a marked affinity for alpha-adrenoceptors and that their actions at this receptor class should be considered when interpreting their pharmacological activity in vivo.

Alpha-adrenergic blockade for variant angina: a long-term, double-blind, randomized trial

Recent reports have shown that beta-adrenergic blockade may exacerbate variant angina. On theoretical grounds, alpha-adrenergic blockade may be beneficial in these patients. To test this hypothesis, we assessed the efficacy of prazosin, an alpha-adrenergic blocking agent, in six men, mean age 49 years, with variant angina. Prazosin, 14.0 +/- 2.4 mg/day (mean +/- SD) in three equal doses, was compared with placebo in a double-blind, randomized, double-crossover trial lasting 4 1/2 months: 2 weeks of open-label prazosin followed by four 1-month periods of blinded alternating therapy. No other vasoactive medications were administered during the study. Prazosin reduced sitting systolic arterial pressure from 145 +/- 18 to 127 +/- 16 mm Hg (p = 0.02), but exerted no effect on diastolic arterial pressure or heart rate. Prazosin did not change the weekly number of episodes of chest pain (2.5 +/- 2.3 with placebo vs 3.1 +/- 3.0 with prazosin, NS), nitroglycerin tablets used (3.9 +/- 3.7 with placebo vs 4.6 +/- 4.2 with prazosin, NS), or transient ST-segment deviations (by calibrated two-channel Holter monitoring for 24 hours/week throughout the study) (6.5 +/- 10.1 with placebo vs 11.8 +/- 17.4 with prazosin, NS). During prazosin therapy, three patients had orthostatic dizziness and one patient had headache. Thus, in a long-term, randomized, double-blind trial, prazosin exerted no obvious beneficial effect in patients with variant angina.

Adrenergic regulation of avian pancreatic polypeptide secretion in vitro

Adrenergic regulation of avian pancreatic polypeptide (APP) secretion from perifused microfragments of chicken pancreas was investigated with catecholamines and adrenoreceptor-specific analogues added to avian Krebs-Ringer perifusion medium containing 11.1 mM glucose. APP release was stimulated by the alpha-adrenoreceptor agonist phenylephrine (threshold 5 microM; maximum effective concn, 50 microM) and the beta-adrenoreceptor agonist isoproterenol (threshold 5 microM; maximum effective concn, 50 microM). Epinephrine (0.164 microM) and 50 microM norepinephrine also stimulated APP release. The results obtained suggest that APP secretion from chicken pancreas is regulated by dual, complementary alpha- and beta-adrenoreceptor mechanisms.

Effects of selective alpha 1 and alpha 2 adrenoceptor active drugs on 86Rb+ efflux from pieces of rat parotid gland

Minute pieces of rat parotid gland were used in studies of adrenergic regulation of K+ efflux using 86Rb+ as a probe for K+. Noradrenaline induced a concentration-dependent RB+ efflux, whereas the beta 1-selective agonist prenalterol was without effect. On the other hand, the beta 2-selective drug, terbutaline, at high concentrations displayed a small enhancement of Rb+-secretion. The selective alpha 1-adrenoceptor drug, phenylephrine, was as potent as noradrenaline, whereas the alpha 2-agonist clonidine had only a small effect. The noradrenaline-induced Rb+-efflux was effectively inhibited in the presence of prazosin, an alpha 1-blocker, whereas the alpha 2-antagonist, yohimbine, was roughly 50 times less potent. The results suggest that catecholamine-induced K+-secretion from the rat parotid gland is mediated via activation of post-synaptic alpha-adrenoceptors of the alpha 1-subtype.