Spare alpha adrenoceptors in the peripheral circulation: excitation-contraction coupling

Postsynaptic alpha adrenoceptors in arteries and veins represent a mixed population of alpha 1 and alpha 2 adrenoceptors, with both subtypes mediating vasoconstriction. In the peripheral arterial circulation, postsynaptic vascular alpha 1 adrenoceptors are found in the adrenergic neuroeffector junction, whereas postsynaptic vascular alpha 2 adrenoceptors are located extrajunctionally. In the venous circulation, it appears that alpha 2 adrenoceptors may be predominantly junctional, whereas alpha 1 adrenoceptors may be predominantly extrajunctional. In general, alpha 1 adrenoceptors play a more important functional role in arteries than in veins, with the converse being true for postsynaptic vascular alpha 2 adrenoceptors. The relationship between alpha-adrenoceptor occupancy and vasoconstrictor response is more favorable for postsynaptic vascular alpha 1 adrenoceptors than for alpha 2 adrenoceptors in both arteries and veins, and there is evidence for a receptor reserve in alpha 1 adrenoceptors in both the arterial and venous circulation. No reserve in postsynaptic vascular alpha 2 adrenoceptors is seen in the arterial circulation, but in isolated venous preparations, a reserve in alpha 2 adrenoceptors has been observed. It has been suggested that spare alpha 2 adrenoceptors found in veins, but not arteries, may be responsible, at least in part, for the exaggerated alpha 2-adrenoceptor-mediated response of veins relative to arteries.     

Interactions of agonists with peripheral alpha-adrenergic receptors

The alpha adrenoceptors may be subdivided based on their anatomical distribution within the synapse. Presynaptic alpha adrenoceptors are generally of the alpha 2 subtype and modulate neurotransmitter liberation via a negative feedback mechanism. Postsynaptic alpha adrenoceptors are usually of the alpha 1 subtype and mediate the response of the effector organ. Although this anatomical subclassification is generally applicable, many exceptions are now known. A more useful classification of alpha-adrenoceptor subtypes is based on a pharmacological characterization in which selective agonists and antagonists are used. Two major classes of alpha-adrenoceptor agonists are known: the phenethylamines, which are structurally related to norepinephrine, and the imidazolines, which are structurally related to clonidine. A number of important differences between these two classes of agonists have been observed and have led to the conclusion that the phenethylamines and imidazolines interact differently with alpha adrenoceptors. Many developments have recently been made in regard to peripheral alpha adrenoceptors in the cardiovascular system. Postsynaptic alpha adrenoceptors in the vasculature represent a mixed population of alpha 1 and alpha 2 adrenoceptors. Both alpha-adrenoceptor subtypes mediate vasoconstriction, but appear to do so through different mechanisms. alpha 1 adrenoceptors also exist in the heart and mediate a positive inotropic response. Renal alpha 1 and alpha 2 adrenoceptors have been identified and subserve a variety of functions such as regulation of renal blood flow, gluconeogenesis, renin release, and sodium and water reabsorption.     

Effects of temperature on alpha adrenoceptors in limb veins: role of receptor reserve

In cutaneous veins of the dog, cooling augments the response to sympathetic nerve stimulation and exogenous norepinephrine (NE). The postjunctional alpha adrenoceptors in this blood vessel belong to both the alpha 1 and alpha 2 subtypes. Cooling augments alpha 2-adrenergic responses (presumably because of an increased receptor affinity), but depresses alpha 1-adrenergic responses (presumably because of a direct inhibitory effect on the contractile process). When agonists of high efficacy such as NE or phenylephrine are used, an alpha 1-adrenoceptor reserve is present that buffers the response from the inhibitory effect of cooling. This allows the potentiating effect of cold on the alpha 2-adrenergic component of the response to catecholamines to predominate, and the contractile response to exogenous NE and sympathetic nerve stimulation is augmented. By contrast, in deep veins of the limb, cold reduces the contractions evoked by alpha 1- and alpha 2-adrenergic activation. This can be explained best by the absence of a receptor reserve for alpha 1-adrenergic agonists of high efficacy, combined with a reduced density of postjunctional alpha 2 adrenoceptors.     

Laminin alpha1-chain shows a restricted distribution in epithelial basement membranes of fetal and adult human tissues

Two novel monoclonal antibodies were raised and used to study the expression of laminin (Ln) alpha1-chain in developing and adult human tissues. In both fetal and adult kidney, a distinct immunoreactivity was seen in basement membranes (BM) of most proximal tubules but not in the distal tubular or glomerular BM or in the basal laminae of blood vessels. Immunoprecipitation of metabolically labeled cultured human renal proximal tubular cells showed an abundant production and deposition of Ln alpha1-chain to the extracellular matrix, suggestive of an epithelial origin of kidney Ln-1. Quantitative cell adhesion experiments with JAR choriocarcinoma cells showed that purified human Ln-1 is a good substrate for cell adhesion that it is differently recognized by integrin receptors when compared to mouse Ln-1. In fetal and adult testes immunoreactivity was solely confined to BM of the seminiferous epithelium. In the airways BM-confined reaction was only seen in fetal budding bronchial tubules (16-19 weeks) at the pseudoglandular stage of development. In the skin a distinct immunoreactivity was confined to BM of developing hair buds but not in epithelial BMs of adult epidermis or of epidermal appendages. In other adult tissues, immunoreactivity was found in BMs of thyroid, salivary, and mammary glands as well as in BMs of endometrium and endocervix, but not of ectocervix or vagina. No immunoreactivity was found in BMs of most of the digestive tract, including the liver and pancreas, except for BMs of esophageal submucosal glands and duodenal Brunner's glands. In fetal specimens, BMs of the bottoms of the intestinal and gastric glands were positive. Basal laminae of blood vessels were generally negative for Ln alpha1 chain with the exception of specimens of both fetal and adult central nervous system in which immunoreactivity for Ln alpha1 chain was prominently confined to capillary walls. The results suggest that outside the central nervous system, Ln alpha1 chain shows a restricted and developmentally regulated expression in BMs of distinct epithelial tissues.     

Evidence for antagonistic activity of endothelin for clonidine induced hypotension and bradycardia.

Effect of endothelin (ET) on clonidine induced cardiovascular effects was studied in male Sprague-Dawley rats. Clonidine (75 micrograms/kg, iv) produced significant decrease in blood pressure and heart rate. ET-1 (50 ng/kg, iv) pretreatment completely antagonized the hypotension and bradycardia induced by clonidine. ET-2 (50 ng/kg, iv) and ET-3 (50 ng/kg, iv) had similar antagonistic effect on clonidine induced hypotension and bradycardia. The antagonistic effect of ET lasted for several hours, however, 4 hours after ET pretreatment only partial blockade of clonidine induced hypotension and bradycardia was observed. This indicated that the antagonistic effect of ET was reversible. Initial hypertensive response induced by high dose of clonidine (750 micrograms/kg, iv) could not be antagonized by ET-1, ET-2 or ET-3, while phenoxybenzamine, an alpha adrenoceptor antagonist, blocked the hypertensive response of clonidine. Thus, ET has no antagonistic effect on the initial hypertensive response but antagonizes the hypotensive and bradycardic effect induced by clonidine. Clonidine induced hypotension and bradycardia are mediated through central alpha 2 adrenoceptors while hypertension is mediated through peripheral alpha 2 adrenoceptors. It is concluded that central alpha 2 adrenoceptors are different from peripheral alpha 2 adrenoceptors and ET antagonizes the effect of clonidine only on central alpha 2 adrenoceptors but has no antagonistic activity on peripheral alpha 2 adrenoceptors.     

Pharmacological properties of A-204176, a novel and selective alpha1A adrenergic agonist, in in vitro and in vivo models of urethral function.

A-204176 (N-[5-(1H-imidazol-4-y1)-5,6,7,8-tetrahydro-1-naphthalenyl]methanesulfonamide) is a potent and selective alpha1A adrenoceptor agonist that binds with 17-fold and 9-fold greater affinity to the alpha1A (Ki=176 nM) than the alpha1b and alpha1d subtypes, respectively. In functional studies A-204176 is potent (pD2=6.4) and efficacious (83% of maximum control phenylephrine response) at rabbit urethra alpha1A receptors, with weaker potency and greatly reduced efficacy at rat spleen alpha1B (pD2=5.3, 11%) and rat aorta alpha1D (pD2=4.4, 10%) subtypes. In anesthetized female dogs, A-204176 is more potent than the non-selective alpha1 adrenoceptor agonist phenylpropanolamine (PPA) to increase measures of urethral tone and is more efficacious to increase pressure in the proximal region of the urethra. Significant increases on parameters of the urethral pressure profilometry were induced at 100 and 300 nmol/kg, i.v., by A-204176 and PPA, respectively. A-204176 was more potent than PPA to increase the abdominal pressure required to produce leakage. In the simultaneous measurement of intraurethral pressure and mean arterial blood pressure, A-204176 displays enhanced urethral selectivity relative to PPA. However, despite its selectivity for alpha1A versus alpha1B and alpha1D adrenoceptors in vitro, A-204176 did not display the degree of urethral selectivity in vivo that would have been expected. The observed effect of A-204176 on blood pressure may be due to the presence of extra-synaptic alpha1A adrenoceptors in the vasculature or to activation of spinal and supraspinal alpha1A adrenoceptors. These data indicate that A-204176 may represent a useful pharmacological tool to investigate the functional role of the alpha1A adrenoceptor in the urethra and to elucidate the lack of uroselectivity observed in vivo.     

Effect of phenylephrine and isoproterenol on mesenteric blood flow in experimental posthaemorrhagic hypotension

In previous reports it was found that in mesenteric blood vessels exist profound alpha-adrenergic, and less profound beta-adrenergic receptors. Our experiments were done on 20 mongrel dogs, both sexes, weighing 6.5-22.5 kg, age from 1-2 years, which were divided into 2 groups: experimental (14 dogs) and control (6 dogs). Experimental group of animals passed through the posthemorrhagic hypotension (180 min. at 5.3 kPa). Aim of investigation was to clarify mesenteric vascular reactivity changes during posthemorrhagic hypotension. In order to reach this goal, phenylephrine and isoproterenol were injected alternatively in one hour intervals through cannulated superior mesenteric artery first proximal branch. Our results indicate that mesenteric blood flow responses to those drugs changed quantitatively, but never ceased. It is apparent from our experiments that those quantitative changes are due to beta 2 adrenoceptors changes during the posthemorrhagic hypotension.     

Adrenoceptor regulation of canine skeletal muscle blood flow during carbon monoxide hypoxia.

We questioned whether carbon monoxide hypoxia (COH) would affect peripheral blood flow by neural activation of adrenoceptors to the extent we had found in other forms of hypoxia. We studied this problem in hindlimb muscles of four groups of anesthetized dogs (untreated, alpha 1-blocked, alpha 1 + alpha 2-blocked, and beta 2-blocked). Cardiac output increased, but hindlimb blood flow (QL) and resistance (RL) remained at prehypoxic levels during COH (O2 content reduced 50%) in untreated animals. When activity in the sciatic nerve was reversibly cold blocked, QL doubled and RL decreased 50%. These changes with nerve block were the same during COH, suggesting that neural activity to hindlimb vasculature was not increased by COH. In animals treated with phenoxybenzamine (primarily alpha 1-blocked), RL dropped (approximately 50%) during COH, an indication that catecholamines played a significant role in maintaining tone to skeletal muscle. Animals with both alpha 1 + alpha 2-adrenergic blockade (phenoxybenzamine and yohimbine added) did not survive COH. RL was higher in beta 2-block than in the untreated group during COH, but nerve cooling indicated that beta 2-adrenoceptor vasodilation was accomplished primarily by humoral means. The above findings demonstrated that adrenergic receptors were important in the regulation of QL and RL during COH, but they were not activated by sympathetic nerve stimulation to the limb muscles.     

Nature of alpha receptors implicated in the hypertensive effect of high doses of isoprenaline in dogs and rats

Isoproterenol injected intravenously in dogs (3 mg/kg-1) and rats (5 mg/kg-1) induced an increase in blood pressure. After alpha 1 blockade (by AR-C 239, 0.1 mg . kg-1 i.v.) or alpha 2 blockade (by yohimbine, 1 mg/kg-1 i.v.) isoproterenol, as adrenaline, again induced an increase in blood pressure. This hypertensive effect was suppressed by an alpha 2 adrenoceptor blocking agent after an alpha 1 adrenoceptor blocking agent, and vice versa. These results are compatible with stimulation by high doses of isoproterenol of both alpha 1 and alpha 2 adrenoceptors to produce increase in blood pressure.     

Postsynaptic alpha adrenoceptors on vascular smooth muscle

A heterogeneous population of alpha adrenoceptors mediates vasoconstriction in the canine saphenous vein (CSV). Studies with isolated strips of venous smooth muscle incubated with selective alpha-adrenoceptor agonists and antagonists revealed that both alpha 1 and alpha 2 adrenoceptors exist independently in this tissue and both subtypes mediate a contractile response. Measurement of contractile responses in reduced or zero external calcium conditions indicates that stimulation of alpha 1 adrenoceptors induces contractions by influx of extracellular calcium and release of calcium from internal stores. In contrast, 45Ca uptake studies suggest that activation of the postsynaptic alpha 2 adrenoceptor produces vasoconstriction dependent only on influx of extracellular calcium. The influx of calcium produced by the selective alpha 2-adrenoceptor agonist BHT-920 is inhibited by calcium entry blockers. Measurements of transmembrane potentials from smooth muscle cells of the CSV suggest that alpha 1-adrenoceptor activation produces depolarization and contraction (electromechanical coupling) whereas alpha 2-adrenoceptor stimulation does not result in concentration-dependent depolarization of the smooth muscle cells (pharmacomechanical coupling).     

Nature of alpha 1 and postjunctional alpha 2 adrenoceptors in the pulmonary vascular bed

The subtypes of postjunctional alpha adrenoceptors in the feline pulmonary vascular bed were studied by using selective alpha-adrenoceptor agonists and antagonists. Under conditions of controlled pulmonary blood flow and constant left atrial pressure, intralobar injections of the alpha 1 agonists phenylephrine and methoxamine, and the alpha 2 agonists UK 14,304 and B-HT 933, increased lobar arterial pressure in a dose-related manner. Prazosin, an alpha 1-adrenoceptor antagonist, reduced responses to phenylephrine and methoxamine to a greater extent than responses to UK 14,304 and B-HT 933. Yohimbine, an alpha 2 blocker, decreased responses to UK 14,304 and B-HT 933 without altering responses to phenylephrine or methoxamine. The same pattern of blockade was observed in animals pretreated with 6-hydroxydopamine, an adrenergic neuronal blocking agent. However, in propranolol-treated animals, prazosin antagonized responses to phenylephrine and methoxamine without altering responses to UK 14,304 or B-HT 933, and the selectivity of the blocking effects of yohimbine were preserved. Responses to intralobar injections of norepinephrine (NE) were markedly decreased by prazosin, whereas yohimbine had only a small effect. These data suggest the presence of both postjunctional alpha 1 and alpha 2 adrenoceptors mediating vasoconstriction in the pulmonary vascular bed. These results also indicate that the vasoconstrictor responses to injected NE in the cat pulmonary vascular bed result mainly from activation of alpha 1 adrenoceptors.     

Characteristics of alpha-adrenoceptors in two human colorectal cancer cell lines.

The DiFi and HT-29 human colorectal cancer cell lines were characterized and compared with respect to binding properties of alpha adrenoceptors present on the cell surface. Both cell lines possessed alpha-1 and alpha-2 adrenoceptors of high affinity; however, DiFi cells were rich in alpha-1 adrenoceptors, whereas HT-29 cells were rich in alpha-2 adrenoceptors. In each cell line, specificity of radioligand binding to alpha-1 or alpha-2 adrenoceptors was proved via competition studies using non-tritiated drugs. We believe this to be the first characterization of alpha-1 adrenoceptors in cell line HT-29 and of alpha-1 and alpha-2 adrenoceptors in DiFi cells. Differences between these cell lines in alpha adrenoceptor expression are discussed in relation to colon carcinogenesis. The high level of alpha-1 adrenoceptors seen in DiFi cells should make this cell line useful in studies of the function and regulation of this adrenoceptor subtype.     

Characterization of adrenergic receptors on proximal tubular basolateral membranes

Alpha adrenergic receptors are identified on basolateral plasma membranes derived from proximal tubular epithelial cells. The density of alpha 2 receptors was over two-fold greater than alpha 1 receptors. The basolateral membranes were devoid of beta receptors. These results support previous demonstrations of alpha adrenergic receptors in rat renal cortex and concur with studies which suggest a limited presence of beta receptors on rat proximal tubules.     

Adrenergic regulation of adipocyte metabolism

Adipocytes can be readily isolated from intact adipose tissue. In adipocytes from hamster and human white adipose tissue it is possible to demonstrate beta, alpha 1, and alpha 2 adrenoceptors. Alpha 2 adrenoceptor activation inhibits while beta adrenoceptor activation stimulates cyclic AMP accumulation and lipolysis. The effects of catecholamines on cyclic AMP accumulation are mediated through regulation of adenylate cyclase activity, which is activated through beta adrenoceptors and inhibited through alpha 2 adrenoceptors. Activation of alpha 1 adrenergic receptors has been shown to be associated with elevations of cytosol calcium and increased turnover of phosphatidylinositol. In white adipocytes, the only known alpha 1 adrenergic effects are inhibition of glycogen synthase and stimulation of glycogen phosphorylase via mechanisms distinct from those by which cyclic AMP produces similar end effects. In brown adipocytes, alpha 1 adrenoceptor activation stimulates respiration. Thyroid hormones primarily regulate the sensitivity of adipocytes to beta-adrenergic amines while having little effect on alpha adrenoceptor sensitivity.     

Noradrenergic agonists and antagonists: effects on avoidance behaviour in rats

The effects of various agonists and antagonists of both alpha and beta adrenoceptors on the acquisition of avoidance behaviour were investigated in the rat. Clonidine, a selective agonist of alpha 2 adrenoceptors depressed avoidance acquisition whilst yohimbine, an antagonist of these receptors produced an opposite effect. Prazosin which showed postsynaptic alpha 1 adrenoceptor blocking activity reduced avoidance behaviour. A similar effect was produced by propranolol, a non-selective antagonist of beta adrenoceptors. On the other hand, salbutamol preferentially stimulating beta 2 adrenoceptors facilitated avoidance behaviour. In general, the results show a fairly good correlation between avoidance acquisition and efficacy of noradrenergic neurotransmission.     

Evidence for the presence of functional beta-adrenoceptor along the proximal tubule of the rat kidney

Evidence for the presence of beta adrenoceptors on proximal tubules from the rat kidney has been obtained using enriched tubule suspensions prepared by Percoll centrifugation. Intact tubules demonstrated simultaneous enrichment of parathyroid hormone and isoproterenol sensitive cAMP production with no enrichment of antidiuretic hormone sensitive cAMP production. Both norepinephrine and epinephrine were less potent than isoproterenol and the stimulatory effect of catecholamines could be blocked with propranolol but not phentolamine. The stimulatory effect of norepinephrine on cellular phenylalanine uptake is blunted by co-addition of isoproterenol suggesting that the beta receptor may modulatory catecholamine stimulated transport.     

Adrenoceptor and local modulator control of cutaneous blood flow in thermal stress

Blood flow to the skin is controlled by body temperatures in two ways: core and mean skin temperature combine in the central nervous system to form a reflex mechanism that controls the frequency of activity in sympathetic nerves to the cutaneous blood vessels; and local mechanisms independent of reflex effects control contractile response to the sympathetic transmitter norepinephrine (NE) at different temperatures. Cutaneous vessels differ in responsiveness to NE across temperatures: in limbs and tails, the superficial vessels constrict more strongly to NE when cooled, while the deep vessels show weaker responses to NE when cooled. This allows the limb to dissipate heat when warm and to conserve heat when cool. The mechanism for this difference in thermal response of deep and superficial vessels is not completely known, but may relate to differences in the adrenoceptors on which NE acts, and/or to the actions of locally produced substances that modulate the responses to NE in different ways at different temperatures. This paper discusses the alpha1- and alpha2-adrenoceptors involved in contraction of deep and superficial cutaneous vessels and also describes the roles of the local modulator nitric oxide, which interacts with adrenoceptors to affect cutaneous blood flow.     

Evidence that noradrenergic transmitter release is regulated by presynaptic receptors

A review is provided of the evidence in support of the existence of prejunctional alpha adrenoceptors on noradrenergic nerve terminals as well as the evidence for their physiological importance. The use of alpha-adrenoceptor agonists and antagonists has provided convincing data in support of the presynaptic receptor hypothesis. Moreover, there is ample evidence for the location of alpha adrenoceptors on nerve terminals. This evidence has often been forgotten in arguments opposing the presynaptic alpha-adrenoceptor hypothesis. The precise physiological role of presynaptic alpha adrenoceptors is still an open question, but there is support from a wide range of experiments in favor of a physiological role. Although it is not known which of these functions is most important, presynaptic alpha adrenoceptors may: regulate the pulse-to-pulse regulation of norepinephrine release during nerve stimulation, prevent noise, and protect the neuroeffector cell from excessive activation by transmitter during periods of rest or as physiological antagonists to the facilitation of transmitter release. In summary, evidence reviewed here strongly supports the existence of presynaptic alpha adrenoceptors. These receptors are clearly important pharmacologically and may play a physiological role in noradrenergic transmission. The exact physiological function must await further experimentation.     

Inverse regulation of alpha-2 and beta-2 adrenoceptors in salt-sensitive hypertension: an hypothesis

A high salt diet leads to up-regulation of alpha-2 adrenoceptors and down-regulation of beta-2 adrenoceptors in normotensive subjects. Although the increase in blood pressure with a high salt diet is not related to the magnitude of the alpha-2 or beta-2 adrenoceptor changes alone, it is related to the increase in the ratio of the receptor changes (operative alpha/beta adrenoceptor ratio). An increase in the operative alpha/beta adrenoceptor ratio with a high salt intake results in vasoconstriction and reduced vasodilatation at resistance vessels, as well as increased renal proximal tubular sodium reabsorption. An influence of heredity on this relationship is supported by four lines of evidence: 1) salt-sensitivity of blood pressure occurs predominantly in subjects with a family history of hypertension; 2) studies in twin children document the influence of genetic variance on salt-sensitivity of blood pressure; 3) subjects with a family history of hypertension have a significantly lower salivary sodium concentration and an altered urinary sodium excretion after salt loading compared to subjects with no such history; 4) salt-sensitivity of blood pressure may be associated with specific genetic markers. On the basis of these observations, we propose the hypothesis that enhanced inverse alpha-beta-adrenoceptor regulation in response to a high salt intake may be responsible for salt sensitivity in the normal population, and may contribute to the development of essential hypertension in susceptible individuals. This alteration is likely to be genetically mediated.