Atypical characteristics of frog and chick erythrocyte beta-adrenoceptors

The characteristics of beta-adrenoceptors on frog and chick erythrocytes have been compared to those present on rat erythrocyte, lung and cerebral cortical membranes. The receptors have been examined using the specific ligand [3H]dihydroalprenolol ([3H]DHA) with conventional filtration techniques. The equilibrium dissociation constant (KD) of [3H]DHA and the affinities of the non-selective beta-adrenoceptor antagonists propranolol, timolol and alprenolol, were very similar in all preparations examined. The relative order of potency of the catecholamines, isoprenaline greater than adrenaline greater than noradrenaline in rat lung, rat erythrocyte and frog erythrocyte; isoprenaline greater than noradrenaline greater than or equal to adrenaline in chick erythrocyte and rat cortex, suggest an overall beta 2-classification for the former and beta 1-classification of the latter tissues. However, the use of highly selective beta 1- or beta 2-agents and computer-assisted iterative curve fitting revealed that whereas rat cortex and lung possess both receptor subtypes (lung 80% beta 2, 20% beta 1: cortex 65% beta 1, 35% beta 2) only beta 2-sites were observed in rat erythrocytes. On the other hand, chick and frog erythrocytes possess a homogeneous population of receptors that do not strictly correspond either to beta 1- or beta 2-adrenoceptors. The atypical nature of these non-mammalian erythrocyte beta-adrenoceptors is discussed in relation to their previous extensive use as model beta-adrenoceptor systems.

Decreased lymphocyte beta-adrenergic-receptor density in patients with heart failure and tolerance to the beta-adrenergic agonist pirbuterol

We compared the initial and long-term effects of the beta-adrenergic agonist pirbuterol in 12 patients with chronic congestive heart failure. The drug's initial effect was a 35 per cent increase in cardiac index, but there was no significant change in heart rate or mean arterial pressure. After one month of therapy, the mean cardiac index and ejection fraction had returned to base-line values, and no clinical effect was evident in most patients. This apparent tolerance was not accompanied by changes in heart rate, blood pressure, or body weight, and it occurred in the presence of therapeutic drug levels during long-term therapy. The density of beta-adrenergic receptors on lymphocytes from patients treated with pirbuterol was significantly depressed as compared with that of patients with heart failure of comparable severity but not treated with pirbuterol. We conclude that tolerance to the hemodynamic and clinical effects of pirbuterol develops during long-term administration; this tolerance may be related to a decrease in myocardial or vascular beta-adrenergic receptors or both.

Fluorescence studies of the beta-adrenergic receptor topology

The nature of the propranolol binding site of the beta-adrenergic receptor has been examined by utilizing the intrinsic fluorescence of propranolol as a probe. Additionally, the spatial relationship between the propranolol binding site and membrane tryptophan has been examined by utilizing I-quenching of intrinsic tryptophan fluorescence, chemical modification of membrane tryptophan, and singlet-singlet energy transfer between membrane-bound propranolol and tryptophan. Propranolol, at concentrations consistent with specific beta-receptor binding, protected approximately 42% of the membrane tryptophan fluorescence from I-quenching. Further, in the presence of propranolol, the apparent quenching constant (kq) was altered from 3.6 to 21.8 M-1. Reaction of the membrane fragments with 2-hydroxy-5-nitrobenzyl bromide (Koshland's reagent I) in the presence and absence of propranolol indicated that low concentrations of propranolol protected approximately 45% of the membrane tryptophan from the reagent. The singlet-singlet energy transfer from tryptophan to propranolol was determined by sensitized emission. The distance between these two species was found to be less than 20 A. These results have been interpreted to indicate that propranolol, when bound to the beta-adrenergic receptor, is situated such that its naphthyl moiety is inserted into a tryptophan-rich hydrophobic pocket of the receptor.

[125I]BE 2254, a new high affinity radioligand for alpha 1-adrenoceptors

[125I]BE 2254 (IBE 2254), a new iodinated radioligand of high specific radioactivity (2175 Ci/mmol) was developed and used to characterize alpha 1-adrenoceptors in rat cerebral cortex membranes. IBE 2254 possessed a high affinity, KD 78 +/- 14 pM with a Bmax of 210 +/- 26 fmol/mg and 90% specific binding at KD value. alpha-Adrenergic antagonists competed for IBE 2254 binding in the following order: prazosin greater than or equal to WB 4101 greater than phentolamine greater than corynanthine greater than yohimbine greater than rauwolscine, a strong indication that IBE 2254 binds to alpha 1-adrenoceptors. IBE 2254 appears to be a very useful tool for studying alpha 1-adrenoceptors.

Identification and characterization of a beta 1-adrenergic receptor in the rat Sertoli cell

The beta 1-adrenergic receptor of rat Sertoli cells was characterized by measurement of the ability of adrenergic receptor agonists and antagonists to stimulate cAMP accumulation in Sertoli cells from 18-day-old rats. Epinephrine, norepinephrine and isoproterenol stimulate cAMP accumulation in Sertoli cells which is not additive with maximal doses of FSH and which is age-dependent, beta-antagonists, alprenolol, hydroxybenzylpindolol or propranolol inhibit isoproterenol-induced cAMP accumulation while alpha-adrenergic antagonists have no effect. Dobutamine and soterenol stimulated cAMP accumulation to a greater extent than albuterol and metoproterenol. Finally, the stimulatory effects of isoproterenol and zinterol are both more sensitive to inhibition by beta 1-antagonists than by beta 2-antagonists. Taken together these data indicate the presence of a beta 1-adrenergic receptor in Sertoli cells which is coupled to adenylate cyclase.

Autonomic innervation of rabbit urinary bladder following estrogen administration

The effects of estrogen administration on the autonomic innervation of the rabbit urinary bladder were studied. Immature female white rabbits were injected twice daily with estrogen (150 microgram./Kg.) for four consecutive days. Control animals received injections of vehicle alone. The adrenergic innervation was identified using the glyoxalic acid method of catecholamine histofluorescence. The cholinergic innervation was visualized utilizing specific acetylcholinesterase staining. Additionally, the effect of estrogen administration on the response of smooth muscle strips of urinary bladder to specific autonomic agonists was determined. Estrogen administration induced a moderate increase in the adrenergic innervation of the rabbit bladder detrusor, whereas no change could be observed in the cholinergic innervation. It should be noted, however, that whereas the adrenergic innervation in the bladder of the control animal was sparse, the cholinergic innervation in the bladder body was quite dense. Estrogen induced a marked increase in the response to alpha-adrenergic (methoxamine) and muscarinic cholinergic (bethanechol) agonists. No alterations were noted in the response to beta-adrenergic agonists (isoproterenol). These findings indicate that the urinary bladder responds as a target organ for estrogen-induced alterations in autonomic innervation.

Human lymphocyte beta-adrenergic receptors are unaltered with age

To investigate the mechanism of receptor changes associated with aging, we developed a beta-adrenergic receptor assay in whole human lymphocytes which fulfills strict affinity, stereospecificity, and specificity criteria. Lymphocyte beta-adrenergic receptors were quantified in 54 (36 males and 18 females) healthy subjects on no medications divided into two age groups, 18 to 30 years (mean: 24) and 60 to 72 years (mean: 64). Using this assay, we report no change in the number of lymphocyte beta-adrenergic receptors in elderly (mean +/- SE: 801 +/- 114 sites/cell) compared to young individuals (680 +/- 47). When the values are examined separately for males and females there again is no significant difference between younger and older groups. We, therefore, suggest that further investigation is necessary before decreased hormonal responsiveness in the elderly is ascribed to decreased hormone receptor number on the basis of lymphocyte beta-adrenergic receptors.

Subtype specificity of alpha-adrenergic receptors in rat heart

Recent studies suggest that two subtypes of alpha-adrenergic receptors (alpha 1 and alpha 2) can be distinguished on the basis of the differential affinities of certain adrenergic ligands for each subtype. We have investigated the binding characteristics of several such ligands for alpha-adrenergic receptors in membrane preparations derived from rat heart. alpha-Adrenergic antagonists competed for binding sites identified by the nonsubtype selective alpha-adrenergic ligand [3H]dihydroergocryptine (DHE) in the order of potency expected for alpha 1-receptors, namely, prazosin (EC50 = 0.35 nM) greater than phentolamine (EC50 = 37 nM) greater than yohimbine (EC50 = 918 nM). Furthermore, the nonsubtype selective radioligand [3H]DHE identified a quantitatively similar number of specific binding sites in rat cardiac membranes as the alpha 1-selective radioligand [3H]prazosin (33 and 36 fmoles/mg protein, respectively), while the alpha 2-selective ligand [3H]clonidine at concentrations up to 20 nM demonstrated negligible specific binding. We conclude that the alpha-adrenergic receptors of rat heart homogenates demonstrate binding characteristics typical of alpha 1-receptors. While we cannot exclude the presence of small numbers of alpha-2 receptors, the similar number of binding sites identified at saturation for the alpha 1-specific ligand [3H]prazosin and for the nonsubtype selective ligand [3H]DHE supports the hypothesis that the alpha-receptors of rat heart are predominantly of the alpha 1-subtype.

The beta-adrenoreceptor in human lymphocytes

The beta-adrenoreceptors of human lymphocytes were investigated by binding assays using a radiolabelled beta-adrenoreceptor agonist, 3H-hydroxybenzylisoproterenol, in an intact cell system. Intact human lymphocytes had binding sites for the ligand, which seemed to represent physiological beta-adrenoreceptors. Thus, assay of binding of the beta-adrenergic agonist, 3H-HBI, by intact lymphocytes can be used to study beta-adrenoreceptors in human tissue and to elucidate possible beta-adrenoreceptor disorders in human diseases.

Postsynaptic alpha-2 adrenergic receptors in isolated rat islets of Langerhans: inhibition of insulin release and cyclic 3':5'-adenosine monophosphate accumulation

Effects of various alpha adrenergic agents on insulin release and cyclic 3':5'-adenosine monophosphate (cAMP) accumulation in pancreatic islets were investigated. Clonidine, epinephrine, alpha-methylnorepinephrine and norepinephrine were most potent and methoxamine and phenylephrine least potent in inhibiting the glucose-stimulated insulin release. Yohimbine and phentolamine were the most effective and prazosin was the least effective in antagonizing the epinephrine-inhibited insulin release. Clonidine markedly inhibited the glucagon-stimulated cAMP accumulation, whereas methoxamine showed weak inhibition. Yohimbine markedly increased cAMP accumulation in the presence of epinephrine, whereas prazosin showed little effect. The effects of alpha adrenergic agents on rabbit aorta contraction were also examined for comparison with alpha adrenergic receptors in pancreatic islets. In the aorta, the order of agonist potency was norepinephrine greater than phenylephrine greater than epinephrine greater than methoxamine greater than alpha-methylnorepinephrine and that of antagonist potency was prazosin greater than WB-4101 greater than phentolamine greater than dihydroergotamine greater than phenoxybenzamine greater than yohimbine. These orders of potencies were markedly different from those in pancreatic islets. These results clearly demonstrate that the alpha adrenergic receptors in rat pancreatic islets are different from those on rabbit aorta (alpha-1) and are typical postsynaptic alpha-2 adrenergic receptors.

Cardiac beta-adrenoreceptors in the thyroxine-treated dog

1. Myocardial beta-adrenoreceptors, assessed in membrane preparations of left ventricle by 3H-dihydroalprenolol binding, were compared in dogs following thyroxine administration (n = 6), aorto-caval fistula (n = 12), and in normal dogs (n = 7). 2. Left ventricular hypertrophy occurred in response to both aorto-caval fistula and thyroxine treatment. Yield of myocardial membrane between the different groups was not significantly different. 3. Binding site concentration (pmol/mg membrane protein) in dogs with aorto-caval fistulae was similar to that in normal dogs and was not significantly influenced by the presence or absence of cardiac failure, degree of hypertrophy or duration of fistula. 4. Thyroxine-treated dogs had an increased concentration of binding sites (c. 1.8-fold) compared with both fistula and normal groups, while binding affinities were similar in all groups. 5. The data suggest that beta-adrenoreceptor concentrations in myocardium increase as a direct result of thyroid hormone action rather than as a result of secondary haemodynamic changes or ventricular hypertrophy.

Beta-adrenergic receptors of human polymorphonuclear leukocytes

125I-iodohydroxybenzylpindolol (IHYP) is a beta-adrenergic antagonist used as a ligand for the characterization of beta-adrenergic receptors in various cell preparations. Since IHYP may be adsorbed to surfaces such as test tubes and pipet tips, it is necessary to measure the actual free concentration of IHYP in the incubation solution. With this modification, the number of beta-adrenergic receptors in human peripheral PMNs can be determined with greater precision.

Relation between brain catecholamine receptors and dopaminergic stereotypy in rat strains

In order to examine the potential relation between behaviour elicited by apomorphine and the density of catecholamine receptors, we studied stereotypy and the binding of [3H]spiperone to D2-(or neuroleptic/dopamine) receptors, of [3H]WB-4101 to alpha1-adrenoceptors, and [3H]clonidine to alpha2-adrenoceptors in two strains of rats. The F344 rats exhibited significantly more stereotypy to apomorphine than the Buffalo strain of rats. Other F344 rats which did not receive apomorphine and 34% more D2-receptors in the striatum and 50% more D2-receptors in the olfactory tubercle. Alpha2-Receptors in the frontal cortex were 20% higher in F344 rats when compared to the Buffalo strain, while alpha 1-receptors were the same across both strains. These results suggest a relation between D2-dopamine/neuroleptic receptor density and behavioural responsiveness to the direct-acting and specific dopamine agonist, apomorphine.

Preponderance of beta-adrenergic binding sites in pancreatic islet cells of the rat

Although the alpha- and beta-adrenergic receptors have opposing effects on insulin secretion, the inhibitory influence of alpha-receptors appears to predominate. To determine if this was due to differences in number and affinity of receptors, isolated rat pancreatic islet cells were incubated with [3H]-dihydroalprenolol and [3H]-dihydroergocryptine as ligands for beta- and alpha-adrenergic binding sites. It was found that the number of beta-adrenergic binding sites was 143 fmol/mg islet protein with a Kd = 0.57 nM. The number of alpha-adrenergic binding sites was 53 fmol/mg protein with a Kd = 0.26 nM. Thus, there are 2.7 times as many beta-adrenergic binding sites as alpha-binding sites, and neither binding site number nor affinity is responsible for the predominant influence of the alpha-adrenergic receptors.

alpha 1- and alpha 2-adrenoceptors in rat cerebral cortex: effect of frontal lobotomy

Surgical noradrenergic denervation of the cortex via frontal lobotomy was used to destroy the noradrenergic nerve endings and thus give some insight into the distribution of alpha-adrenoceptors. Frontal lobotomy caused a reduction in noradrenaline content in rat cerebral cortex (2.1 +/- 0.4 ng/mg protein for lesioned side, 6.0 +/- 0.3 mg/mg protein for non-lesioned side), indicating an effective noradrenergic denervation. The differences in 3H-clonidine and 3H-prazosin binding observed following surgery were a significant decrease in the number of alpha 2-adrenoreceptors (115.0 +/- 4.5 to 91.7 +/- 3.2 fmol/mg protein, n = 7, P less than 0.001) and a smaller but significant increase in the number of alpha 1-adrenoceptors (119.7 +/- 2.5 to 131.6 +/- 5.4 fmol/mg protein, n = 7, P less than 0.05) in the lesioned cortex. Results of this study indicate that alpha 2-adrenoceptors located on presynaptic noradrenergic terminals represent only a small proportion of the total alpha 2-adrenoceptors in rat cerebral cortex.

Quantitation of beta adrenergic receptors in rat liver: confounding effect of displaceable but nonstereospecific antagonist binding

In rat liver membranes three types of ligand binding were seen using [3H]-dihydroalprenolol (DHA) and [125I]-hydroxybenzylpindolol (HYP): binding stereospecifically displaced by beta-adrenergic agonists and antagonists, binding nonstereospecifically displaced by beta-adrenergic antagonists, and binding which was not displaced by beta-adrenergic agonists or antagonists. The magnitude of the nonstereospecific displaceable binding varied with the physiological state of the animal. It was sufficient to prevent the quantitation of the stereospecific displaceable binding in some preparations from young rats but in all preparations of rats greater than 150 g or more than about 6 weeks of age. In adrenalectomized weanling rats 10-30% of the total binding was of nonstereospecific displaceable type while in control rats it comprised up to 60% of the total binding. Addition of 5 X 10(-6) M phentolamine to the assay eliminated a large proportion of the nonstereospecific displaceable binding. When phentolamine was included in the assay, liver membranes from weanling rats stereospecifically bound 30-35% of total binding; membranes from adrenalectomized rats showed stereospecific binding of up to 50 to 80%. Because the amount of displaceable, nonstereospecific binding varied greatly depending on the physiologic state of the animals, stereospecific displacement should be monitored for every type of liver membrane preparation. Furthermore, animal age is an important variable. Using the published antagonist binding methodology (DHA or HYP) in liver membranes, it is not presently possible to quantitate liver beta-adrenergic receptors in normal rats that have reached maturity.

Do the intracellular beta adrenoreceptors exist in the rabbit auricle

Propranolol introduced by a cut-end method into atrial trabeculae evoked negative inotropic response without any significant changes in basic electrical parameters. This effect was resistant to noradrenaline but not to dibutyryl cAMP. These results suggest that existence of intracellular beta adrenoreceptors involved in the control of the contraction.

Decreased concentration of myocardial alpha-adrenoceptors with increasing age in foetal lambs

Using [3H]-dihydroergocryptine, we identified myocardial alpha-adrenoceptor binding sites in foetal lambs and demonstrated that the concentration of receptors decreased with increasing foetal age. The presence of the receptor in the foetus correlated with the presence of myocardial alpha-adrenergic responsiveness. However, we found neither the alpha-receptor binding site nor responsiveness to alpha-adrenoceptor stimulation in the myocardium of adult sheep.

Adrenergic alpha receptors outnumber beta receptors in human penile corpus cavernosum

Using specific adrenergic receptor binding assays, we characterized both alpha and beta adrenergic receptors in human penile corpus cavernosum tissue. The density of alpha adrenergic receptors in corporal tissue was found to be almost ten times greater than the density of beta adrenergic receptors. Our results are compatible with studies that suggest an important role for adrenergic innervation in penile erection.

Characterization of alpha-1 adrenergic receptors in the heart using [3H]WB4101: effect of 6-hydroxydopamine treatment

To identify and characterize the cardiac alpha-adrenoceptors, a radioreceptor binding assay using the potent alpha adrenergic antagonist, [3H]WB4101 was performed in rat hearts. Specific [3H]WB4101 binding to rat left ventricular homogenates was saturable, reversible and of high affinity (Kd = 0.18 nM) with a Bmax of 2.57 fmol/mg of tissue (27.7 fmol/mg of protein). Adrenergic agonists competed for specific [3H]WB4101 binding in the order: (-)-epinephrine > (-)-norepinephrine greater than (-)-isoproterenol. Stereospecificity of the [3H]WB4101 binding sites was also demonstrated with (-)-epinephrine, (Ki = 90) nM being 270 times as potent as (+)-epinephrine, (K1 = 24 microM). Adrenergic antagonists competed for the binding in the order: WB4101 = prazosin greater than yohimbine greater than (-)-propranolol. WB4101 and prazosin exhibited a markedly greater (2000 times) affinity for [3H]WB4101 binding sites than yohimbine. The affinities (pKi) of alpha agonists and antagonists for [3H]WB4101 binding sites in the rat heart closely correlated with their pharmacological potencies in the heart. Scatchard analysis for [3H]WB4101 binding, performed in five regions from control and 6-hydroxydopamine-treated rat hearts, revealed specific [3H]WB4101 binding (Bmax) significantly greater in the ventricles and intraventricular septae than in atria. At 1 week after 6-hydroxydopamine treatment, there was a significant increase (40%) in the Bmax for [3H]WB4101 binding to ventricles and intraventricular septae without a change in Kd. We conclude: 1) [3H]Wb4101 selectively labels postsynaptic alpha-1 adrenoceptors in the rat heart; 2) there is a definite regional variation for cardiac alpha-1 adrenoceptors; and 3) 6-hydroxydopamine treatment caused a significant increase in the density of alpha-1 adrenoceptors in ventricles and intraventricular septae, compatible with a postsynaptic localization of the [3H]WB4101 binding site.

A radioreceptor assay for propranolol and 4-hydroxypropranolol

A radioreceptor assay for the measurement of propranolol and 4-hydroxypropranolol levels in plasma is described. Maximum sensitivity for propranolol was 1.2 +/- 0.15 ng/ml and for 4-hydroxypropranolol 4.2 +/- 0.4 ng/ml. Interassay and intra-assay variations for both were under 10%. Modifications in the radioreceptor assay permitted the measurements of total beta-adrenergic blocking activity and the separate contributions of parent drug and metabolite. When 4-hydroxypropranolol was stabilized, a composite level of total beta-adrenergic blocking activity in plasma was obtained. When the 4-hydroxy metabolite was oxidized, only the stable parent drug was detected. The difference in values between measurements made under these conditions was equivalent to the amount of 4-hydroxypropranolol in the sample. The radioreceptor assay was also used to measure the amount of free propranolol and 4-hydroxypropranolol. Under identical experimental conditions, more 4-hydroxypropranolol than propranolol circulated in the free form. These observations establish the feasibility of adapting the radioreceptor assay for propranolol to the measurement of total beta-adrenergic blocking activity and its components in plasma as well as to the measurement of free drug and metabolite levels.

Beta-adrenergic-receptor localization by light microscopic autoradiography

beta-Receptors were identified in rat brain by a light microscopic autoradiographic technique. The procedure involved binding 3H-labeled dihydroalprenolol to beta-receptors in intact slide-mounted tissue sections and generating autoradiograms by the apposition of emulsion-coated cover slips, Biochemical analysis of the binding indicated that these conditions provided a high degree of selective labeling of beta-receptors. High densities of receptors were found in superficial layers of the cerebral cortex, throughout the caudate-putamen, in the periventricular nucleus of the thalamus, in the molecular layer of the cerebellum, and in other areas. These results are in agreement with other electrophysiological and histochemical data. This radiohistochemical approach should be an important addition to other methods for mapping functional catecholamine neuronal pathways and sites of hormonal action.

Postsynaptic alpha 1- and alpha 2-adrenoceptors in the circulatory system of the pithed rat: selective stimulation of the alpha 2-type by B-HT 933

The antagonism by yohimbine (1 mg/kg, i.v.) of vasopressor responses in pithed rats was most pronounced towards B-HT 933 (dose ratio 18.3) and moderate towards clonidine (dose ratio 3.7) and especially L-phenylephrine (dose ratio 2.5). Prazosin (0.1 mg/kg, i.v.) had no effect on the pressor responses to B-HT 933, moderately affected those to clonidine (dose ratio 3.9), but strongly diminished those to L-phenylephrine (dose ratio 53). Phentolamine (1 mg/kg, i.v.) was devoid of a differential antagonism. The results obtained suggest a subclassification of postsynaptic alpha-adrenoceptors into alpha 1- and alpha 2-subtypes mediating pressor effects. B-HT 933 is a selective agonist and yohimbine an antagonist of postsynaptic alpha 2-adrenoceptors. L-Phenylephrine preferably stimulates and prazosin preferentially occupies the alpha 1-adrenoceptors. Clonidine is a potent agonist of both types and phentolamine behaves as a non-selective antagonist.

Changes in tissue alpha- and beta-adrenergic receptors in renal hypertension in the rat

Myocardial membranes prepared from renal hypertensive rats contained reduced concentrations of both alpha- and beta-adrenergic receptors. The decrease in alpha-receptor concentration measured by [3H]-dihydroergocryptine binding was from 80 +/- 6 (SEM) to 52 +/- 2 fmol/mg. Beta-receptor concentration measured by 125I-iodohydroxybenzylpindolol binding also decreased by about half from 80 +/- 16 to 41 +/- 9 fmol/mg. The affinities of the receptors were unchanged. There was no change in either concentration or affinity of beta receptors in membranes prepared from the lungs or kidneys of these hypertensive rats. There results demonstrate that the observed receptor changes are tissue-specific. Cardiac adrenergic receptor alterations are therefore not part of a generalized adrenergic receptor decrease associated with elevated circulating plasma catecholamine concentrations, but probably reflect a specific increase in cardiac sympathetic drive.

Beta-adrenergic receptors in ciliary processes of the rabbit

Identification and characterization of beta-adrenergic receptors were attempted in particulate membrane fractions derived from isolated ciliary processes (CP) of rabbit eyes. High-affinity binding sites for 125I-hydroxybenzylpindolol (125I-HYP), a beta-adrenergic antagonist, were identified in particulate membrane fractions of homogenized CP that were recovered from discontinuous sucrose density gradients. Adenylate cyclase activity was recovered in the same fraction as the 125I-HYP binding sites. The dissociation constant of 125I-HYP for the high affinity site is 0.25 nM, with a minimum capacity of about 35 fmol/mg of protein. Adrenergic agonists and antagonists, including timolol, 1-alprenolol, d,1-propranolol, 1-isoproterenol, 1-epinephrine, and phentolamine, were examined for their ability to displace 125I-HYP from its binding site. The results were consistent with the identification of the high-affinity 125I-HYP binding sites as beta-adrenergic receptors. This is the first report which identifies by ligand binding techniques beta-adrenergic receptors in CP exclusive of iridial or other uveal tissue and supports the possibility of direct action of beta-adrenergic agents on the formation of aqueous humor.

Decreased number of beta-adrenoceptors in cerebral cortex of hypothyroid rats

The influence of thyroid hormone deficiency on beta-adrenoceptors in the rat cerebral cortex was investigated using the 3H-dihydroalprenolol binding assay. The maximal number of binding sites (Bmax) and the dissociation constants (KD) were determined in the brain tissue from euthyroid animals and from rats made hypothyroid by feeding 6-propyl-2-thiouracil. Hypothyroidism resulted in a 37% decrease in beta-adrenoceptor density (Bmax). The dissociation constants (KD) of both groups were not significantly different.

Autoradiographic visualization of beta-adrenergic receptors in normal and denervated skeletal muscle

Autoradiographic localization of beta-adrenergic receptors in rat skeletal muscle in vivo was achieved utilizing [125I]-iodohydroxybenzylpindolol, a potent beta-adrenergic blocker with high affinity and specificity for those receptors. In normal muscle the beta-adrenergic receptors were localized mainly to blood vessels, arterioles greater than venules, with much less concentration of grains over the fascicles of muscle fibers. One week after denervation there was an increase in binding both to blood vessels and muscle fibers, more so in soleus and gactrocnemius than in extensor digitorum longus. While these results parallel in vitro biochemical studies, they dictate caution when inferring cellular localization of beta-adrenergic receptors (and other molecules) solely on the basis of biochemical techniques applied to subcellular fractions of whole-organ homogenates.

Direct binding studies of adrenergic receptors: biochemical, physiologic, and clinical implications

Recently developed radioligand binding techniques permit direct investigation of the alpha- and beta-adrenergic receptors for catecholamines in a wide variety of tissues. These techniques allow the receptors to be quantitated, characterized, and studied under varying conditions of physiologic and pathophysiologic interest. They are providing fresh insights into the mechanisms by which endogenous catecholamines and other hormones regulate the properties of the adrenergic receptors and, in turn, control tissue sensitivity to catecholamine action.