Glucocorticoid regulation of beta-adrenergic receptors in 3T3-L1 preadipocytes

Treatment of 3T3-L1 preadipocytes (fibroblasts) with 250 nM dexamethasone for 48 hr caused a doubling of total beta-adrenergic receptors and an increase in beta 2-adrenergic receptor subtype proportion from approximately 50% in controls to 85% in treated cells. The responses to epinephrine and norepinephrine in a whole cell cAMP accumulation assay reflected these changes. The effects of dexamethasone on beta-adrenergic receptors were mediated through the glucocorticoid receptor and were time and dose dependent with an EC50 of 2.77 +/- 0.73 nM for an increase in the proportion of beta 2-adrenergic receptors. The rank order of potency of steroids to effect these changes (betamethasone = dexamethasone greater than fludrocortisone greater than hydrocortisone = triamcinolone greater than aldosterone) correlated with their glucocorticoid potency. [3H]Dexamethasone binding to intact cells yielded a KD value of 3.47 +/- 0.38 nM for binding to the glucocorticoid receptor which correlated well with the EC50 for dexamethasone to alter beta-adrenergic receptors. Inhibition of [3H]dexamethasone binding by other steroids confirmed that the ability of steroids to regulate beta-adrenergic receptors correlated with the affinity of each compound for the 3T3-L1 glucocorticoid receptor. Progesterone, which can bind to the glucocorticoid receptor but has only weak agonist activity, competitively inhibited the ability of dexamethasone to alter beta-adrenergic receptors. Protein synthesis, RNA synthesis, and N-linked glycosylation appeared to be necessary for the change in receptor subtype expression and the increase in beta-adrenergic receptor number induced by dexamethasone. The present study suggests that regulation of beta-adrenergic receptor expression in 3T3-L1 preadipocytes by dexamethasone is a glucocorticoid-specific effect which may require gene activation.     

Interactions between adenosine and alpha 1-adrenergic agonists in regulation of respiration in hamster brown adipocytes

Respiration in brown adipocytes can be increased by beta-adrenergic receptor agonists or by alpha 1-adrenergic receptor agonists (phenylephrine and norepinephrine). Previous studies have shown that beta receptor-stimulated respiration is inhibited by adenosine and that enzymatic removal of adenosine produced by fat cells under normal incubation conditions enhances the respiratory response to beta receptor activation. The present experiments were performed to determine the effect of adenosine on the respiratory response elicited by agonists of alpha 1 receptors. The alpha-adrenergic agonists phenylephrine and norepinephrine (in the presence of the beta-adrenergic antagonist propranolol) stimulated respiration and the respiratory response to each agent was enhanced when endogenous adenosine was removed with adenosine deaminase. Addition of hydrolysis-resistant analogues of adenosine inhibited phenylephrine-stimulated respiration, and, since N6-phenylisopropyladenosine was more effective than was 5'-N-ethylcarboxamidoadenosine, we conclude that an A1 receptor is involved. In contrast, the P site agonist 2',5'-dideoxyadenosine did not inhibit phenylephrine-stimulated respiration but did cause some inhibition of isoproterenol-stimulated respiration. These results suggest that adenosine, acting via A1 receptors, modulates alpha 1-adrenergic effects on thermogenesis in brown fat cells, an action that is analogous to its inhibition of beta-adrenergic receptor-stimulated thermogenesis.     

Postsynaptic alpha 1- and alpha 2-adrenergic mechanisms in coronary vasoconstriction

This study examined the relative importance of postsynaptic alpha 1- and alpha 2-adrenoceptors in mediating coronary vasoconstriction in open chest dogs in which the left circumflex coronary artery was cannulated and perfused at a constant rate. The cervical vagus nerves and central connections of the stellate ganglia were transected, and beta-adrenergic blockade was produced with propranolol. Coronary vasoconstriction occurred in response to intraarterial administration of both the alpha 1-agonist phenylephrine and the alpha 2-agonist BHT 933. The response to phenylephrine was partially blocked with prazosin and nearly completely eliminated by yohimbine. The response to BHT 933 was resistant to prazosin, but almost completely blocked by yohimbine. Coronary vasoconstriction produced by norepinephrine was resistant to prazosin, but was blunted by alpha 2-adrenergic blockade with yohimbine or idazoxan. Prazosin produced some blunting of coronary vasoconstriction in response to small doses of epinephrine, while yohimbine markedly attenuated epinephrine-induced vasoconstriction at all doses used. Measurements of regional myocardial blood flow with radioactive microspheres demonstrated no transmural redistribution of perfusion during vasoconstriction produced by either alpha 1- or alpha 2 stimulation. Thus, although stimulation of both alpha 1- and alpha 2-adrenoceptors is capable of causing coronary vasoconstriction, vasoconstriction in response to norepinephrine and epinephrine is mediated principally by postsynaptic alpha 2-adrenoceptors.     

Synthesis and characterization of a high affinity radioiodinated probe for the alpha 2-adrenergic receptor

The availability of radioiodinated probes has facilitated the localization and molecular characterization of cell membrane receptors for hormones and neurotransmitters. However, such probes are not available for the study of the alpha 2-adrenergic receptor. This report describes the synthesis and characterization of functionalized derivatives of the selective alpha 2-adrenergic antagonists, rauwolscine and yohimbine, which can be radiolabeled to high specific activity with 125I. Following demethylation of rauwolscine or yohimbine, the resultant carboxylic acid derivatives were reacted with 4-aminophenethylamine to yield the respective 4-aminophenethyl carboxamides, 17 alpha-hydroxy-20 alpha-yohimban-16 beta-[N-4-amino-phenethyl]carboxamide (rau-pAPC) and 17 alpha-hydroxy-20 beta-yohimban-16 alpha-[N-4-aminophenethyl]carboxamide. In competitive inhibition studies using rat renal membranes and the radioligand [3H]rauwolscine, rau-pAPC (Ki = 11 +/- 1 nM) exhibited a 14-fold greater affinity than the corresponding yohimbine derivative (Ki = 136 +/- 45 nM). The higher affinity compound, rau-pAPC, was radioiodinated by the chloramine T method, and the product, 125I-rau-pAPC [17 alpha-hydroxy-20 alpha-yohimban-16 beta-(N-4-amino-3 -[125I]iodophenethyl)carboxamide], was purified by reverse phase HPLC to high specific activity (2175 Ci/mmol) and its binding characteristics were investigated in rat kidney membranes. Specific binding of 125I-rau-pAPC was saturable and of high affinity as determined by Scatchard analysis (KD = 1.8 +/- 0.3 nM) or from kinetic studies (KD = k2/k1 = 0.056 +/- 0.013 min-1)/4.3 +/- 0.2 X 10(7) M-1 min-1 = 1.3 +/- 0.3 nM). In competition studies, alpha-adrenergic antagonists and agonists inhibited the binding of 125I-rau-pAPC with a potency order consistent with an interaction at alpha 2-adrenergic receptors (rauwolscine greater than phentolamine greater than prazosin; clonidine greater than (-)-epinephrine greater than (-)-norepinephrine greater than dopamine greater than (+)-epinephrine). In rat liver and human platelet membranes, high affinity binding of 125I-rau-pAPC was also observed (liver, KD = 1.2 +/- 0.4 nM; platelet, KD = 3.2 +/- 1.5 nM). In addition, the density of alpha 2-adrenergic receptors identified from binding studies with 125I-rau-pAPC in kidney, liver, and platelet membranes was similar to that observed in parallel studies with [3H]rauwolscine. These findings indicate that 125I-rau-pAPC is a high affinity probe that selectively identifies alpha 2-adrenergic binding sites. Availability of this radioligand should facilitate the localization and biochemical characterization of this alpha-adrenergic receptor subtype.     

Effects of alpha 2-adrenergic agonists on carbachol-stimulated catecholamine synthesis in cultured bovine adrenal medullary cells

We examined the effects of alpha 2- and alpha 1-adrenergic agonists on synthesis of catecholamines in cultured bovine adrenal medullary cells. Clonidine, an alpha 2-adrenergic agonist, inhibited carbachol-stimulated synthesis of [14C]catecholamines from [14C]tyrosine in a concentration-dependent manner. Clonidine also inhibited carbachol-induced uptake of 45Ca2+ into cells at concentrations similar to those that inhibited the synthesis of [14C]catecholamines. Other alpha 2-adrenergic agonists, oxymetazoline and guanfacine, also strongly inhibited carbachol-stimulated synthesis of [14C]catecholamines. alpha 1-Adrenergic agonists, phenylephrine and norfenefrine, did not affect the synthesis. Tyrosine hydroxylase (EC 1.14.16.2) activity in a soluble fraction of cultured bovine adrenal medullary cells was assayed after gel filtration on a Sephadex G-25 column. Stimulation of the cells with carbachol increased the activity of tyrosine hydroxylase. Clonidine, oxymetazoline, and guanfacine all suppressed the carbachol-induced increase in activity of tyrosine hydroxylase in the cells. These results suggest that alpha 2-adrenergic agonists inhibit carbachol-stimulated synthesis of catecholamines by suppression of tyrosine hydroxylase activity, probably through the inhibition of Ca2+ uptake. However, the involvement of alpha 2-adrenoceptors in the inhibitory effects of alpha 2-agonists on catecholamine synthesis is still unsettled, since yohimbine failed to antagonize the inhibitory effect of clonidine on the synthesis in cultured bovine adrenal medullary cells.     

Renal catecholamines and alpha 2-adrenergic receptors in salt-related and genetic hypertension

Increased dietary salt intake alters renal function which often leads to deleterious cardiovascular consequences. Studies were carried out to characterize the effects of high-salt diets on renal catecholamines and alpha 2-adrenergic receptors. These parameters were evaluated in both genetic and acquired forms of hypertension and also in normotensive rats on high-salt diets. Renal catecholamine content was determined by high-performance liquid chromatography with electrochemical detection. Renal alpha 2-adrenergic receptor-binding studies were performed on whole kidney homogenates using 3H-p-aminoclonidine to label both high- (0.5 nM) and low-affinity (5.0 nM) renal alpha 2-adrenergic receptors. Increased salt intake elevated blood pressure, decreased renal norepinephrine stores and resulted in renal alpha 2-adrenergic receptor up-regulation in deoxycorticosterone acetate salt hypertensive rats, Dahl-S rats and COX-SHR. The decreased renal stores of norepinephrine (NE) appeared to reflect increased renal NE utilization. In contrast, SHR (Charles River) had elevated NE stores and alpha 2-adrenergic receptors while on normal salt diets. Short-term (10-14 days) exposure to high-salt diets had modest effects in normotensive rats or COX-SHR, although it was sufficient to increase low affinity renal alpha 2-adrenergic receptor number. Renal dopamine metabolism was also altered by high-salt diets. These studies demonstrated a relationship between renal NE content and renal alpha 2-adrenergic receptors. The implications of this relationship and other salt-related changes in renal catecholamine metabolism were discussed as they pertained to hypertension and renal function.     

Involvement of alpha 1-adrenergic receptors in stimulation of phosphatidylinositol metabolism by catecholamines in mouse thyroids

The effects of alpha-adrenergic agonists and thyroid stimulating hormone on the incorporation of radioactive phosphate into phosphatidylinositol were investigated in mouse thyroids in vitro. The incorporation of 32P orthophosphate into phosphatidylinositol was stimulated by thyroid stimulating hormone, norepinephrine (a mixed alpha 1- and alpha 2-adrenergic agonist), methoxamine and phenylephrine (alpha 1-agonists) and slightly by clonidine and oxymetazoline (alpha 2-agonists) but not by isoproterenol (beta-agonist). Prazosin (alpha 1-antagonist) inhibited the stimulation by norepinephrine of 32P incorporation into phosphatidylinositol, but yohimbine (alpha 2-antagonist) was less effective. Although norepinephrine inhibits the thyroid stimulating hormone-induced release by activating alpha-, especially alpha 1-adrenoceptors in mouse thyroids [M. L. Maayan et al., Metabolism 26, 473 (1977); M. L. Maayan et al., Endocrinology 101, 284 (1977); T. Muraki et al., Endocrinology 110, 51 (1982)] alpha 1-agonists did not decrease the stimulation of turnover elicited by thyroid stimulating hormone and did not have additive action with it. These results suggest that (1) the stimulation of phosphatidylinositol turnover of mouse thyroids elicited by adrenergic agonists is mediated by activation of alpha 1-adrenoceptors and (2) the inhibitory effect of norepinephrine on the thyroid stimulating hormone-induced release of thyroxine is not mediated by norepinephrine-inhibition of phosphatidylinositol-turnover stimulated by thyroid stimulating hormone.     

Syntheses of (R)- and (S)-2- and 6-fluoronorepinephrine and (R)- and (S)-2- and 6-fluoroepinephrine: effect of stereochemistry on fluorine-induced adrenergic selectivities

Several routes to the enantiomers of fluoronorepinephrines (1) and fluoroepinephrines (2) were explored. A catalytic enantioselective oxazaborolidine reduction and a chiral (salen)Ti(IV) catalyzed asymmetric synthesis of silyl cyanohydrins proved efficacious in the key stereo-defining steps of two respective routes. Binding studies of the catecholamines with alpha(1)-, alpha(2)-, beta(1)-, and beta(2)-adrenergic receptors were examined. The assays confirmed that fluorine substitution had marked effects on the affinity of (R)-norepinephrine and (R)-epinephrine for adrenergic receptors, depending on the position of substitution. Thus, a fluoro substituent at the 2-position of (R)-norepinephrine and (R)-epinephrine reduced activity at both alpha(1)- and alpha(2)-receptors and enhanced activity at beta(1)- and beta(2)-receptors, while fluorination at the 6-position reduced activity at the beta(1)- and beta(2)-receptors. The effects of fluorine substitution on the S-isomers were less predictable.     

Specificity of the beta 2-adrenergic receptor stimulating cyclic AMP accumulation in the intermediate lobe of rat pituitary gland

Changes of cyclic AMP levels were used to assess the specificity of the beta-adrenergic receptor in primary cultures of cells prepared from the intermediate lobe of rat pituitary gland. During a 4 min incubation, beta-adrenergic agonists led to a 4 to 6 fold stimulation of cyclic AMP concentration with the following order of potency (Kd values): zinterol (0.75 nM) greater than hydroxybenzylisoproterenol (1.0 nM) greater than (--)-isoproterenol (4.6 nM) greater than soterenol greater than (7.7 nM) greater than (--)-epinephrine (10 nM) greater than OPC 2009 (procaterol, 11 nM) much greater than (--)-norepinephrine (300 nM). The potent antagonists cyanopindolol, (--)-propranolol and hydroxybenzylpindolol reversed the stimulatory effect of (--)-isoproterenol at Kd values of 0.4-0.6 nM. Other beta-adrenergic antagonists had the following order of potency: pindolol = (--)-alprenolol = timolol (0.9-1.0 mM) much greater than metoprolol (100 nM) greater than dichloroisoproterenol (300 nM) greater than butoxamine (1100 nM). The beta 1-selective antagonist practolol had a low potency at 700 nM. The stereoselectivity of the receptor is indicated by the 400 to 70 fold higher potency of the (--)-isomers of isoproterenol, epinephrine and propranolol as compared to their (+)-stereoisomers. The data show that the beta-adrenergic receptor in the intermediate lobe of the rat pituitary gland is mainly of the bet 2-subtype. Study of this pure population of postsynaptic beta-adrenergic receptors where binding could be correlated with other parameters of cellular activity (cyclic AMP formation and alpha-MSH secretion) should yield useful information about the less accessible adrenergic systems of the brain.     

Regulation of autonomic receptors in rat submandibular gland

The binding of receptor specific radioligands to autonomic receptors in the rat submandibular gland was characterized after chronic drug administration and surgical sympathetic denervation. Reserpine administration resulted in an up-regulation of both alpha 2-adrenergic receptors labeled by [3H]clonidine and beta 1-adrenergic receptors labeled by [3H]dihydroalprenolol. The increase in alpha 2-receptors was half-maximal 24 hr after a single injection of reserpine, and was about 10-fold greater than control after seven daily injections. By contrast, the beta-adrenergic receptor density was the same as control after 3 days of reserpine administration, but within 7 days was about 2-fold greater than control. Guanethidine or yohimbine administration also resulted in an up-regulation of alpha 2-adrenergic receptors. Reserpine administration or unilateral superior cervical ganglionectomy increased the density of alpha 1-adrenergic receptor binding sites 24-80%. Norepinephrine and methoxamine, but not clonidine, caused potassium to be released from submandibular gland slices. Prazosin, but not yohimbine, blocked this response to norepinephrine, indicating that the response was mediated by alpha 1-adrenergic receptors. Potassium release elicited by alpha 1-agonists was augmented in slices from animals that received reserpine. Neither drug treatment nor sympathetic denervation altered muscarinic cholinergic receptor binding. The densities of muscarinic and beta-adrenergic receptors were found to be 23-51% higher in glands from female rats than in glands from male rats.     

alpha 2-Adrenergic receptors in human polymorphonuclear leukocyte membranes

Human polymorphonuclear cell membranes contain alpha 2-adrenergic receptors which are measured by binding of the alpha 2-adrenergic antagonist [3H]yohimbine. The alpha 1-adrenergic antagonist [3H]prazosin showed no specific binding. High and low affinity sites were detected which had Kd values of 2.38 +/- 0.4 and 139 +/- 12 nM, respectively, and which bound maximally 4.82 +/- 0.9 and 81 +/- 9 fmoles of [3H]yohimbine/mg membrane protein. The high and low affinity sites were also detected by competition studies with phentolamine, epinephrine and norepinephrine and by dissociation kinetics of bound [3H]yohimbine. [3H]Yohimbine binding was stereospecifically inhibited by (-)- and (+)-epinephrine and norepinephrine. [3H]Yohimbine binding to intact cells showed about 500 high affinity sites per cell (Kd 0.5 nM) and approximately 4000 lower affinity sites per cell (Kd 3-4 nM). Yohimbine enhanced the (-)-norepinephrine stimulation of cAMP production in intact cells.     

Bucindolol, a nonselective beta 1- and beta 2-adrenergic receptor antagonist, decreases beta-adrenergic receptor density in cultured embryonic chick cardiac myocyte membranes

Bucindolol and carvedilol, nonselective beta1- and beta2-adrenergic receptor antagonists, have been widely used in clinical therapeutic trials of congestive heart failure. The aim of the current study was to investigate long-term effects of bucindolol or carvedilol on beta-adrenergic receptor protein and gene expression in cardiac myocytes. Embryonic chick cardiac myocytes were cultured and incubated with bucindolol (1 microM), carvedilol (1 microM), or norepinephrine (1 microM) for 24 h. 125I-iodocyanopindolol binding assays demonstrated that incubation with norepinephrine or bucindolol, but not carvedilol, significantly decreased beta-adrenergic receptor density in crude membranes prepared from the myocytes. Neither bucindolol nor carvedilol significantly stimulated adenylyl cyclase activity in membranes from drug-untreated cells. Unlike by norepinephrine, the receptor density reduction by bucindolol incubation was not accompanied by a change in beta1-adrenergic receptor messenger RNA abundance. A decrease in membrane beta-adrenergic receptor density without a change in cognate messenger RNA abundance was also observed in hamster DDT1 MF2 cell line incubated with bucindolol (1 microM, 24 h). We conclude that incubation with bucindolol, but not carvedilol, results in true reduction of beta-adrenergic receptor density in chick cardiac myocyte membranes by mechanisms that are distinct from those responsible for receptor density reduction by the agonist norepinephrine.     

Moxonidine, an antihypertensive agent, is permissive to alpha1-adrenergic receptor pathway in the rat-tail artery

To investigate whether alpha1-adrenergic receptors were involved in the contractile response of tail arteries to moxonidine, isolated ring segments of tail arteries from male adult Sprague-Dawley rats were studied. Moxonidine (EC50 = 1.3 microM) and the alpha1-agonist phenylephrine (EC50 = 2.5 microM) increased tension development in the rat-tail artery similarly. The response to moxonidine (1 microM) could be blocked by both alpha1-adrenoceptor blockers prazosin (IC50 = 1 nM), and urapidil (IC50 = 14 nM), and also by alpha2-adrenoceptor blockers, yohimbine (IC50 = 49 nM) and efaroxan (IC50 = 49 nM). Combination drug treatment (urapidil and yohimbine, or yohimbine and prazosin) was more effective in blocking the contractile response to moxonidine, than treatment with prazosin or urapidil alone. Comparison of pA2 values for prazosin in the presence of moxonidine (9.35) or phenylephrine (10.2) confirm that alpha1-adrenergic receptors are involved in the contractile response of rat-tail artery to moxonidine.     

Human beta 1- and beta 2-adrenergic receptor binding and mediated accumulation of cAMP in transfected Chinese hamster ovary cells. Profile of nebivolol and known beta-adrenergic blockers.

The interaction of nebivolol and its SRRR and RSSS enantiomers, and of known beta-adrenergic blockers, with human beta 1- and beta 2-adrenergic receptors expressed separately in Chinese hamster ovary cells in culture (CHO-Hu beta 1 and CHO-Hu beta 2), was investigated. We studied [3H]CGP-12177 binding to the intact cells and the accumulation of cAMP induced by isoproterenol. Each of the receptor subtypes displayed saturable [3H]CGP-12177 binding on intact cells with sub-nanomolar affinity. The density of beta 1- and beta 2-adrenergic receptor sites was 1.1 x 10(6) receptor binding sites per CHO-Hu beta 1 cell and 0.2 x 10(6) receptor binding sites per CHO-Hu beta 2 cell, respectively. The beta-adrenergic antagonists CGP 20712-A, ICI 118-551 and propranolol showed the same binding properties as beta-adrenergic receptors in previously described tissues or cells. The potencies of these compounds in inhibiting beta-adrenergic receptor mediated accumulation of cAMP corresponded well with their binding affinities. d-Nebivolol (SRRR) and nebivolol showed combined high affinity and selectivity for inhibition of beta 1-adrenergic receptor coupled accumulation of cAMP in CHO-Hu beta 1 cells (0.41 and 0.42 nM for d-nebivolol and nebivolol, respectively). l-Nebivolol (RSSS) was 1460 times less potent than d-nebivolol in CHO-Hu beta 1 cells. The binding affinities of d-nebivolol and nebivolol for human beta 1-adrenergic binding sites correlated well with their potencies in inhibiting beta 1-adrenergic receptor coupled accumulation of cAMP. CHO cells transfected with human beta 1- and beta 2-adrenergic receptors are a valid model system for studying the interaction of compounds with human beta-adrenergic receptors.     

Turnover of beta 1- and beta 2-adrenergic receptors after down-regulation or irreversible blockade

The turnover of beta 1- and beta 2-adrenergic receptors was measured after both isoproterenol-induced down-regulation and irreversible blockade of receptors. Changes in the density of receptors were quantified using the radioligands 125I-iodopindolol and 125I-iodocyanopindolol. Treatment of intact L6 myoblasts or C6 glioma cells with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) inactivated beta-adrenergic receptors on membranes prepared from these cells. At a concentration of 100 microM EEDQ, more than 90% of beta 1- and beta 2-adrenergic receptors were inactivated within 2 hr of treatment. Recovery of beta-adrenergic receptors on intact cells after inactivation by EEDQ required more than 24 hr and was prevented by cycloheximide, an inhibitor of protein synthesis. The kinetics of recovery of the density of receptors were analyzed in terms of a model that allows estimation of the rate constants for receptor appearance in and disappearance from the membrane, assuming that the rate of appearance of receptors is constant and the rate of disappearance of receptors is proportional to the number of receptors. Beta 2-Adrenergic receptors on L6 myoblasts were incorporated into the membrane at a rate of 28 fmol/mg of protein/hr and had a half-life of 12.6 hr. On C6 glioma cells, Beta 1- and beta 2-adrenergic receptors appeared at rates of 13.3 and 6.6 fmol/mg of protein/hr, respectively, with half-lives of 9.4 and 6.4 hr. Recovery of receptors on C6 cells after isoproterenol-induced down-regulation was inhibited by cycloheximide. The rate of recovery of beta 1- and beta 2-adrenergic receptors was reduced after treatment with isoproterenol for 8 hr when compared to recovery after treatment with EEDQ. The major effect of treatment with isoproterenol was a persistent decrease in the rate of appearance of beta 1- and beta 2-adrenergic receptors (rate of synthesis and insertion into the membrane after treatment with isoproterenol = 4.0 fmol/mg of protein/hr). Since treatment with isoproterenol did not alter the rate of cell division or total protein synthesis, the isoproterenol-induced alteration was probably a specific effect on the rate of synthesis of beta-adrenergic receptors.     

Characterization of beta-adrenergic receptors in rat ovary

Specific beta-adrenergic receptors were characterized in rat ovary, using an in vitro receptor binding assay with [3H]dihydroalprenolol. The ligand was bound to a single class of saturable and high affinity receptors (dissociation constant = 3.5 nM and maximum capacity = 3.4 pmol/g tissue). In competition studies, the binding was stereoselective for the (-)-isomer of isoproterenol. Known beta-adrenergic agonists displaced the ligand from ovarian receptors with the following order of potency: isoproterenol greater than epinephrine much greater than norepinephrine, which is typical for a beta 2-adrenergic receptor.     

Agonist-mediated regulation of alpha 1- and beta 2-adrenergic receptor metabolism in a muscle cell line, BC3H-1

We have compared the metabolism of alpha 1- and beta 2-adrenergic receptors which are both expressed in BC3H-1 muscle cells. During growth of the cells to confluence, the number of alpha 1-receptors per mg of membrane protein increases, whereas that of the beta 2-receptors remains constant. Experiments using cycloheximide and irreversible alpha 1- and beta 2-receptor antagonists, phenoxybenzamine and N-[2-hydroxy-3-(1-naphthoxy)-propyl]-N'-bromoacetylethylenediamine , respectively, yield disparate turnover rates (t1/2) for the two receptors: alpha 1 congruent to 25 hr, beta 2 congruent to 200 hr. These experiments suggest that synthesis of beta 2-receptors virtually ceases in confluent cells. Maximally effective doses of agonists down-regulated both receptor types 80-90% and enhanced the rates of loss of both receptors (t1/2 = 1-5 hr). The rates of down-regulation were not affected by cycloheximide, implying that agonists enhance receptor clearance rather than decrease receptor appearance. The rank orders of potencies of agonists for promoting receptor down-regulation were those characteristic of alpha 1- and beta 2-receptors. However, concentrations of agonists that resulted in down-regulation of each receptor subtype were 10- to 100-fold lower than those required for occupancy of receptors as assessed in radioligand binding studies. Receptor recovery following removal of agonists was blocked by cycloheximide and was much faster than the recovery that followed treatment of cells with irreversible antagonists. Therefore, protein synthesis (but perhaps not receptor synthesis per se) appears necessary for recovery from down-regulation. In addition, the rates of recovery of alpha 1- and beta 2-receptor-mediated functions (phosphatidylinositol turnover and cyclic AMP synthesis, respectively) following receptor down-regulation or irreversible blockade parallel the rates of receptor recovery. These data indicate that basal metabolism of alpha 1- and beta 2-receptors in BC3H-1 cells is substantially different, but that agonist-mediated changes in metabolism of the two receptor subtypes are similar. Thus, common mechanisms appear to mediate the regulation by agonists of alpha 1- and beta 2-receptors in these cells.     

Alpha 1- and beta-adrenergic and muscarinic cholinergic receptors in guinea-pig nasal mucosa

The alpha 1- and beta-adrenergic and muscarinic cholinergic receptors in guinea-pig nasal mucosa were measured for the first time by direct binding techniques using [3H]prazosin, [3H]dihydroalprenolol ([3H]DHA) and [3H]quinuclidinyl benzilate ([3H]QNB). The maximum binding capacities of [3H]prazosin, [3H]DHA and [3H]QNB in guinea-pig nasal mucosa were 20.1, 42.3, 159 fmol/mg protein, respectively. The dissociation constants of [3H]prazosin, [3H]DHA and [3H]QNB in guinea-pig nasal mucosa were 0.37, 0.77 and 1.5 nM, respectively. After the removal of the superior cervical ganglion in the guinea-pig, the number of alpha 1-adrenergic receptors was increased in the nasal mucosa while the number of beta-adrenergic and muscarinic receptors remained unchanged. Thus, alpha 1-adrenergic receptors are probably postsynaptic receptors in the target cells of sympathetic nerves, while beta receptors relate to the circulating catecholamines.     

Conformational effects on the activity of drugs. 13. A revision of previously proposed models for the activation of alpha- and beta-adrenergic receptors.

The alpha 1-, alpha 2-, beta 1-, and beta 2-adrenergic properties of the 2-(3,4-dihydroxyphenyl)morpholines 3 and 4 (2-DPMs), of the 3-(3,4-dihydroxyphenyl)-3-piperidinols 5 and 6 (3-DPPs), and of the trans-2-amino-5,6-dihydroxytetrahydronaphthalen-1-ols 7 and 8 and the trans-2-amino-6,7-dihydroxytetrahydronaphthalen-1-ols 9 and 10 (2-ADTNs) were evaluated in vitro both by radioligand binding assays and by functional tests on isolated preparations and compared with those of norepinephrine (NE, 1) and isoprenaline (ISO, 2). Through a comparison of the stereostructures of the compounds examined with their biopharmacological properties, it was possible to revise previously proposed molecular models for the direct activation of alpha- and beta-adrenergic receptors. The revised models (A-C) provided information about the conformational requirements of adrenergic drugs, which substantially fit in with the results of several published studies involving conformationally-restricted adrenoceptor agonists. The different position of the catecholic hydroxyl groups in model B, which refers to the alpha 2 receptors, and in model C, which refers to the beta receptors, confirms the importance of the rotameric position of the aromatic ring of catecholamines in the interaction with the alpha- and beta-adrenergic receptor.     

Selective down regulation of vascular beta 1 adrenergic receptors after prolonged isoproterenol infusion

Prolonged isoproterenol infusion (400 micrograms/kg/h for 4 days) in rats was previously shown to produce a reduction in the sensitivity of both cardiac and vascular beta-adrenergic receptors without affecting responsiveness to alpha 1 agonists or phosphodiesterase inhibitors in either vascular or cardiac muscle. The present study was designed to determine if the loss in beta receptor responsiveness was similar for both beta 1 and beta 2 vascular receptors. The rat jugular vein was previously shown to relax in response to both norepinephrine and isoproterenol with norepinephrine-induced relaxation being mediated by interaction with beta 1 adrenergic receptors and isoproterenol-induced relaxation being mediated by its interaction with beta 2 vascular receptors. Using this preparation, tissues from isoproterenol-infused rats were approximately threefold less responsive to isoproterenol when compared to responses in tissues from saline-treated rats. Relaxation to norepinephrine in jugular veins from isoproterenol-infused rats was virtually abolished relative to the response in saline-treated animals. These data suggest that beta 1-adrenergic receptors in blood vessels are considerably more susceptible to down regulation than are beta 2-adrenergic receptors. This observation may have importance in both the therapy of congestive heart failure, where down regulation of beta-adrenergic receptors has been observed, and in our understanding of the mechanism for the inotropic effects of beta receptor agonists.