Genetic influence on the regulation of beta adrenergic receptors in mice

The regulation of beta adrenergic receptors was investigated in inbred mouse strains in which previous studies revealed differences in the regulation of dopamine receptors. The density of beta adrenergic receptors in the cerebral cortex of BALB/J mice was about one-third of that in CBA/J and C57BL/6J mice. Strain differences in the binding of [125I]iodohydroxypindolol to beta adrenergic receptors were due to changes in the density of beta-1 adrenergic receptors. Chronic administration of propranolol did not result in an increase in the density of beta adrenergic receptors receptors in cortices of C57BL/6J and BALB/cJ mice were observed. In contrast, pretreatment with 6-hydroxydopamine resulted in increases in the density of beta adrenergic receptors in the cerebral cortex of all three strains. Analysis of the effects of these treatments on the subtypes of beta adrenergic receptors revealed that the changes were restricted to changes in the density of beta-1 receptors. The failure to observe a response to propranolol in CBA/J mice expands the extent of deficits reported previously in this strain for striatal dopamine receptor supersensitivity after chronic treatment with haloperidol (Severson et al., Brain Res. 210: 201-215, 1981). CBA/J mice may be a useful model for genetic analysis of mechanisms for the control of receptor sensitivity and to investigate the impairments of the regulation of catecholaminergic receptors observed in aged rodents.     

Selective regulation of beta-2 adrenergic receptors by the chronic administration of the lipophilic beta adrenergic receptor agonist clenbuterol: an autoradiographic study

Many antidepressant drugs, when administered chronically to rats, have been shown to produce decreases in the density of beta adrenergic receptors in the central nervous system. The centrally active beta adrenergic receptor agonist clenbuterol is currently being evaluated clinically as an antidepressant. The chronic administration of this drug to rats resulted in a large decrease in the density of beta adrenergic receptors in some areas of the rat brain but not in others. Thus, autoradiographic studies revealed that the total density of beta adrenergic receptors in the molecular layer of the cerebellum, but not in layers 1 to 3 or layer 4 of the cerebral cortex, was decreased. To examine whether this regional selectivity occurred because of differences in plasticity of cerebellum and cortex or because cerebellum contains mainly beta-2 adrenergic receptors and cortex contains mainly beta-1 adrenergic receptors, separate analyses of the subtypes of beta adrenergic receptors were performed in each area. These experiments indicated that the decrease in receptor density was entirely specific for beta-2 adrenergic receptors, whereas the density of beta-1 receptors was unchanged. Thus, even in layers 1 to 3 and layer 4 of the cerebral cortex, beta-2 receptor density was decreased, with no change in beta-1 receptor density. Using the autoradiographic assay for ligand binding, it was shown that clenbuterol has equal affinity for beta-1 and beta-2 adrenergic receptors, indicating that the selective effect of this drug was not due to a selective affinity for beta-2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of pindolol and propranolol on beta adrenergic receptors on human lymphocytes

The cardiovascular supersensitivity observed after discontinuation of administration of beta adrenergic receptor antagonists may be explained by an increase in the density of beta adrenergic receptors. Thus, a change in the density of receptors has been observed in human lymphocytes after administration of propranolol (Aarons et al., 1980). The effects of pindolol, a beta adrenergic receptor antagonist with intrinsic sympathomimetic activity, were compared with those of propranolol or placebo. Pindolol (10 mg q.i.d.), propranolol (40 mg q.i.d.) or placebo were administered to 12 subjects for 8 days. The density of beta adrenergic receptors was determined by Scatchard analysis of the specific binding of [125I]iodopindolol on membranes prepared from human lymphocytes. Administration of pindolol resulted in a 30 to 50% decrease in the density of beta adrenergic receptors. This decrease was apparent within 1 day of beginning pindolol administration and it persisted for at least 8 days after discontinuation of drug administration. The reversibility of the decrease in receptors observed after pindolol administration was studied in 27 subjects given propranolol, pindolol or placebo for 4 days in a double-blind cross-over trial. Propranolol consistently induced a small increase in the density of beta adrenergic receptors. The density of receptors returned to predrug values within 2 days after discontinuation of propranolol administration. Pindolol induced a 30 to 50% decrease in the density of receptors which, as observed previously, persisted for at least 10 days after discontinuation of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Noradrenergic denervation alters serotonin2-mediated behavior but not serotonin2 receptor number in rats: modulatory role of beta adrenergic receptors

Recent behavioral evidence suggests that enhancement of noradrenergic neurotransmission may alter the functional sensitivity of serotonin2 (5-HT2) receptors in the central nervous system. The present studies have examined the effects of two types of noradrenergic denervation [neurotoxic: via N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) treatment; and pharmacologic: via chronic beta adrenergic receptor blockade] on the 5-HT2-mediated head shake response and cortical beta adrenergic and 5-HT2 receptor number in the rat. No changes in quipazine-induced head shakes were observed 3 days after DSP4 lesion. However, the frequency of head shakes was significantly enhanced 10 days after DSP4 treatment in the presence of a 39% up-regulation of beta adrenergic receptors. Pretreatment with propranolol 10 days after DSP4 lesion selectively antagonized the enhancement of the behavioral response to quipazine without altering base-line response rate, whereas pretreatment with the 5-HT2 antagonist ketanserin totally blocked head shakes in both control and DSP4-treated rats. Pharmacologic denervation achieved by continuous (14 day) administration of the beta adrenergic antagonist propranolol also resulted in a potentiation of the head shake response (274% of control) and an upregulation of beta adrenergic receptors (44%). Conversely, continuous treatment with the beta adrenergic agonist clenbuterol resulted in a marked reduction in head shakes (36% of control) with a concomitant 29% down-regulation of beta adrenergic receptors. 5-HT2 receptor binding was not modified by either DSP4 lesion or continuous administration of beta adrenergic agonists or antagonists. These studies demonstrate that changes in cortical beta adrenergic receptor density may modify 5-HT2-mediated behavior in a manner that is independent of changes in 5-HT2 receptor number.     

Coexistence of beta-1 and beta-2 adrenergic receptors in the human heart: effects of treatment with receptor antagonists or calcium entry blockers

The properties of the binding of [125I]iodopindolol ([125I]IPIN) to beta adrenergic receptors on plasma membranes prepared from right atrial tissue removed during cardiac bypass surgery were investigated. Some of the patients from whom the tissue was removed had been treated before surgery with either a beta adrenergic receptor antagonist or a calcium entry blocker or both. The specific binding of [125I]IPIN to beta adrenergic receptors was saturable, stereoselective and rapidly reversible. Studies of the inhibition of the specific binding of [125I]IPIN by drugs selective for beta-1 or beta-2 adrenergic receptors suggested that both beta-1 and beta-2 adrenergic receptors are present in the tissue, with approximately 55% of the receptors having the properties of beta-2 adrenergic receptors. The density of receptors in patients not treated with beta adrenergic receptor antagonists or calcium entry blockers was approximately 80 fmol/mg of protein, whereas the density of beta adrenergic receptors in treated patients was increased by approximately 50%. The relative proportion of beta-1 to beta-2 adrenergic receptors in subjects treated with beta adrenergic receptor antagonists and/or calcium entry blockers was not significantly different from that in untreated subjects. Studies were also carried out with a limited number of samples of human ventricular muscle obtained from untreated subjects at the time of surgery. The density of receptors was lower than that observed in studies with atrial tissue. However, as with atrial tissue, approximately half of the receptors appeared to be beta-2 adrenergic receptors.     

Presynaptic modulation of beta adrenergic receptors in rat cerebral cortex after treatment with antidepressants.

Treatment with desmethylimipramine (DMI), a tricyclic antidepressant, for 7 to 21 days resulted in a 35 to 45% decrease in the accumulation of adenosine cyclic 3':5'-monophosphate (cAMP) in response to a maximally effective concentration of (-)-isoproterenol (ISO) in rat cerebral cortical slices. The EC50 for ISO-stimulated cAMP accumulation was not affected by DMI administration. The diminution in responsiveness to catecholamines was accompanied by a 35 to 40% decrease in the density of beta adrenergic receptors as measured by the binding of [125I]iodohydroxybenzylpindolol. Decreases in ISO-sensitive cAMP accumulation and in beta adrenergic receptor density were temporally correlated, maximal decreases being observed within 5 to 7 days. Within 7 days after cessation of chronic DMI treatment ISO-stimulated cAMP accumulation and beta adrenergic receptor density returned to normal. The role of presynaptic nerve terminals in mediating these phenomena was also investigated. Treatment of newborn rats with 6--hydroxydopamine inhibited the development of noradrenergic nerve terminals in the cerebral cortex and blocked the effects of DMI on cortical cAMP accumulation and on beta adrenergic receptor density. The administration of the beta adrenergic receptor antagonist propranolol led to increases in maximal ISO-stimulated cAMP accumulations and beta adrenergic receptor density in the rat cerebral cortex. This increase was not affected by the simultaneous administration of propranolol and DMI. Thus, the effect of DMI appears to be mediated through an action of norepinephrine at beta adrenergic receptors. Chronic treatment with two other clinically effective antidepressants, pargyline and iprindole, led to effects similar to those observed with DMI administration. Pretreatment of neonates with 6-hydroxydopamine blocked the effect of iprindole on beta adrenergic receptors. Preincubation of cortical membranes with guanosinetriphosphate before determination of the density of beta adrenergic receptors had no effect on the decreased number of receptors had no effect on the decreased number of receptors seen in DMI-treated animals. These experiments suggest that antidepressants, acting presynaptically, increase the concentration of transmitter at noradrenergic synapses and induce a compensatory decrease in the density of beta adrenergic receptors.     

Differential inhibition of beta adrenergic receptors in human and rabbit ciliary process and heart

Prior biochemical studies have suggested that beta adrenergic receptors in the ciliary process are mostly of the beta-2 subtype. The present experiments evaluate a number of beta adrenergic antagonists, including several recently developed drugs, for their ability to block rabbit and human ciliary process and heart beta adrenergic receptors activating adenylate cyclase. Three of these agents (alpha-methylpropranolol, IPS 339 and ICI 118,551) demonstrated a high degree of oculoselectivity in both rabbit and human. The other agents (S 37-429, S 32-468, ICI 78,462,H35/25, butoxamine, propranolol, timolol, atenolol and practolol) showed either modest or no oculoselectivity. Structure-activity studies suggested that, among antagonists of the aryloxymethyl type, methylation of the side-chain alpha-carbon or the aromatic ring may enhance oculoselectivity primarily by decreasing potency at cardiac beta adrenergic receptors. Additional physiological studies of cardiac chronotropic response revealed that, compared with nonselective beta blockers, compounds with biochemical oculoselectivity demonstrate decreased physiological effects on cardiac function. This was true when the selective agents were applied either systemically or topically to the eye. On the other hand, the systemic absorption of topical timolol was sufficient to block cardiac chronotropic effects completely. These findings, identifying relatively specific blockers of rabbit and human ciliary process beta adrenergic receptors, have implications for the development of ocular hypotensive agents with fewer systemic side effects on tissues enriched in beta-1 adrenergic receptors.     

Psychopharmacological consequences of activation of beta adrenergic receptors by SOM-1122

SOM-1122 was found to be a high-affinity, partial agonist for beta adrenergic receptors. SOM-1122 inhibited the binding of [125I]iodopindolol to membranes prepared from rat cerebral cortex and cerebellum. GTP regulated the binding of SOM-1122 by increasing the Hill coefficient in both tissues and reducing the affinity of the receptor for SOM-1122 in the cerebellum. SOM-1122 increased the concentration of cyclic AMP in slices of rat cerebral cortex in a dose-dependent manner; this effect was antagonized by propranolol. Two lines of evidence suggested that SOM-1122 was centrally active after peripheral administration. First, SOM-1122 inhibited the binding of [125I]iodopindolol in vivo in a dose-dependent manner. Second, after chronic infusion with SOM-1122 for 7 days, the density of beta adrenergic receptors in the cerebellum was reduced; receptor density also was reduced 18 hr after acute administration of SOM-1122, although to a lesser extent. SOM-1122 was found to be behaviorally active. It reduced locomotor activity and reduced response rate under a multiple fixed-interval, fixed-ratio schedule in a dose-dependent manner. SOM-1122 also reduced response rate and increased reinforcement rate under a differential-reinforcement-of-low-rate schedule. These behavioral actions of SOM-1122 appeared to be due to an interaction of the agonist with beta adrenergic receptors, as they were antagonized by propranolol. The behavioral changes produced by stimulation of beta adrenergic receptors with SOM-1122 were generally similar to those caused by other centrally acting beta adrenergic agonists and by antidepressant drugs.     

Effects of ethanol administration and withdrawal on neurotransmitter receptor systems in C57 mice

C57BL/6 mice were treated with 7% (v/v) ethanol in a Bio-Serve liquid diet for 7 days. Some animals were then allowed to withdraw from ethanol for a period of 24 hr. The severity of the ethanol withdrawal was assessed by monitoring behavioral changes and by quantitating the decrease in body temperature that occurred during the first 16 hr of withdrawal. Animals withdrawn from ethanol for 24 hr showed a decreased hypothermic response to apomorphine suggesting that changes in dopaminergic systems had occurred. This possibility was further examined in homogenates of striatum by measuring dopamine-stimulated adenylate cyclase activity and the binding of [3H]spiroperidol. However, there were no changes observed in either basal- or dopamine-stimulated adenylate cyclase activity or in the density or affinity or receptors for [3H]spiroperidol. The affinity of apomorphine for the dopamine receptor was also unchanged. In other experiments, alpha and beta adrenergic receptor-mediated increases in cyclic AMP accumulation were assessed in slices of cerebral cortex. There was no change in cyclic AMP accumulation due to either alpha or beta adrenergic receptors. There was, however, a significant decrease in the density of beta adrenergic receptors in both the ethanol-treated mice and in the withdrawn mice. This decrease was restricted to the beta-2 receptor subtype with no change being observed in the density of beta-1 adrenergic receptors. Ethanol administration was also associated with a significant increase in the density of muscarinic cholinergic receptors in the hippocampus and cerebral cortex. The effect was not observed in animals allowed to withdraw for 24 hr.     

Adrenergic and serotonergic receptor binding in rat brain after chronic desmethylimipramine treatment.

The effects of chronic administration of the tricyclic antidepressant agent desmethylimipramine (DMI) on brain adrenergic and serotonergic receptor binding processes were studied. We examined the kinetic properties of alpha adrenergic, beta adrenergic and serotonergic receptor binding sites in cortical and subcortical brain regions of rats treated chronically for various time periods with DMI(6 mg/kg i.p. daily). After 1 week of daily injections, beta receptor binding density in the cortex was significantly decreased. The reduced density of the cortical beta receptors was evident throughout a 12-week administration period. It was not until after 6 weeks of DMI administration that a significant reduction in the subcortical beta receptors was evident. Compared to saline-injected controls, chronic DMI administration lowered [3H]dihydroalprenolol binding in the hippocampus but not in the striatum. After 12 weeks of DMI we detected no differences in alpha adrenergic binding characteristics in the cortex or subcortical forebrain using [3H]dihydroergocryptine as the binding ligand. There was no consistent alteration in the cortical serotonin receptor densities throughout the 12 weeks of DMI administration, and DMI had no effect on the serotonergic binding characteristics in the subcortical forebrain region. We conclude that chronic DMI administration selectively decreases the density of beta adrenergic receptors in rat brain.     

Denervation-induced changes in alpha and beta adrenergic receptors of the rat submandibular gland.

Denervation resulted in a marked increase in the beta adrenergic response and isoproterenol-stimulated cyclic adenosine 3':5'-monophosphate accumulation of dispersed cells prepared from a rat submandibular gland. This increase in beta adrenergic response was paralleled by an increase in the density of beta adrenergic receptors in membranes prepared from these glands. Denervation also produced a moderate increase in alpha adrenergic receptor density in membranes prepared from whole glands. However, the alpha adrenergic response in cells, epinephrine-induced release of potassium, appeared unaltered by denervation. Furthermore, membranes prepared from denervated dispersed cells did not show the increase in alpha adrenergic receptor density seen in membranes from an intact denervated gland. These data demonstrate that removal of noradrenergic nerve terminals affects alpha and beta adrenergic receptors differently. While it is clear that beta adrenergic membrane receptors participate in denervation-induced supersensitivity, the changes in alpha adrenergic membrane receptors are more complex and may not contribute to the supersensitivity seen after denervation. On the basis of competitive binding studies, the adrenergic receptors in membranes from intact submandibular glands were subclassified as beta-1 and alpha-2. Denervation did not alter the binding characteristics of the alpha-2 receptors in this gland, demonstrating that alpha-2 adrenergic membrane receptors can be postsynaptic in this adrenergically innervated tissue.     

Elevation of beta-adrenergic receptor density in human lymphocytes after propranolol administration.

Abrupt withdrawal after the chronic administration of propranolol has resulted in clinical syndromes that suggest adrenergic hypersensitivity. The effect of propranolol administration and withdrawal on beta-adrenergic receptors was studied in human lymphocyte membranes. Receptor density was quantitated by direct binding assays with the radioligand [125I]iodohydroxybenzylpindolol. Administration of propranolol (160 mg/d) for 8 d resulted in trough plasma levels of approximately 35 ng/ml. By day 5 of propranolol administration the density of beta-adrenergic receptors had increased 43 +/- 4% (P less than 0.01) above pretreatment levels. Abrupt withdrawal of propranolol was followed by the disappearance of propranolol from the plasma within 24 h. The density of beta-adrenergic receptors did not return to pretreatment level for several days. Physiologic supersensitivity of beta-adrenergic receptor-mediated responses was suggested by the appearance of significant increases in the orthostatic change in heart rate (P less than 0.05) and the orthostatic change in the heart rate-systolic blood pressure product (P less than 0.01) during the first 48 h after propranolol withdrawal. These data show that propranolol administration leads to an increase in the density of beta-adrenergic receptors in human tissue. The results are consistent with the hypothesis that some of the untoward effects observed after abrupt discontinuation of propranolol are caused by beta-receptor-mediated adrenergic hypersensitivity.     

Beta adrenoceptor subtype binding activity in plasma and beta blockade by propranolol and beta-1 selective bisoprolol in humans. Evaluation with Schild-plots

In the present study we investigated whether the beta adrenoceptor subtype binding activity in plasma samples can predict selective and nonselective beta blockade in humans. From the right shifts of isoprenaline dose-response curves 0 to 84 hr after administration of propranolol and the beta-1 selective bisoprolol, in vivo beta blockade was assessed. In an in vitro radioreceptor assay with membrane preparations of beta-1 or beta-2 adrenoceptors, plasma samples were assayed for subtype selective blocking activity. After propranolol administration, in vitro beta-1 and beta-2 adrenoceptor occupancy declined from initially 97% to less than 10% within 48 hr. An isoprenaline dose ratio (DR)-1 of 1 coincided with a 50% occupancy of the beta-1 or the beta-2 subtype in vitro. In Schild-plots using plasma concentrations (radioreceptor assay) and the isoprenaline DR-1 for heart rate, diastolic blood pressure and inotropy (QS2C), slopes of unity were observed. After bisoprolol administration, in vitro beta-1 occupancy shifted from initially 95% to less than 10% within 72 hr. For the beta-2 subtype, an occupancy of greater than 10% was detectable only within the first 12 hr. An isoprenaline DR-1 of 1 coincided with a 50% occupancy of beta-1 adrenoceptors. The bisoprolol Schild-plots yielded a slope of unity for inotropy, but less than unity for the heart rate and diastolic blood pressure. From an extended analysis of subtype selective antagonism in Schild-plots, the fractions of the beta-2 adrenoceptor subtype participating in the isoprenaline response were calculated: heart rate 0.45 +/- 0.12 and diastolic blood pressure 0.23 +/- 0.13. It is concluded that in vitro receptor occupancy can predict beta blockade in humans for propranolol. Beta adrenoceptor subtype-mediated effects in humans can be evaluated with a selective antagonist and a refined analysis of Schild-plot data.     

Interaction of beta adrenergic agonists and antagonists with brain beta adrenergic receptors in vivo

There is interest in knowing whether beta adrenergic antagonists or agonists, when administered systemically, can enter the brain to interact with central beta adrenergic receptors. To study this, the reduction in the radioactive content in the brain of rats after administration of (-)-[125I]iodopindolol (IPIN) by systemically administered beta agonists or antagonists was measured. Previous studies show that after the i.v. administration of IPIN the binding in vivo to various areas of the central nervous system has the characteristics expected of binding to beta adrenergic receptors. Of the antagonists tested, pindolol and butylpindolol showed potent interactions with beta receptors in both cortex and cerebellum whereas atenolol and practolol did not interact at doses up to 30 mg/kg. CGP-12177 showed moderate potency in inhibiting IPIN binding in vivo. We have shown previously that propranolol and alprenolol inhibit IPIN binding with high potency in cortex and cerebellum. At high doses, butoxamine, a beta-2 antagonist, reduced the binding of IPIN in the cerebellum but not in the cortex. Of the agonists tested, clenbuterol and prenalterol caused a significant dose-dependent reduction of the binding of IPIN, with clenbuterol being more potent. Isoproterenol, salbutamol, salmefamol and dobutamine had no effect. With the exception of CGP-12177, the affinity of the drugs for central beta adrenergic receptors measured in vitro was correlated significantly with their ability to inhibit IPIN binding in vivo whereas their degree of lipophilicity was not correlated significantly with potency in vivo. The inhibition of IPIN binding in vivo from brain areas can be used to evaluate whether drugs penetrate into brain and interact with central beta adrenergic receptors.     

Quantitative analysis of the selectivity of radioligands for subtypes of beta adrenergic receptors

In tissues with two classes of binding sites for a drug, it is common to estimate the proportion of each class of binding site by inhibiting the binding of a radioligand with a selective unlabeled ligand. Accurate estimates of the density of each class of binding site, however, will be obtained only if the radioligand is nonselective or used at a concentration that saturates both classes of binding sites. A method of simultaneous regression analysis of multiple inhibition curves, using the program MLAB on the PROPHET system, was used to quantify the selectivity of radioligands for beta-1 or beta-2 adrenergic receptors. The selectivity of [125I]iodopindolol, [125I]iodocyanopindolol, [125I]iodohydroxybenzylpindolol and [3H]dihydroalprenolol for beta-1 and beta-2 adrenergic receptors was assessed by inhibiting the binding of each radioligand with the beta-1-selective unlabeled ligand ICI 89,406 at increasing concentrations of the radioligand, using membranes prepared from C6 glioma cells, which have both beta-1 and beta-2 adrenergic receptors. Scatchard plots for all four radioligands were linear, with correlation coefficients greater than 0.95. [125I]Iodopindolol and [125I]iodocyanopindolol were 3.2- and 2-fold selective, respectively, and [125I]iodohydroxybenzylpindolol and [3H]dihydroalprenolol were 5.8- and 2.3-fold selective, respectively, for beta-2 adrenergic receptors. Values obtained for the densities of beta-1 and beta-2 adrenergic receptors and the affinities of the receptors for ICI 89,406 were independent of the radioligand used.(ABSTRACT TRUNCATED AT 250 WORDS)     

Species differences in the localization and number of CNS beta adrenergic receptors: rat versus guinea pig

The localization and number of beta adrenergic receptors were directly compared in the brains of rats and guinea pigs. The time course of association and saturability of [125I]cyanopindolol (CYP) binding to slide-mounted tissue sections was similar in rats (Kd = 17 pM) and guinea pigs (Kd = 20 pM). The beta-1 and beta-2 receptor subtypes were examined through the use of highly selective unlabeled receptor antagonists, ICI 118,551 (50 nM) and ICI 89,406 (70 nM). Dramatic species differences between rats and guinea pigs were observed in the neuroanatomical regional localization of the beta adrenergic receptor subtypes. For example, in the thalamus prominent beta-1 and beta-2 receptor populations were identified in the rat; however, the entire thalamus of the guinea pig had few, if any, beta adrenergic receptors of either subtype. Hippocampal area CA1 had high levels of beta-2 adrenergic receptors in both rats and guinea pigs but was accompanied by a widespread distribution of beta-2 adrenergic receptors only in rats. Quantitative autoradiographic analyses of 25 selected neuroanatomical regions 1) confirmed the qualitative differences in CNS beta adrenergic receptor localization, 2) determined that guinea pigs had significantly lower levels of beta adrenergic receptors than rats and 3) indicated a differential pattern of receptor subtypes between the two species. Knowledge of species differences in receptor patterns may be useful in designing effective experiments as well as in exploring the relationships between receptor and innervation patterns. Collectively, these data suggest caution be used in extrapolation of the relationships of neurotransmitters and receptors from studies of a single species.     

Beta adrenergic receptor subtypes in the atria: evidence for close coupling of beta-1 and beta-2 adrenergic receptors to adenylate cyclase

The relationship between occupancy of beta adrenergic receptors and stimulation of adenylate cyclase in dog atrial tissue was examined by studying the binding of [125I]iodopindolol and the activation of adenylate cyclase. Computer-assisted nonlinear regression analysis was used to analyze the inhibition of isoproterenol-stimulated adenylate cyclase activity by beta-1- or beta-2-selective antagonists. The Ki values for each subtype of receptor for the selective antagonists resulting from studies of the inhibition of adenylate cyclase activity were similar to those determined in studies of the inhibition of the binding of [125I]iodopindolol. To compare further the occupancy of beta-1 or beta-2 adrenergic receptors with the activation of adenylate cyclase mediated by each class of receptor, computer modeling of the stimulation of adenylate cyclase by the beta-1-selective agonist norepinephrine was carried out. The EC50 values of norepinephrine for each receptor subtype, as measured in studies of norepinephrine-stimulated adenylate cyclase activity, were similar to the Ki values for the inhibition by norepinephrine of the binding of [125I]iodopindolol to each receptor subtype. The data led to the conclusion that beta-1 adrenergic receptors make up about 70% of the total number of beta adrenergic receptors and mediate 70% of the increase in adenylate cyclase activity produced by isoproterenol. These results suggest that the relationship between occupancy of each class of receptor and activation of adenylate cyclase is linear and that, when agonist-stimulated adenylate cyclase activity is used as a functional response, neither spare beta-1 nor spare beta-2 adrenergic receptors exist in the atrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of adrenergic receptor binding in rat lung: physiological regulation

The ontogeny, circadian rhythm and sex differences of adrenergic receptors were studied in a crude particulate fraction prepared from rat lung. The density of alpha-2 adrenergic receptors ([3H]yohimbine binding sites) decreased rapidly with age from 304 fmol/mg of protein on the 1st day to 123 fmol/mg after the 1st week and then to an undetectable level at 5 weeks of age. Alpha-1 adrenergic receptors ([3H]prazosin binding) showed a moderate variation in density, with the greatest density at the 3rd week of age (311 fmol/mg). The density at birth (194 fmol/mg) was higher than the adult (126 fmol/mg). Beta adrenergic receptors ([3H]dihydroalprenolol binding) demonstrated changes in density with a sharp rise in the 3rd week (to 813 fmol/mg from 315 fmol/mg at birth), which was maintained through the 10th week. Unlike receptor densities, the affinity constants were not significantly altered during postnatal development. Alpha-1 and beta receptors in the adult rat lung did not show any significant changes in their densities or their affinities in the two other physiological variables studied (circadian rhythm and sex). During postnatal development, the highest concentration of norepinephrine in the rat lung is reported to be at about 3 weeks of age, which correlates with the changes in both alpha-1 and beta adrenergic receptors, but not alpha-2 receptors. The presence of alpha-2 receptors in the lungs of young, but not mature animals, may suggest an important developmental role, which is not yet understood.     

A beta adrenoceptor with atypical characteristics is involved in the relaxation of the rat small intestine

In several studies in guinea pig ileum or rat colon a beta adrenoceptor with characteristics distinct from beta-1 or beta-2 receptors has been observed and has been denoted as "atypical" beta adrenoceptor. In this study the relaxation of the rat small intestine was investigated, using isolated segments of the rat jejunum. Several beta-1 or beta-2 agonists and antagonists were tested on the rat jejunum preparation, and it was found that nonselective and selective antagonists for beta-1 or beta-2 receptors showed a relatively low affinity, compared to their affinity for beta-1 or beta-2 receptors. BRL 37344, an agonist which has been reported to be selective for the atypical beta adrenoceptor, was more potent although a partial agonist compared to isoprenaline, whereas it is clearly less active than isoprenaline on beta-1 or beta-2 receptors. These findings indicate that beta adrenergic relaxation of the rat small intestine is mediated via atypical beta adrenoceptors. Efforts were made to confirm these findings with binding studies on small intestinal 45,000-g membranes. Competition radioligand binding experiments were performed with the radiolabeled ligand [125I]iodocyanopindolol and the various antagonists which were also tested in the intact rat jejunum preparations. Receptor binding experiments only revealed beta adrenoceptors of the beta-2-subtype, which does not correspond with the results obtained in the jejunum relaxation. Probably the beta-2 receptors found in the binding studies are located on circular smooth muscle cells or on epithelial cells, whereas longitudinal smooth relaxation is mediated by atypical beta adrenoceptors. Atypical beta adrenoceptors were not measured in binding studies probably because [125I]iodocyanopindolol is an unsuitable ligand to label atypical intestinal beta adrenoceptors.     

Regulation of alpha-1 adrenergic receptor density and functional responsiveness in rat brain

The effects of changes in norepinephrine availability on alpha-1 adrenergic receptor density and responsiveness were determined in several regions of rat brain. Receptor density was determined from specific 125I-BE 2254 binding, and responsiveness was determined by norepinephrine-stimulated increases in inositol phosphate accumulation and in cyclic AMP accumulation in the absence or presence of adenosine in slices where beta adrenergic receptors had been inactivated. Adrenergic input was reduced by destroying noradrenergic neurons with DSP-4, depleting amines with chronic reserpine treatment or blocking alpha-1 adrenergic receptors with chronic prazosin treatment. All three treatments caused similar increases in alpha-1 adrenergic receptor density in cerebral cortex but not in striatum or hippocampus. DSP-4 treatment increased the maximal cyclic AMP response to both norepinephrine and phenylephrine but did not alter the maximal inositol phosphate response or the maximal potentiation of the cyclic AMP response to adenosine. DSP-4 treatment also increased the potency of norepinephrine in activating all three responses in cerebral cortex. Adrenergic input was increased by chronic treatment with desmethylimipramine to block norepinephrine reuptake, chronic i.c.v. infusion of norepinephrine or chronic yohimbine treatment to block presynaptic autoreceptors. None of these treatments caused significant changes in alpha-1 adrenergic receptor density or functional responsiveness in any region studied. The results suggest that alpha-1 adrenergic receptor density and responsiveness in rat brain can be increased but not easily decreased. They also suggest that different brain regions are affected differently by alterations of adrenergic input and that the different responses are not coregulated.