In vitro interactions of agonists and antagonists with beta-adrenergic receptors

Forskolin reverses tachyphylaxis to the bronchodilator effects of salbutamol: an in-vitro study on isolated guinea-pig trachea

The objective of this study was to assess the relaxant responses of salbutamol, a beta2 agonist, and forskolin, an activator of adenylate cyclase, and the possible role of forskolin in reversing tachyphylaxis to salbutamol. The in-vitro bronchodilator effects of salbutamol and forskolin (10(-9)-10(-5) M) were tested on isolated guinea-pig tracheal rings precontracted with carbachol (10(-7) M). Both salbutamol and forskolin elicited concentration-dependent relaxation. Potency (EC50; the dose resulting in 50% relaxation) was determined from cumulative concentration-response curves. Salbutamol was more potent than forskolin in relaxing the tracheal preparations (-log molar EC50 7.68+/-0.14 and 6.3+/-0.17, respectively). Reproducible relaxant responses to salbutamol could be elicited after 24 h incubation in Krebs solution. Tachyphylaxis to the relaxant effects of salbutamol was experimentally induced by incubation (24h) of the preparations in Krebs solution containing salbutamol (10(-6) 3x10(-6) or 10(-5) M). This pretreatment of the tissues resulted in a significant reduction in the potency of salbutamol. The potency of salbutamol was reduced to 6.85+/-0.2, 6.8+/-0.1 and 5.9+/-0.27 after 24h incubation with salbutamol 10(-6), 3x10(-6) or 10(-5) M, respectively. The potency of salbutamol was increased from 7.35+/-0.2 to 7.76+/-0.28 by addition of forskolin (3x10(-7) M) under control conditions. Moreover, forskolin (3x10(-7) M) reversed the development of tachyphylaxis to salbutamol-induced relaxation in tissues pretreated with salbutamol. The potency of salbutamol was increased to 7.29+/-0.41, 7.37+/-0.17 and 7.23+/-0.35 after the addition of forskolin (3x10(-7) M) to preparations pre-incubated (24h) with salbutamol 10(-6), 3x10(-6) or 10(-5) M respectively. These results show that in guinea-pig tracheal ring preparations, forskolin shares with salbutamol the ability to relax airway smooth muscle and produces an apparent reversal of tachyphylaxis to the bronchodilator effects of salbutamol, particularly in the low concentration range. This effect could provide an alternative therapy for long term use, particularly with high doses of beta2 agonists in bronchial asthma.

Thermodynamic analysis of isoproterenol binding to beta-adrenoceptors in rat lung membranes

The thermodynamic properties of the binding of the beta-adrenoceptor agonist isoproterenol and of the antagonist propranolol to beta-adrenoceptors of rat lung were investigated. We found that in our experimental conditions, the high- and low-affinity binding sites for the agonist displayed different properties: the binding to the high-affinity binding site was entropy-driven with a small increase in enthalpy, while agonist binding to the low-affinity binding site was enthalpy-driven. Binding of isoproterenol in the presence of GTP or its non-hydrolyzable analogue GppNHp, and the binding of propranolol were enthalpy-driven with a small increase in entropy.

Modulation by isoproterenol and propranolol of somatostatin receptors in synaptosomes from rat frontoparietal cortex

DL-Propranolol (PRO), a beta-adrenergic blocking agent, and the neuropeptide somatostatin (SS) have central nervous system depressant and anticonvulsive properties. To investigate a possible relationship between these two components, we studied the influence of PRO and DL-isoproterenol (ISO), a beta-adrenergic agonist, on the somatostatinergic system in the rat frontoparietal cortex. The short- (5 h) and long-term (14 days) administration of ISO (5 mg/kg, intraperitoneally (i.p.)), or of PRO (10 mg/kg, i.p.) did not affect somatostatin-like immunoreactivity (SLI) content in the frontoparietal cortex of male Wistar rats. Both short- and long-term ISO administration decreased the number of specific [125I]Tyr11-SS receptors in synaptosomes from frontoparietal cortex (31%, P < 0.05, and 26%, P < 0.02, after short- and long-term administration, respectively) without changing the affinity constant. This decrease in the number of [125I]Tyr11-SS receptors was not due to a direct effect of ISO on these receptors since no decrease in binding was produced by high concentrations of ISO (10(-5) M) when added in vitro. This decrease could be blocked by pretreatment with PRO. Short- and long-term administration of PRO alone produced an increase in the [125I]Tyr11-SS binding in frontoparietal cortex (26%, P < 0.02, and 40%, P < 0.001, after short- or long-term administration, respectively) without changing the affinity constant.

Investigations into factors determining the duration of action of the beta 2-adrenoceptor agonist, salmeterol

1. This study has explored the mechanism underlying the long duration of action of the beta 2-adrenoceptor agonist, salmeterol. 2. Salmeterol, salbutamol and isoprenaline caused a concentration-related inhibition of electrically-induced contractile responses of the guinea-pig superfused trachea preparation. The effects of both isoprenaline and salbutamol were rapid in onset and rapidly reversed upon removal of the agonist. In contrast, the effects of salmeterol were slower in onset and could not be reversed by superfusion of the tissue with agonist-free Krebs solution even for periods of up to 10 h. 3. The effects of salmeterol were, however, readily reversed by a number of beta-adrenoceptor blocking drugs, as was the effect of a continuous infusion of isoprenaline. Upon removal of the antagonist, however, the effects of salmeterol and of the isoprenaline infusion were reasserted at a rate which was inversely related to the lipophilicity of a beta-adrenoceptor blocking drugs. 4. Salmeterol inhibited the binding of [125I]-(-)-iodopindolol (100 pM) to rat lung membranes (pIC50 7.1), with isoprenaline (pIC50 6.2) and salbutamol (pIC50 5.1) having lower potencies. The inhibition of binding by salmeterol was apparently non-competitive, whereas that produced by salbutamol and isoprenaline was competitive in nature. 5. Isoprenaline and salbutamol rapidly dissociated from their binding sites, whereas in marked contrast, the binding of salmeterol showed no dissociation for periods of up to 1 h. 6. These data are consistent with the mechanism in which salmeterol binds adjacent to the active site of the beta 2-adrenoceptor, such that the drug cannot be washed out of the tissue, yet can interact with and activate the receptor. This latter property is susceptible to antagonism by beta-adrenoceptor blocking drugs but is reassured when the antagonists are removed.

Binding of beta-adrenergic receptors in human skin

Radioligand-binding experiments were performed with crude membrane homogenates (CMH) from human skin in order to investigate the epidermal beta-adrenergic receptor (beta AR) density. CMH were prepared from leftovers of split-thickness skin grafts by sequential homogenization and centrifugation procedures to yield essentially epidermal fragments. Saturation experiments with the non-selective beta-adrenoceptor antagonist (-)-[125I]-iodocyanopindolol (ICYP) as radioligand showed saturable specific binding isotherms. Scatchard transformation of the data demonstrated high-affinity binding of ICYP to a single class of beta AR (Bmax = 80 +/- 10 fmol/mg protein; KD = 8 pM +/- 0.9; n = 8). beta AR antagonists displaced ICYP in a monophasic displacement pattern. The IC50 values were (nmol/1) propranolol (non-selective) 24.8; ICI 118,551 (beta 2 selective) 14.7; CGP-12177 (non-selective) 28.9; bisoprolol (beta 1 selective) 1500; CGP-20712A (beta 1 selective) 8990. beta AR agonists displaced ICYP with a potency ranking isoprenaline greater than adrenaline greater than noradrenaline. We conclude that epidermal crude membrane homogenates prepared from human split-thickness skin contain a high population of beta 2-adrenergic receptors. These receptors may be studied to further investigate the nature of human epidermal beta-adrenergic mechanisms.

Metabolic and cardiovascular benefits deriving from beta-adrenergic blockade in chronic congestive heart failure

Ten patients with congestive heart failure were given metoprolol (50 mg/day) or placebo during a double-blind, crossover, randomized study. After a run-in period of 4 weeks, metoprolol and placebo were administered over a period of 3 months, which was separated by a washout period of 4 weeks. At the end of the run-in, metoprolol, and placebo periods, all patients underwent metabolic (oral glucose tolerance and hyperinsulinemic glucose clamp tests) and noninvasive cardiologic (New York Heart Association classification, bimodal echocardiographic left ventricular end-diastolic determination, maximal oxygen consumption, left ventricular radionuclide ejection fraction) tests. Our results show that beta-adrenergic blockade significantly enhances insulin-mediated suppression of hepatic glucose output (p less than 0.005) and increase in glucose uptake (p less than 0.01) with a concurrent improvement in New York Heart Association functional class (p less than 0.05) and the multistage exercise treadmill test score (p less than 0.05). After administration of metoprolol all changes in glucose turnover parameters were found to correlate with the decrease in basal plasma free fatty acid levels. In conclusion, our findings confirm the beneficial cardiologic effects of beta-adrenergic blockade in congestive heart failure and demonstrate that metoprolol is also useful for reversing the metabolic damage caused by exaggerated plasma norepinephrine levels.

The specific binding of broxaterol, a new beta 2-selective agonist, to beta-adrenoceptors

The specific binding of broxaterol, a potent new orally active antiasthmatic drug, to beta 1- and beta 2-adrenoceptors was characterized by receptor binding studies with rat heart and lung membrane preparations. Broxaterol showed high affinity and selectivity for the beta 2-component of [3H]dihydroalprenolol binding in both lung (58% beta 2-sites, Ki = 130 nM) and heart membranes (19% beta 2-sites, Ki = 98 nM), whereas the binding to the beta 1-component was at lower affinity (42% beta 1-sites, Ki = 4100 nM in the lung and 81% beta 1-sites, Ki = 3460 mM in the heart). The influence of temperature changes on the binding properties of broxaterol towards beta-adrenoceptors was also investigated. A marked increase in the affinity of broxaterol for lung beta-receptors was observed on lowering the assay temperature, whereas the affinity for heart beta-receptors was little affected by temperature changes. Thermodynamic analysis of the binding data showed that the binding of broxaterol as well as isoproterenol to lung beta-receptors was associated with a large decrease in enthalpy, which correlates well with the full agonistic properties of this compound at beta 2-receptors.

Roles of Ca2+ in human neutrophil responses to receptor agonists

Previous studies have concluded that cytosolic Ca2+ [( Ca2+]i) transients are essential for neutrophils (PMN) to degranulate and make superoxide anion when challenged with the receptor agonists N-formyl-methionyl-leucyl-phenylalanine, platelet-activating factor and leukotriene B4. This view is based on the profound unresponsiveness of PMN that have their [Ca2+]i fixed at resting levels by removing storage Ca2+ and loading the cells with greater than or equal to 20 microM of a Ca2+ chelator, quin2 AM. We too observed this unresponsive state in PMN loaded with 10-32 microM-quin2 AM, fura-2 AM or 1,2-bis-(2-aminophenoxy) ethane-NNN'N'-tetra-acetic acid (BAPTA). When loaded with less than or equal to 1 microM fura-2 AM, however, Ca(2+)-depleted PMN failed to alter [Ca2+]i appreciably, yet still had substantial degranulation and superoxide-anion-generating responses to the receptor agonists. Function thus did not require [Ca2+]i transients. Moreover, Ca(2+)-depleted PMN had 20-35% decreases in receptor numbers for each of the three agonists, and chelator loading of these cells decreased receptor availability by 30-50%. All receptor losses were reversed by incubating PMN with Ca2+ at 37 degrees C, but not at 4 degrees C, and agonist binding at 4 degrees C was not influenced by the presence or absence of extracellular Ca2+. Ca2+ thus caused PMN to up-regulate their agonist receptors at 37 degrees C, and the effect persisted at 4 degrees C regardless of ambient Ca2+. We conclude that Ca2+ acts in at least three ways to regulate responses to receptor agonists. First, some pool of (probably cellular) Ca2+ maintains receptor expression. Second, [Ca2+]i transients potentiate, but are not required for, function. The [Ca2+]i pool may or may not be the same as that influencing receptors. Finally, another pool(s) of Ca2+ signals or permits responses. This last pool, rather than [Ca2+]i transients, appears essential for the bioactions of standard Ca(2+)-mobilizing stimuli.

Biological activity of parathyroid hormone antagonists substituted at position 13

Lysine occupies position 13 in the parathyroid hormone (PTH) antagonist, [Nle8,18,Tyr34]bPTH(7-34)NH2. Acylation of the epsilon-amino group in lysine 13 by a hydrophobic moiety is well tolerated in terms of bioactivity: the analog [Nle8,18, D-Trp12,Lys 13 (epsilon-3-phenylpropanoyl),Tyr34]bPTH(7-34)NH2 is equivalent to the parent peptide in its affinity for PTH receptors and its ability to inhibit PTH-stimulated adenylate cyclase in both kidney- and bone-based assays. Truncation of this peptide by deletion of phenylalanyl7 with concomitant removal of the amino-terminal alpha-amino group yielded the analog desamino[Nle8,18,D-Trp12,Lys13 (epsilon-3-phenylpropanoyl),Tyr34]bPTH(8-34)NH2, an antagonist of high potency in vitro (Kb = 4 and 9 nM, Ki = 73 and 3.5 nM in kidney- and bone-based assays, respectively). Also this analog is potentially stable to aminopeptidases present in many biological systems.

Effects of hydrophobic substitutions at position 18 on the potency of parathyroid hormone antagonists

Position 18 in a parathyroid hormone (PTH) antagonist, [Nle8,18,Tyr34]bPTH(7-34)NH2 (ii), was shown to tolerate substitutions by a range of amino acids with retention of inhibitory activity. The effects of hydrophobic substitutions at this position as a means of enhancing binding interactions with the receptor were evaluated. Substitution of Nle at position 18 with either D-Ala, D-Trp, or L-Trp in analog ii or with Trp (D or L) in the recently reported, highly potent antagonist, [Nle8,18,D-Trp12,Tyr34]bPTH(7-34)NH2 (in vitro activities; Kb = 15 nM and Ki = 125 nM), was performed. In terms of activity on renal receptors, one antagonist, [Nle8,D-Trp12,18,Tyr34]bPTH(7-34)NH2, is the most active in vitro PTH antagonist yet reported (Kb = 4 nM; Ki = 30 nM). The rationale for design of this antagonist and the conclusions regarding PTH-receptor interactions are discussed.

Histamine H3 receptor binding sites in rat brain membranes: modulations by guanine nucleotides and divalent cations

The binding of [3H](R)alpha-methylhistamine, a potent and specific agonist at histamine H3 receptors, was investigated with membranes of rat cerebral cortex. In phosphate buffer the specific binding defined with thioperamide, an H3 receptor antagonist, displayed characters of reversibility and saturability with a Bmax of approximately 30 fmol/mg protein. The KD, derived from either dissociation/association rates or saturation kinetics at equilibrium, was approximately 0.5 nM at 25 degrees C. Competition studies indicated that the binding occurred with a stereoselectivity and pharmacological specificity similar to that of functional H3 autoreceptors regulating histamine release in brain slices. However, whereas the potency of antagonists was closely similar in the two assay systems, that of agonists was approximately 10-fold higher in the binding assay. Among antagonists burimamide was the only one to compete with a pseudo-Hill coefficient significantly different from unity (nH = 0.48 +/- 0.03), indicating a possible heterogeneity among binding sites. The presence of 2.6 mM Ca2+, in a modified Krebs-Ringer medium, promoted the conversion of a larger fraction of sites into a low-affinity component with a KD of 16 nM. The presence of guanylnucleotides in the Krebs-Ringer medium with Ca2+ abolished the binding to this low-affinity component whereas in a phosphate buffer only the KD was slightly increased. It is concluded that the H3 receptor, like many other amine receptors, is coupled to its still unidentified effector system via a G-protein and regulated by Ca2+. However, unlike the latter, the H3 receptor is down-regulated by the divalent cation.

Stereoselectivity in beta-adrenomimetic and beta-adrenolytic actions of carteolol, a beta-adrenoceptor blocker with intrinsic sympathomimetic action in guinea-pig taenia caecum

1. The beta-adrenomimetic and beta-adrenolytic activities of S(-) and R(+) isomers of carteolol, a beta-adrenergic partial agonist (a beta-adrenoceptor blocker with intrinsic sympathomimetic action) were tested in the guinea-pig taenia caecum. 2. The beta-adrenoceptor blocking activities (pA2 values) of S(-) and R(+) isomers of carteolol were significantly larger than the corresponding beta-adrenomimetic activities (pD2 values), supporting our views that beta-adrenoceptors contain two different binding sites; high and low affinity sites. 3. In beta-adrenoceptor blocking action S(-) carteolol was about 10 times as potent as R(+) carteolol while beta-adrenomimetic action of S(-) carteolol was about 2 times as potent as that of R(+) carteolol. Further, intrinsic activity for S(-) carteolol was slightly but significantly larger than that for R(+) carteolol. 4. These results suggest that the binding site for competitive antagonism between S(-) isoprenaline and S(-), R(+) and RS(+/-) carteolol is more stereoselective than the binding site to induce beta-adrenomimetic action.

GTP-independent stimulation of rabbit heart adenylate cyclase by isoproterenol at physiological ATP concentrations

Isoproterenol increased the activity of the adenylate cyclase of rabbit heart sarcolemmal membranes in the absence of added GTP. ATP, the ATP-regenerating system, and the sarcolemmal membrane preparation were eliminated as possible sources of contaminating GTP. Isoproterenol-stimulation increased as ATP was raised. At 0.5 mM ATP, isoproterenol increased activity by 19% whereas at 5 mM ATP isoproterenol increased activity by 121%. There was no change in basal activity between 0.5 and 5 mM ATP. Stimulation by Gpp(NH)p and NaF increased slightly between 0.5 and 5 mM ATP; stimulation by KCl was unaffected. GTP does not activate cyclase d. GTP does not activate cyclase to the same extent as Gpp(NH)p even though the two act at the same site on Ns (the stimulatory guanine nucleotide-binding protein). GTP decreased cyclase activation by Gpp(NH)p in a concentration-dependent fashion when the two were added to the assay simultaneously. Increasing ATP from 0.5 to 5 mM did not reduce activation by Gpp(NH)p when both were added simultaneously to the assay. This suggests that ATP does not interact with the same site as Gpp(NH)p. ATP gamma S, an analogue of ATP which irreversibly thiophosphorylates proteins, did not irreversibly support activation by isoproterenol. The effect of ATP in supporting isoproterenol stimulation is not, therefore, thought to be due to phosphorylation of a protein.

Irreversible binding and recovery of the norepinephrine uptake system using an alkylating derivative of norepinephrine

The effects of bromoacetylaminomenthylnorepinephrine (BAAN) on the sodium-dependent, high-affinity norepinephrine (NE) uptake system in rat brain synaptosomes and CNS neuronal cultures were investigated. BAAN inhibited [3H]NE uptake into synaptosomes in a dose- and time-dependent manner (IC50, 6.5 microM). Pretreatment of cortical synaptosomes or neuronal cells with BAAN alone, followed by washing to remove free drug, reduced the Vmax but did not alter the Km value for [3H]NE uptake. The BAAN-induced reduction in Vmax was attenuated by concurrent pretreatment with desipramine and blocked by the reaction of BAAN with dithiothreitol or cysteine. In contrast, BAAN was 19-fold less potent at inhibiting [3H]dopamine uptake in striatal synaptosomes, and no change in the Vmax or Km value for [3H]dopamine uptake was observed after a pretreatment with BAAN followed by washing. Furthermore, the irreversible beta-antagonist, bromoacetylalprenololmentane, was equipotent to BAAN for inhibiting [3H]NE uptake into cortical synaptosomes, but did not alter the Vmax or Km for [3H]NE after pretreatment. In neuronal cultures, BAAN inhibited sodium-dependent uptake of [3H]NE (IC50, 5.6 microM) with no effect on sodium-independent uptake. After pretreatment of cultures with 30 microM BAAN followed by washing, there was a 74% decrease in the Vmax for [3H]NE uptake. Following a 24-h lag period, uptake recovered to the control level within 48 h; however, recovery was completely blocked by cycloheximide. The data indicate that BAAN irreversibly binds to the [3H]NE uptake system in both CNS synaptosomes and neuronal cultures and may be a useful probe for studying the turnover of the [3H]NE uptake system.

Lipolysis and beta-adrenergic receptor binding on adipocytes of spontaneously hypertensive rats

Adipocytes from spontaneously hypertensive rats demonstrated a blunted lipolytic response to isoproterenol and dibutyryl cyclic AMP. (-)-[3H]Dihydroalprenolol binding was examined in adipocytes from normotensive and spontaneously hypertensive rats. Increasing concentrations of isoproterenol decreased total (-)-[3H]dihydroalprenolol binding to intact cells from normotensive rats, and the efficacy of competition was decreased in adipocytes from spontaneously hypertensive rats. Scatchard analysis indicated that the number of (-)-[3H]dihydroalprenolol binding sites and the affinity of dihydroalprenolol binding were comparable between normotensive and spontaneously hypertensive rats. Isoproterenol- and Gpp(NH)p-stimulated adenylate cyclase activity was consistently depressed in adipocyte membranes from spontaneously hypertensive rats as compared to normotensive rats. No difference in fluoride-stimulated adenylate cyclase activity was observed. The blunted lipolytic and cyclic AMP response to isoproterenol in these cells suggest a postreceptor lesion of the lipolytic pathway (possibly the guanine nucleotide regulatory protein) in adipocytes from spontaneously hypertensive rats. The blunted lipolytic response to dibutyryl cyclic AMP suggests defective regulation of lipolytic enzymes at the protein kinase-hormone-sensitive lipase level.

Two beta-adrenergic pharmacophores on the same molecule. A set of agonist-antagonist combinations

A series of compounds containing combinations of one or two pharmacophores of the agonist type (isoproterenol) or the antagonist type (propranolol or alprenolol) on the same molecule were prepared. The pharmacophores were connected by a derivative of polyethylene glycol with an average length of six atoms (carbon and oxygen). Furthermore, compounds containing two alprenolol residues, separated by chains of average lengths of 70 or 145 atoms, were synthesized. The abilities of these compounds to interact with beta-adrenoceptors of rat heart and lung tissues were examined by measuring the following parameters: competitive binding with [3H]dihydroalprenolol, activation of adenylate cyclase, and inhibition of isoproterenol-stimulated adenylate cyclase. The affinity of the compound with two isoproterenol pharmacophores for receptor was about the same as that with one isoproterenol pharmacophore and between 30 and 200 times weaker than that of (+/-)isoproterenol. Both mono- and bis-pharmacophore compounds partially stimulated catecholamine sensitive adenylate cyclase and at high concentrations inhibited the stimulation produced by (-)isoproterenol. The affinity of the compound with antagonist (propranolol) and agonist (isoproterenol) pharmacophores on the same molecule was intermediate between that of propranolol and isoproterenol. The compound was only able to inhibit adenylate cyclase activity. Compounds containing two antagonist (alprenolol) pharmacophores bound to receptors with affinities from an order of magnitude lower to about equal to that of the compound containing one pharmacophore. When membranes were preincubated with compounds containing two antagonist pharmacophores and then washed extensively, there were persistent effects of all of these compounds on the binding constants of [3H]dihydroalprenolol. All of these compounds were only able to inhibit adenylate cyclase activity and none exhibited any subtype selectivity at beta-adrenoceptors. The results suggest that, in the beta-adrenergic system, compounds with agonist and antagonist substituents on the same molecule exhibit properties of the substituent with the higher affinity for beta-adrenoceptor, and no agonist activity is evident when two antagonist pharmacophores are linked on the same molecule. All of the above results may be explained without recourse to cross-linking of beta-adrenoceptors with two pharmacophores, a phenomenon cited in similar studies of receptors for opiates and gonadotropin-releasing hormone.

Agonist-induced isomerization of the alpha 1-adrenergic receptor: kinetic analysis using broken-cell and solubilized preparations

The affinity of agonists but not antagonists at hepatic membrane alpha 1-adrenergic receptors is temperature dependent; a 100-fold higher affinity is observed at 4 degrees C than at 37 degrees C. The relationship between these two agonist affinity states was investigated by using a strategy that allows the kinetics of this transition to be examined under equilibrium conditions. When competition assays are performed at 37 degrees C for varying intervals and the reaction mixture is then rapidly cooled by freezing, allowed to thaw, and further equilibrated at 4 degrees C, a rapid and progressive decrease (t1/2 of 1-2 min) in agonist affinity occurs, the extent of which is directly related to the incubation time at 37 degrees C. This decrease in agonist affinity is sustained as long as agonist is present but can be reversed by its subsequent removal. In contrast, no change in affinity is seen in identical experiments when antagonists are employed as the competing ligand. High-affinity binding of agonists is also demonstrated in short-term nonequilibrium experiments, indicating that the low-temperature incubations do not induce, but rather stabilize, a receptor conformation of high affinity for agonists. These findings suggest that the predominantly low-affinity binding of agonists to alpha 1-adrenergic receptors demonstrated in equilibrium studies at physiological temperatures may be the result of a ligand-driven decrease in affinity. Since the transition in receptor affinity for agonists occurs not only in broken-cell preparations but also after detergent solubilization of the membrane receptor, it most likely is due to an agonist-induced change in the conformation of the receptor protein per se.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of membrane histamine H1-receptor binding sites by guanine nucleotides, mono- and divalent cations

Agonist interaction with histamine H1-receptor in [3H] mepyramine bovine aortic membranes labeled with [3H] mepyramine is selectively regulated by cations and guanine nucleotides. GTP and his nonhydrolisable analog Gpp(NH)p' markedly decrease histamine affinity for [3H] mepyramine binding sites. The effect of GTP is reversed in the presence of divalent cation, magnesium. Calcium and sodium ions have little effect on histamine binding whereas magnesium ions decrease the affinity of histamine for the radioantagonist binding sites about tenfold. GTP has little effect on [3H] mepyramine binding and the interaction of H1-antagonist triprolidine with histamine H1-receptors. The above results indicate that the effect of guanine nucleotides, mono and divalent cations involves the effect on membrane signal transducing mechanism probably GTP-binding protein(s) cation regulatory site(s) rather than receptor binding site directly.

Influence of assay buffer on dissociation constants of drugs at beta-adrenoceptor subtypes

The present investigation has assessed the influence of assay buffer (Tris versus Krebs) on the abilities of several beta-adrenoceptor agonists and antagonists to displace the radioligand (-)-[125I]iodocyanopindolol [( 125I]CYP) from beta 1-(left atrial) and beta 2- (uterine) adrenoceptor sites. Saturation studies indicated that the dissociation constant (KD) for [125I]CYP at both beta-receptor sites was approximately 2-fold greater in Krebs as opposed to Tris buffer, and that the maximal density of binding sites (Bmax) in atria (but not uterus) was also reduced 2-fold. In general, the KD values for beta-adrenoceptor agonists were more influenced by the type of buffer used than were KD values for antagonists. Agonist KD values at beta 2-adrenoceptor sites were higher in Krebs than in Tris buffer, while at beta 1-adrenoceptor sites, variable changes resulted. The selective affinity of agonists at beta 1- and beta 2-adrenoceptor sites was therefore markedly influenced by the buffer used and could not be predicted from the established beta 1-/beta 2-adrenoceptor selectivity of the agonists as found in organ bath studies.

Comparison of the effects of forskolin and isoprenaline on tracheal, cardiac and skeletal muscles from guinea-pig

Forskolin, a potent activator of adenylate cyclase, and isoprenaline, an unselective beta-adrenoceptor agonist, were studied in vitro on tissues from guinea-pig with respect to relaxation of the carbachol-contracted trachea, increase in the force of contraction of the papillary muscle and depression of subtetanic contractions of the soleus muscle, three well-characterized beta-adrenoceptor-mediated effects. Forskolin and isoprenaline relaxed the trachea and increased the force of contraction of the papillary muscle. Isoprenaline but not forskolin caused a depression of the subtetanic contraction of the soleus muscle. Forskolin did not seem to potentiate the effects of isoprenaline on the tissues studied; the combined effects appeared to be a mere addition. Forskolin did not increase the efficacy of the partial agonist prenalterol either. It is concluded that there is no simple relation between c-AMP generation and the functional response to beta-adrenoceptor agonists. Forskolin should not be used uncritically to probe beta-adrenoceptor-mediated effects.

Characterization of vascular histamine H1-receptors: multiple affinity states of aortic histamine H1-receptor

We have shown that [3H]mepyramine labels histamine H1-receptor-binding sites in bovine aortic membranes. Further characterization of H1-receptors in this tissue was done by the interaction of an unlabelled histamine receptor agonist or antagonist, with the radioantagonist [3H]mepyramine-binding sites. The competition-binding assays have uncovered differences in the characteristics of the agonist/receptor interaction not shared by antagonists. Agonists interact in the heterogeneous manner with the radioantagonist-labelled sites, showing shallow competition curves with the nH 0.50-0.72, whereas antagonists were devoid of this effect (steeper slopes of the inhibition curves nH approximately 1). The results suggest the presence in this tissue of multiple affinity states of histamine H1-receptor, differentiated by high and low affinity for agonists and the same affinity for antagonists.