Comparison of pharmacological and binding assays for ten beta-adrenoceptor blocking agents and two beta-adrenoceptor agonists

Dose effects of butoxamine, a selective β2-adrenoceptor antagonist, on bone metabolism in spontaneously hypertensive rat

Recent studies have shown that osteoblasts and osteoclasts express β2-adrenoceptor, and increased sympathetic nervous activity causes bone loss via an increase in osteoclastic bone resorption and a decrease in osteoblastic bone formation. We previously demonstrated that non-selective β-adrenoceptor antagonist propranolol at low doses (0.1 and 1mg/kg), but not at a higher dose (10mg/kg), prevented a decrease in bone mass and an increase in bone fragility in spontaneously hypertensive rat (SHR), an animal model of osteoporosis with hyperactivity of the sympathetic nervous system, without affecting blood pressure. In the present study, the dose effects of butoxamine, a selective β2-adrenoceptor antagonist, on bone metabolism were examined in SHR by analysis of microcomputed tomography, bone histomorphometry, biomechanical testing and plasma biochemistry. Treatment of SHR with butoxamine at 0.1, 1 and 10mg/kg (per os) for 12 weeks increased bone mass indices and biomechanical parameters of strength and toughness of the lumbar vertebrae, suggesting antiosteoporotic activity. Butoxamine dose-dependently decreased osteoclast number and surface per bone surface with decreases in plasma tartrate-resistant acid phosphatase-5b level, a biochemical index of osteoclastic activity. On the other hand, histomorphometry indices of bone formation and plasma osteocalcin concentration reflecting osteoblastic activity were increased in SHR treated with butoxamine at 0.1 and 1mg/kg, but not at 10mg/kg. These results suggest that β-adrenoceptor antagonists at a low dose may improve osteoporosis with hyperactivity of the sympathetic nervous system via β2-adrenoceptor blocking action, while they may have a somewhat inhibitory effect on osteoblastic activity at a high dose.

The relative importance of the time-course of receptor occupancy and response decay on apparent antagonist potency in dynamic assays

1. The potency of the beta1-adrenoceptor antagonist atenolol was measured as an inhibitor of responses to isoprenaline in guinea-pig left atria. Measurements were made in two ways, firstly, by pre-incubating the atria with a given concentration of atenolol followed by an isoprenaline dose-response curve and, secondly, by measuring the response to isoprenaline followed by addition of atenolol. 2. It was found that the estimation of atenolol potency as an antagonist of beta1-adrenoceptors by these two methods gave divergent results. Specifically, it was found that the isoprenaline-induced increased rate of myocardial relaxation was resistant to receptor blockade. Thus, the rate-limiting step in the relaxation response was dissociated from receptor activation and therefore, could not be used for the measurement of receptor occupancy. 3. In contrast, the positive inotropic response was very responsive to receptor occupancy. However, when atenolol was used to block a steady-state isoprenaline response, there was a complicating depression of basal inotropy after receptor blockade that obfuscated measurement of receptor blockade. 4. In general, these data indicated that the blockade of a steady-state agonist response to measure the potency of an antagonist might in some cases yield erroneous results. These studies indicate some caution in the interpretation of blockade responses in pre-contracted or pre-stimulated pharmacological preparations.

Potential role of the autonomic nervous system in the immunosuppressive effects of acute morphine administration

These studies investigated the role of the autonomic nervous system in mediating the immunosuppressive effect of morphine on blood lymphocyte proliferation in rats. To determine the contribution of the autonomic nervous system, rats were pretreated with the ganglionic blocker chlorisondamine (5 mg/kg) prior to morphine (7 mg/kg) administration. Ganglionic blockade with chlorisondamine completely antagonized the inhibitory actions of morphine, suggesting that intact ganglionic transmission was required for the inhibition to occur. Blockade of postganglionic parasympathetic neurotransmission with atropine methylbromide (1 mg/kg) or blockade of sympathetic neurotransmission with the alpha-adrenoceptor antagonist phentolamine (1 mg/kg) did not attenuate the suppressive effect of morphine. Blockade of beta-adrenoceptors with propranolol (2.5 mg/kg) resulted in partial antagonism, but this action was not shared by the peripherally acting beta-adrenoceptor antagonist nadolol (6 mg/kg). These results suggest that the inhibitory effect of morphine on blood lymphocyte proliferation may be mediated through activation of the autonomic nervous system; however, individual blockade of either the parasympathetic or sympathetic division of the autonomic nervous system was not sufficient to antagonize this immunosuppressive effect.

Blood pressure reflexes following activation of capsaicin-sensitive afferent neurones in the biliopancreatic duct of rats

1. Inflammatory diseases of the pancreas or diseases which cause obstruction within the biliary or within the biliary or pancreatic duct system are associated with severe pain. Although neuropeptides such as substance P are present in the biliary tree, only few capsaicin-sensitive, substance P-positive nerve fibres have been found in the ducts. In order to obtain functional evidence whether capsaicin-sensitive afferent neurones transmit nociceptive information arising from the biliopancreatic duct, blood pressure reflexes following electrical stimulation of the duct or increases in intraductal pressure were determined in barbiturate-anaesthetized rats. 2. Electrical stimulation of neurones in the biliopancreatic duct was carried out at 30 V, 3 ms, 50 Hz for 20s. In untreated animals the electrical stimulation resulted in rises in blood pressure by up to 25 mmHg, but in about a quarter of all animals tested this response was absent. Following the administration of phentolamine (7 mumol kg-1, i.p.) the blood pressure responses were changed to pronounced and reproducible depressor reflexes of -5 to -30 mmHg. Retrograde injections into the biliopancreatic duct of 300 microliters of a 154 mM sodium chloride solution produced increases in intraductal pressure of approximately 10 mmHg. This elicited shortlasting falls in blood pressure of 3-15 mmHg. Phentolamine significantly augmented the fall in blood pressure to 8-30 mmHg. 3. The depressor reflexes observed in both models after the administration of phentolamine were abolished by morphine (1 mumol kg-1, i.v.). The inhibition by morphine was reversed by naloxone (3 mumol kg-1, i.v.). Naloxone given before morphine did not affect the depressor reflex but prevented the inhibitory action of subsequently injected morphine.4. Acute s.c. injection of capsaicin (30 mg kg-1) abolished the depressor reflexes in response to both types of nociceptive stimulation in phentolamine-treated rats. The initial pressor effects of electrical stimulation were only partly inhibited by capsaicin whereas the basal depressor reflexes in response to elevation of intraductal pressure were abolished. In rats which had received capsaicin on the day before the experiment or had been treated with capsaicin as neonates, only minor rises in blood pressure were induced by electrical stimulation at the beginning of the experiment and no changes in blood pressure occurred after the administration of phentolamine. After adult or neonatal pretreatment with capsaicin the depressor reflexes in response to increased intraductal pressure were only small and were unchanged by phentolamine.5. The depressor reflexes following either electrical stimulation or increases in intraductal pressure were abolished by the unselective Beta-blocker, (-)-propranolol (3 micromol kg-1, i.p.), and greatly reduced by the Beta 1-blocker, metoprolol (6 micromol kg- 1, i.p.). The Beta2-preferring adrenoceptor antagonist, butoxamine(3 micromol kg-1, i.p.), had no effect on the depressor responses. The reflex falls in blood pressure were also abolished by hexamethonium (10 micromol kg-1, i.p.) but not by atropine (3 micromol kg-1, i.p.).6. Both models of stimulation of nociceptive afferents caused identical patterns of blood pressure responses following adrenalectomy or chemical sympathectomy. In adrenalectomized rats, the initial responses consisted of depressor reflexes which were not augmented but significantly reduced by phentolamine and further inhibited by metoprolol. In rats that had been pretreated with 6-hydroxydopamine(total dose 0.6 mmol kg-1) to accomplish chemical sympathectomy, nociceptive stimulation caused rises in blood pressure. Phentolamine treatment abolished these pressor effects but revealed only minor, if any, depressor responses that were unaffected by metoprolol.7. In summary, the hypotensive effects in both models constitute nociceptive reflexes since they are abolished by morphine and restored by naloxone. The afferent part of the reflex is mediated by nerve fibres sensitive to capsaicin. Both experimental procedures seem to elicit two, presumably separate, reflex mechanisms. Firstly, catecholamines released from the adrenal medulla elevate blood pressure or limit hypotensive responses via activation of vascular alpha receptors. Secondly, the reflex inhibition of the sympathetic nerve activity in the heart and the vasculature causes the nociceptive depressor reflexes.

Relationship between in vitro relaxation of the costo-uterine smooth muscle and mesovarial leiomyoma formation in vivo by beta-receptor agonists

The three beta-agonists, salbutamol, ritodrine, and terbutaline have been shown to possess differing potentials to induce leiomyomas in rat costo-uterine muscle following chronic exposure (salbutamol greater than terbutaline greater than ritodrine). It has been suggested that the potential to induce leiomyomas is related to the relaxant properties of these agonists in the costo-uterine muscle. In order to test this hypothesis, the potencies of salbutamol, terbutaline, and ritodrine were compared to isoproterenol and norepinephrine in vitro in the rat costo-uterine smooth muscle, a beta 2-adrenergic receptor rich tissue. All compounds produced relaxation of potassium chloride (KCl) contracted costo-uterine smooth muscle. Significant differences in potency were observed, with isoproterenol being the most potent, followed in rank order by salbutamol, terbutaline and ritodrine. The relative potency of the non-selective beta-blocker propranolol in inhibiting the agonist mediated relaxant activity was similar for all agonists examined, indicative of interactions at the same receptor site (Tallarida and Jacob 1979). When tested for beta-agonist activity in the guinea pig atria, salbutamol and ritodrine were less potent in these tissues compared to the costo-uterine muscle. In summary, the in vitro pharmacological potency of salbutamol, terbutaline and ritodrine correlated with the potential to induce leiomyoma formation in rat costo-uterine muscle following chronic exposure to the respective beta-agonists. These results indicate that the isolated rat costo-uterine muscle is a sensitive model for comparing the potency of beta-agonists, and may assist in establishing the risk of costo-uterine leiomyoma formation in chronic rat studies relative to agents such as salbutamol.

Possible activation of intracellular beta-adrenoceptors by extraneuronally accumulated isoprenaline in perfused rat heart

Two rat hearts were perfused in series by a modified Loewi's method. The recipient heart was perfused with the perfusate collected from the donor heart. 1. After perfusion with 3H-isoprenaline in the presence of tropolone, an inhibitor of catechol-O-methyltransferase, the donor heart was washed out with amine-free medium containing tropolone and corticosterone. The heart rate of the recipient heart increased after the change to the perfusate from the donor heart during the wash-out. After wash-out the heart rate of the donor heart (which had accumulated 43.4 pmol.g-1 3H-isoprenaline) was higher than that of the recipient heart (which had accumulated 0.44 pmol.g-1 3H-isoprenaline), and the rates of efflux of 3H-isoprenaline from both hearts were similar. 2. After perfusion with 3H-isoprenaline and corticosterone in the absence of tropolone, the enhanced heart rate of the donor heart decreased during wash-out with amine-free medium in the presence of corticosterone. The heart rate of the recipient heart increased after the medium change to the perfusate from the donor heart, and the heart rates in both hearts were similar after wash-out. Only small amounts of 3H-isoprenaline remained in both hearts after wash-out, and the rates of efflux of 3H-isoprenaline from both hearts were similar. 3. After perfusion with 3H-isoprenaline in the presence of tropolone, the effects of propranolol and atenolol on the heart rate during wash-out with amine-free medium containing tropolone and corticosterone were compared. The inhibitory effect of propranolol on the heart rate was significantly greater than that of atenolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic delta-9-THC treatment on cardiac beta-adrenoceptors in rats

This study was designed to determine if chronic treatment with delta-9-tetrahydrocannabinol (THC) alters cardiac beta-adrenoceptors in the rat. Following daily oral administration of 10 or 20 mg/kg THC or an equivalent volume of control solvent for 90 days, rats were sacrificed, and sarcolemmal membranes were prepared from ventricular myocardium. Beta-adrenoceptor density and binding affinity estimated with (-)[3H]dihydroalprenolol; a beta-adrenergic antagonist, were not significantly affected by treatment with THC when compared to vehicle controls. These results suggest that the tolerance to cardiovascular effects of THC which develops during chronic exposure in the rat is not associated with alterations in cardiac beta-adrenoceptors as monitored by radiolabeled antagonist binding.

Effects of various antianginal drugs on sodium influx in rat brain synaptosomes and in rat heart muscle cells in culture

This paper describes the inhibitory effects of several antianginal drugs on 22Na uptake of the fast Na+ channel in rat brain synaptosomes and in rat heart muscle cells in culture. Calcium antagonists like verapamil, flunarizine, perhexiline, two perhexiline derivatives IPS 629 and IPS 672, and beta-adrenoceptor antagonists like propranolol and practolol were tested. IPS 672 was the most active compound on synaptosomes and heart muscle cells (IC50 = 2.0 X 10(-6) and 2.4 X 10(-6) M respectively). The relative potencies of the Ca2+ antagonists tested on heart muscle cells were found to be IPS 672 greater than IPS 629 greater than perhexiline greater than flunarizine greater than verapamil. Verapamil was 55 and 10 times less active than IPS 672 on synaptosomes and heart cells respectively. Propranolol had an inhibitory activity comparable to that of flunarizine and was 100 times more active than practolol. It can be concluded that several antianginal drugs seems to interfere with the Na+ fast channel on rat brain and heart.

Displacement analysis of binding inhomogeneities in crude extracts of receptors

Guidelines are given to distinguish different kinds of binding inhomogeneities of non-radiolabeled ligands in crude extracts of receptors if an appropriate 'binding analogue' of the displacers is available in radiolabeled form. Three minimal models for the simplest types of binding inhomogeneities are analysed theoretically. These models include a cooperative system (with two interacting sites on the same receptor molecule) and two non-cooperative systems (one of them with a single-site receptor having two conformational states in equilibrium and the other with two single-site receptors independent of each other). In certain cases one can distinguish these systems experimentally. Furthermore, if a group of displacers is already classified according to the above models, then dissociation constants can be determined. The quantitative comparison of these displacers on the basis of their dissociation constants is more appropriate (e.g. in Quantitative Structure Activity Relationship studies) than on the basis of their ID50 and Ki values or Hill coefficients, which is often done.

Characterization of human platelet beta-adrenoceptors

The widespread use of beta-adrenoceptor antagonists against hypertension, angina pectoris and migraine or as a preventive treatment after myocardial infarction has encouraged us to investigate the effects of these drugs on platelet function. The aim of this study was to examine whether beta-blocking drugs interfere with platelet beta- adrenoceptors and whether this dependency is related to their selectivity for beta-adrenoceptor subtypes. Beta-adrenoceptor stimulation of human platelets with isoprenaline increased cyclic AMP (cAMP), which is known to inhibit platelet aggregation. Furthermore, our studies showed that cAMP formation in vitro was stimulated by non-selective and beta 2-selective agonists, but not by the predominant beta 1-agonist prenalterol. Isoprenaline- stimulated cAMP formation was blocked by the non- selective beta-adrenoceptor antagonists propranolol, timolol, and alprenolol, while the beta 1-selective antagonists atenolol and metoprolol had no influence on an isoprenaline-induced cAMP formation. Receptor binding studies using (3H)-dihydroalprenolol revealed an IC50 value for propranolol of 85 nM, while metoprolol only displaced the bound (3H)-dihydroalprenolol at far higher concentrations (IC50, 20 microM). We conclude that the human platelet beta-adrenoceptors are mainly of the beta 2- subtype and that beta-adrenoceptor antagonists, especially the non-selective antagonists interfere with platelet function assessed as platelet cAMP formation.

Pharmacological evidence for differences in the beta-adrenoreceptor populations mediating atrial and ventricular chronotropic responses in the unanaesthetized dog

The beta-adrenoreceptor populations mediating atrial and ventricular chronotropic responses were studied comparatively in the unanaesthetized dog with chronic atrio-ventricular block. With this aim, we first compared the increases in atrial and ventricular rate induced by isoprenaline. We then compared the reductions of this isoprenaline-induced atrial and ventricular cardio-acceleration, observed after administration of cumulative doses of propranolol and alprenolol, both non-selective beta-adrenoreceptor blocking agents, and metoprolol, a blocking agent selective for beta 1-adrenoreceptors. Isoprenaline induced a much more intense stimulation of atrial chronotropic beta-adrenoreceptors than of ventricular chronotropic beta-adrenoreceptors, whether or not the cholinoreceptors were blocked. Similarly, all three beta-blocking agents induced a more intense blockade of atrial chronotropic beta-adrenoreceptors than of ventricular chronotropic beta-adrenoreceptors, whatever the dose of isoprenaline considered and whether or not the cholinoreceptors were blocked. These results cannot be readily explained by quantitative differences in the distribution of beta 1- and beta 2-adrenoreceptors in the heart, given the identical effects obtained with metoprolol, propranolol and alprenolol. Therefore, beta-adrenoreceptor populations are not identical throughout the nodal tissue, and in particular, differences occur between beta-adrenoreceptors in the sinus node mediating atrial chronotropic responses and those in the His bundle mediating ventricular chronotropic responses, which might be linked to differing sensitivities of these beta-adrenoreceptor populations.

Beta 2-receptors in the rat anterior pituitary mediate adrenergic stimulation of prolactin release

As previously shown, the beta-adrenergic agonists isoproterenol, epinephrine and norepinephrine stimulate prolactin (PRL) release from superfused rat anterior pituitary cell aggregates. In order to further characterize the beta-adrenergic response in this tissue preparation, the effects of various beta-adrenergic agents were investigated. The beta 2-agonist, zinterol, stimulated PRL release at concentrations more than 4 orders of magnitude lower than prenalterol, a beta 1-agonist with high potency in rat heart. The order of potency of the antagonists IPS 339 (beta 2), ICI 118.551 (beta 2), propranolol, sotalol, practolol (beta 1), metoprolol (beta 1) and H 35/25 for inhibition of beta-agonist-stimulated PRL release provided additional support for a beta 2-stimulatory effect. beta-Agonists were also capable of stimulating PRL release from superfused intact pituitaries. The beta-adrenergic response desensitized rapidly during prolonged exposure of the aggregates to beta-agonists.

Adrenergic regulation of avian pancreatic polypeptide secretion in vitro

Adrenergic regulation of avian pancreatic polypeptide (APP) secretion from perifused microfragments of chicken pancreas was investigated with catecholamines and adrenoreceptor-specific analogues added to avian Krebs-Ringer perifusion medium containing 11.1 mM glucose. APP release was stimulated by the alpha-adrenoreceptor agonist phenylephrine (threshold 5 microM; maximum effective concn, 50 microM) and the beta-adrenoreceptor agonist isoproterenol (threshold 5 microM; maximum effective concn, 50 microM). Epinephrine (0.164 microM) and 50 microM norepinephrine also stimulated APP release. The results obtained suggest that APP secretion from chicken pancreas is regulated by dual, complementary alpha- and beta-adrenoreceptor mechanisms.

Effect of labetalol and YM-09538 on neuronal uptake of (3H)norepinephrine in the rat vas deferens

Labetalol and YM-09538 are combined alpha and beta receptor antagonists with demonstrated antihypertensive activity. Both compounds inhibited the uptake of (3H)norepinephrine into nerves in the rat vas deferens. However, labetalol was approximately 5 fold more potent than YM-09538 as an inhibitor of neuronal uptake. Inhibition of neuronal uptake occurred at concentrations 43 and 3800 times higher than necessary for inhibition of alpha receptors by labetalol and YM-09538, respectively. Thus YM-09538 shows a greater separation between neuronal uptake blocking properties and alpha receptor blocking properties than labetalol. This separation of activities may contribute to greater antihypertensive efficacy of YM-09538 relative to labetalol.

High and low (Gpp(NH)p-sensitive) affinity sites for beta 2-adrenergic blockers as antagonists of isoproterenol in the field-stimulated rat vas deferens

Schild plots for (-)-propranolol, timolol and IPS 339, but not penbutolol or carazolol, as antagonists of isoproterenol in the isolated, field-stimulated rat vas deferens exhibited biphasic curves indicating the presence of both high and low affinity sites for these agents in this preparation. Schild plots determined for (-)-propranolol in the presence of prazosin (0.3 micron) or using the prostatic end of the rat vas deferens yielded two similar affinity sites suggesting the lack of involvement of postsynaptic alpha 1-adrenergic mechanisms in the observed response. In the presence of Gpp(NH)p(4 micron), (-)-propranolol, timolol and IPS 339 exhibited only a single affinity site which corresponded to their high affinity site in antagonizing isoproterenol in the absence of Gpp(NH)p. Similarly, only a single high affinity site was observed for (-)-propranolol when salbutamol, rather than isoproterenol, was used as the agonist. The data suggest the two affinity sites observed with some beta-adrenergic blockers in this preparation reflects a possible action of isoproterenol and the antagonists upon two subpopulations of beta 2-adrenergic receptors, one of which is sensitive to Gpp(NH)p.

Mapping of mammalian beta-adrenoreceptors by use of macromolecular alprenolol derivatives. A comparison with amphibian erythrocyte receptors

THe interaction of macromolecular alprenolol derivatives with beta-adrenoreceptors of rat heart, lung, and erythrocytes and frog erythrocytes has been studied. Macromolecular derivatives were prepared by covalently coupling alprenolol to dextrans containing a homologous series of spacer arms of various lengths. The affinity of these macromolecules for frog erythrocyte membranes increased with increasing length of spacer arm. In contrast, the affinity of these macromolecules for all mammalian membrane preparations was weak and insensitive to the length of the spacer arm. The inhibition of [3H]dihydroalprenolol binding to rat heart, lung, and erythrocyte membrane preparations by these macromolecular derivatives was more than 1000-fold less potent than inhibition by alprenolol. The results suggest different structural characteristics between mammalian and amphibian beta-adrenoreceptors; however, apparently only small differences between mammalian receptors could be distinguished with these probes.

Comparison of the apparent irreversible beta-adrenoceptor antagonist Ro 03-7894 with propranolol in cardiac ventricular muscle by pharmacological and radioligand binding techniques

Membrane fractions were prepared from guinea-pig ventricular muscle and the specific binding of [3H]dihydroalprenolol ([3H]DHA) assessed. The dissociation constant (Ki) of (+/-)-propranolol was determined (6.9 nM) from its ability to displace [3H]DHA binding. This compared with the pA2 value of propranolol of 8.32 (dissociation constant, 4.8 nM) determined for the antagonism of isoprenaline-induced positive inotropic responses of papillary muscles from guinea-pig hearts. Scatchard analysis of the saturation curves for specific [3H]DHA binding showed that in the presence of propranolol, the displacement was characteristic of competitive antagonism. That is, there was no change in the total beta-adrenoceptor was characteristic of competitive antagonism. That is, there was no change in the total beta-adrenoceptor binding sites (Bmax) but an apparent reduction of the dissociation constant (KD) of [3H]DHA. This antagonism was fully reversed by washing membranes that had been previously incubated with propranolol. In contrast, in the presence of the beta-adrenoceptor antagonist, Ro 03-7894, the Scatchard plots were displaced in a manner characteristic of irreversible antagonism. The Bmax was significantly reduced. This antagonism was resistant to washout, with the Scatchard plots still showing a reduced Bmax and no change in the dissociation constant (KD) of [3H]DHA. This apparent irreversible antagonism by Ro 03-7894 was also demonstrated in guinea-pig isolated papillary muscles. The maximum of the dose-response curve to isoprenaline, constructed after incubation with Ro 03-7894 and a 3 hr bath-washout, was depressed by 89.5 +/- 7.5%

(+/-)[125Iodo] cyanopindolol, a new ligand for beta-adrenoceptors: identification and quantitation of subclasses of beta-adrenoceptors in guinea pig

(+/-)[125Iodo] cyanopindolol (ICYP) is a radioligand which binds with an extraordinarily high affinity and specificity to beta-adrenoceptors. In contrast to (+/-) [125Ido]-hydroxybenzylpindolol (IHYP), the new ligand has neither affinity to alpha-nor to 5-HT-receptors. The dissociation constants of ICYP for beta- adrenoceptors in various tissues range from 27 to 40 pM, thereby exceeding the affinity of IHYP by a factor of approximately 3. ICYP does not discriminate between beta 1- and beta 2-adrenoceptors. Therefore, the densities of the two receptor subtypes can be determined from competition curves of ICYP by drugs previously found to show in vitro selectivity for beta 1-adrenoceptors. The guinea pig left ventricle contains only beta 1-adrenoceptors, whereas in a lung tissue, the ratio of beta 1-to beta 2-adrenoceptors is 1 to 4. The calculated affinities of five beta 1-selective antagonists for beta 1-adrenoceptors were nearly identical in the ventricle and the lung. Kinetic studies of ICYP binding to guinea pig lung membranes indicated that the dissociation reaction consists of two components, a fast process (t 1/2 = 9 min) and a slower process (t 1/2 = 8.8 h). A mathematical treatment revealed two possibilities of interpretation: 1. Two forms of the receptor exist which are interconvertible. 2. The (+)- and (-)- enantiomers of ICYP dissociate with different rate constants. The low dissociation constant of ICYP in combination with its high specific radioactivity (2175 Ci mmole -1) allows binding studies to be carried out with small protein and ligand concentrations, e.g. 3 microgram protein per assay in guinea pig lung membranes.