Effect of thiols on beta 2-adrenoceptors in human mononuclear leucocytes

Roles of sulfhydryl and disulfide groups in the binding of CP-55,940 to rat brain cannabinoid receptor

The roles of sulfhydryl and disulfide groups in the specific binding of synthetic cannabinoid CP-55,940 to the cannabinoid receptor in membrane preparations from the rat cerebral cortex have been examined. Various sulfhydryl blocking reagents including p-chloromercuribenzoic acid (p-CMB), N-ethylmaleimide (NEM), o-iodosobenzoic acid (o-ISB), and methyl methanethiosulfonate (MMTS) inhibited the specific binding of [3H]CP-55,940 to the cannabinoid receptor in a dose-dependent manner. About 80-95% inhibition was obtained at a 0.1 mM concentration of these reagents. Scatchard analysis of saturation experiments indicates that most of these sulfhydryl modifying reagents reduce both the binding affinity (Kd) and capacity (Bmax). On the other hand, DL-dithiothreitol (DTT), a disulfide reducing agent, also irreversibly inhibited the specific binding of [3H]CP-55,940 to the receptor and about 50% inhibition was obtained at a 5 mM concentration. Furthermore, 5 mM DTT was abelt to dissociate 50% of the bound ligand from the ligand-receptor complex. The marked inhibition of [3H]CP-55,940 binding by sulfhydryl reagents suggests that at least one free sulfhydryl group is essential to the binding of the ligand to the receptor. In addition, the inhibition of the binding by DTT implies that besides free sulfhydryl group(s), the integrity of a disulfide bridge is also important for [3H]CP-55,940 binding to the cannabinoid receptor.

A partially automated radioligand binding assay system for use in clinical and pharmaceutical research

Using a Tecan robotic sample processor and IBM compatible PCs we have developed a flexible, partially automated radioligand binding assay system. It handles pipetting parameters of up to 16 saturation or competition experiments at a time with up to 24 radioligand- or competitor-concentrations in a range over 4 orders of magnitude per experiment. The system provides enough flexibility so that all pipetting parameters including different tube-, rack-sizes, sample volumina and pipetting sequences may be easily adapted to the large variety of experimental requirements in binding assays. It rationalizes and increases assay throughput (up to 70% spare of working time), improves reliance and reproducibility of results. Radioactive exposure is minimized to the time preparing the radioligand working solution and transferring the sample tubes to and from the sample processor. The system has proven effective in various investigations on binding interactions, as well as in clinical studies on receptor expression under physiologic, pathological and therapeutic conditions.

Evolving concepts of partial agonism

The exact mechanism of receptor activation at the molecular level are still not known, nor do we completely understand the precise factors that distinguish agonist- and partial agonist-induced activation. Nevertheless, recent years have brought forth an explosion of new information regarding beta-adrenergic receptor structure and ligand-induced activation. Partial agonists are likely intermediate in their ability to interact with crucial serine residues (Ser204 and Ser207) on the beta-adrenergic receptor; these interactions allow either incomplete stimulation of the entire receptor population, or full stimulation of only a portion of the entire receptor population. From the work presented by Tota and Schimerlik for the muscarinic cholinergic receptor (another G-protein coupled receptor), it is likely that partial agonists induce or stabilize receptor conformations that have a lower affinity for their G protein compared to receptors stimulated by a full agonist. Molecular cloning of beta-adrenergic receptors and analyses of mutated and chimeric receptors expressed in transfected systems have indicated that domains of the receptor that bind agonists may be different from those with which antagonists interact. Thus, the ability of a partial agonist to interact with these two different domains may be a determinant of efficacy. Agonists alter the sulfhydryl redox status of the beta-adrenergic receptors in the presence of Gs. Disulfide rearrangement has been postulated to provide a structural constraint which biases G-protein-linked receptors in the "ground state" and may be important for stabilizing the active state of the receptor and holding the agonist/receptor/Gs ternary complex in the high-affinity state. Partial agonists induce this state less efficaciously or are less capable of holding the receptor in the active conformation to allow disulfide exchange to take place. The extent of receptor stimulation may dictate which G proteins are activated by a particular receptor, and thus which cellular effectors are stimulated. Alternatively, the level of activation of a receptor may translate into varying states of activation of a particular G protein (stabilized in part by disulfide bonds). Techniques such as fluorescence energy transfer in reconstitution systems or nuclear magnetic resonance spectroscopy should prove useful in distinguishing among these possible mechanisms. Ultimately, as a long-term goal, X-ray crystallography of unoccupied receptors and receptors liganded by partial or full agonists may provide definitive insights. Although definitive answers are not yet possible, the rapid progress in understanding aspects of receptor structure allows a reformulation of ideas regarding the molecular basis of efficacy and partial agonism.(ABSTRACT TRUNCATED AT 400 WORDS)

Nocturnal asthma. Beta 2-adrenoceptors on peripheral mononuclear leukocytes, cAMP- and cortisol-plasma concentrations

To evaluate pathophysiologic mechanisms of the predominantly nocturnal complaints in atopic bronchial asthma, the expression and function of beta 2-adrenoceptors on peripheral mononuclear leukocytes (pMNL), the cAMP--as well as the cortisol--plasma concentrations were studied in eight healthy men and ten so far untreated male asthmatic patients at 4-h intervals for 24 h. No difference was seen in the beta 2-adrenoceptor density (Bmax) on pMNL between healthy and asthmatic men (24-h means +/- SE: 908 +/- 59 sites per cell and 821 +/- 54 sites per cell, respectively). The equilibrium dissociation constant (Kd), however, was significantly higher in the asthmatic patients (24-h mean +/- SE: 8.8 +/- 1.2 pmol/L vs 3.0 +/- 0.2 pmol/L in healthy men, p less than 0.0001), which is equivalent to a lower affinity of the beta 2-adrenoceptors for the radioligand 125iodocyanopindolol. Bmax showed a statistically significant circadian variation, but Kd did not. The circadian variation in Bmax was reflected in the basal intracellular cyclic adenosine-monophosphate (cAMP) content of the cells investigated. High Kd values (equivalent to low receptor affinities) tended to be associated with small increases of the intracellular cAMP content after in vitro stimulation by 10(-7) mol/L isoprenaline (isoproterenol) (24-h mean +/- SE: 1.4 +/- 0.2 pmol/10(6) cells; r = -0.529, p = 0.05 at r = -0.549, n = 10). Plasma cAMP concentrations were found to be significantly lower in the asthmatic patients (24-h means +/- SE: 22.9 +/- 1.3 nmol/L vs 29.1 +/- 1.1 nmol/L, p less than 0.0001). Plasma cortisol concentrations were significantly higher in the asthmatic patients (24-h means +/- SE: 0.500 +/- 0.084 mumol/L vs 0.319 +/- 0.063 mumol/L). The results support the hypothesis that a lesion of the beta-adrenergic system contributes to the pathophysiology of atopic bronchial asthma. In the patients investigated in this study, such a lesion could be demonstrated in the affinity rather than in the number of beta 2-adrenoceptors expressed on peripheral cells of the immune system (pMNL). According to present-day knowledge of adrenergic effects on pMNL, such an affinity decrease of beta 2-adrenoceptors could account for overshooting immune responses. In association with other factors influencing respiratory function, it could be responsible for the predominantly nocturnal complaints in atopic bronchial asthma. Plasma cortisol concentrations did not appear to be related to the principal cause of "nocturnal asthma;" they rather reflected an endogenous defense mechanism to the disease.

Quantitation of mRNA for the beta-adrenoceptor gene in human mononuclear leucocytes by in situ hybridization with fluorochrome labeled cloned DNA

Determination of the transcriptional activity of the beta-adrenoceptor (beta-AR) gene in human mononuclear leucocytes (MNL) may provide important information concerning variations in receptor expression in clinical studies. We used a new method with highly fluorochrome-labeled gene probes for in situ hybridization which allows a rapid detection of mRNA at the cellular level. MNL from venous blood of healthy volunteers were permeabilized, fixed and attached to adhesive glass slides. After incubation with fluorochrome-labeled cloned hamster beta AR-DNA probes the amount of DNA-mRNA hybrids formed in the cytoplasm of individual cells was analyzed by microfluorimetry of specifically bound fluorochrome. In nine of ten normal subjects studied so far a fraction of beta AR-mRNA positive MNL could be detected. The proportion of positive cells as well as their signal intensity revealed a high interindividual variance.

Revised radioreceptor assay for beta 2-adrenoceptors expressed on peripheral mononuclear leukocytes

Peripheral mononuclear leukocytes (pMNL) bear a population of beta 2-adrenoceptors. Radioreceptor assays with (-)-125Iodocyanopindolol (125I-CYP) are often used to determine the expression of these hormone receptors under physiological and pathological conditions. Doubts on the occurrence of just one class of binding sites as well as the availability of new laboratory equipment prompted us to revise the procedure employed for investigation of these receptors. pMNL were harvested from venous human blood by density centrifugation with LymphopaqueR, LymphoprepR, or FicollR yielding immunologically distinct pMNL fractions. Receptor binding assays were performed semi-automatically with 125I-CYP in the range from 0.6-600 pmol/l. Analysis of the data (modified affinity spectra, Scatchard plot) revealed two classes of binding sites (high- and low affinity binding). The binding isotherms were sigmoidal in the concentration range from 0.6-3.0 pmol/l. Parameters estimated for the high affinity binding site may vary by a factor of 10, depending on the mathematical model employed.

The use of robots and computers in the organisation of studies on the circadian variation of beta 2-adrenoceptor sites in peripheral mononuclear leucocytes

We have developed a partially automated method for the performance of equilibrium radioligand binding studies which is applied by our group in investigations on circadian variations and stimulation studies on beta 2-adrenoceptor sites in human peripheral mononuclear leucocytes (pMNL). Using a Tecan Robotic Sample Processor, binding assays with 12 concentrations of 125iodocyanopindolol (1-150 pmol/l, total binding in triplicates, unspecific binding in the presence of 10(-5) mol/l timolol in duplicates) are prepared automatically with all titer tubes per experiment arranged in a microtiterplate-sized rack. After incubation in a waterbath for 2hr at 37 degrees C, the whole rack is centrifuged at 5000g and transferred back to the lab robot. Bound radioactivity is separated from the unbound ligand by removing the supernatant by the machine. The radioactive counts are evaluated using personal computers. The lab robot enhances reproducibility of experimental results and frees lab workers from time-consuming pipetting jobs. Radioactive exposure is minimized to the time preparing the radioligand working solution and transferring the sample tubes from the robot to the waterbath, to the centrifuge and back to the robot. The variability of our software allows easy adaptation to other binding studies with intact cells.

Oxygen radicals in lung pathology

Pulmonary tissue can be damaged in different ways, for instance by xenobiotics (paraquat, butylated hydroxytoluene, bleomycin), during inflammation, ischemia reperfusion, or exposure to mineral dust or to normobaric pure oxygen levels. Reactive oxygen species are partly responsible for the observed pulmonary tissue damage. Several mechanisms leading to toxicity are described in this review. The reactive oxygen species induce bronchoconstriction, elevate mucus secretion, and cause microvascular leakage, which leads to edema formation. Reactive oxygen species even induce an autonomic imbalance between muscarinic receptor-mediated contraction and the beta-adrenergic-mediated relaxation of the pulmonary smooth muscle. Vitamin E and selenium have a regulatory role in this balance between these two receptor responses. The autonomic imbalance might be involved in the development of bronchial hyperresponsiveness, occurring in lung inflammation. Finally, several antioxidants are discussed which may be beneficial as therapeutics in several lung diseases.

Beta-adrenergic blood pressure regulation in Shy-Drager syndrome and pheochromocytoma

Both Shy-Drager syndrome and pheochromocytoma are characterized by an abnormal catecholamine secretion, e.g. a reduced secretion in Shy-Drager syndrome, and an excessive stimulation in pheochromocytoma resulting in adrenergic dysfunction and in adrenergic hyperactivity, respectively. The relationship between extreme variations in circulating catecholamines and beta-adrenergic receptor activity was studied in two patients with severe orthostatic hypotension (Shy-Drager syndrome) and in a patient with pheochromocytoma with excessive spontaneous catecholamine increases using the lymphocyte beta 2-adrenoceptor assay. In both patients with Shy-Drager syndrome, basal plasma concentrations of epinephrine and dopamine were low under resting conditions and could not be stimulated in the upright position. Norepinephrine was low in the first patient, and could not be stimulated; whereas the second patient had a normal basal concentration of norepinephrine, which could be moderately stimulated. There was no beta-adrenoceptor abnormality in the first patient: however, in the second patient, there were no measurable beta-adrenoceptors on membrane fractions, whereas a population of receptors only in the low affinity state could be identified on intact cells. Alpha-adrenoceptor density on thrombocyte membranes was slightly increased in both patients with Shy-Drager syndrome and showed no substantial change during upright posture. Catecholamine increases in the pheochromocytoma patient were accompanied by a rise in blood pressure, bradycardia, and an acute up-regulation of beta-adrenoceptors. Plasma concentrations of cAMP paralleled the increase in receptor density and blood pressure. The findings in pheochromocytoma add support to the theory that an acute catecholamine stimulation gives rise to an acute beta-adrenergic sensitization leading to blood pressure elevation.(ABSTRACT TRUNCATED AT 250 WORDS)