Ascorbic acid-induced binding of [125I]-iodocyanopindolol to non-beta-adrenoceptor sites in guinea-pig trachea

Classical and atypical binding sites for beta-adrenoceptor ligands and activation of adenylyl cyclase in bovine skeletal muscle and adipose tissue membranes

1. The radioligand [125I]-iodocyanopindolol ([125I]-ICYP) was used under standard ligand binding conditions, to detect beta 1- and beta 2-adrenoceptors in membrane preparations from bovine skeletal muscle and adipose tissue. High concentrations of [125I]-ICYP were also used, to identify an 'atypical' binding site in skeletal muscle. Finally, adenosine 3':5'-cyclic monophosphate (cyclic AMP) production was measured in the same membrane preparations, to determine the relationship between the beta-adrenoceptor sub-types present and the production of this second-messenger. 2. According to the results of radioligand binding studies, both skeletal muscle and adipose tissue membranes have beta 2-adrenoceptors, characterized by a high affinity for the beta 2-selective antagonist, ICI 118551 (pK 8.3 and 8.6 respectively); and a low affinity for the beta 1-selective antagonist CGP 20712A (pK 5.2 in both tissues). Antagonism of (-)-isoprenaline-stimulated cyclic AMP production by low concentrations of ICI 118551, yielded pseudo pA2 values in muscle and adipose tissue of 7.6 and 8.7 respectively, confirming that beta 2-adrenoceptors in these tissues are linked to the production of the second-messenger. 3. Although beta 1-adrenoceptors could not be detected in either skeletal muscle or adipose tissue membranes by use of ligand binding techniques, high pseudo pA2 values were obtained (8.0 and 8.2 respectively), when CGP 20712A was used to block the stimulation of cyclic AMP production by (-)-isoprenaline. This finding is consistent with the presence in both tissues of a population of beta 1-adrenoceptors which is small, but efficiently coupled to the second-messenger. 4.In addition to identifying standard beta 1- and beta 2-adrenoceptors, it was also established that skeletal muscle membranes have an 'atypical' binding site which has a relatively low affinity for [125]-ICYP(pK8.84), but which exists in abundance. At high concentrations of radioligand, the 'atypical' site accounted for 89% of the total [125I]-ICYP binding sites present.5. The results of second-messenger studies do not support the hypothesis that skeletal muscle or adipose tissue membranes contain functional beta 3-adrenoceptors: based on the failure of a beta 3-adrenoceptor-selective agonist (BRL 37344) to stimulate cyclic AMP production, the absence of a biphasic response to (-)-isoprenaline, and the observation that cyclic AMP production was not resistant to blockade by either ICI 118551 or CGP20712A.6. It is concluded that data from radioligand binding studies do not accurately reflect the contribution made by beta 1- and beta 2-adrenoceptors to cyclic AMP production in bovine skeletal muscle and adipose tissue membranes. Furthermore, the 'atypical' [125I]-ICYP binding site identified in bovine skeletal muscle does not represent a functional bovine beta 3-adrenoceptor.

Eosinophil peroxidase and 125I- ion binding in guinea-pig trachea

The present study further characterizes ascorbic acid-dependent 125I- ion binding in guinea-pig trachea. Binding of 125I- ion in the presence of ascorbic acid was detected at the epithelial/submucosal interface, apparently involving individual cells containing peroxidase enzyme. A similar binding pattern was observed when hydrogen peroxide was substituted for ascorbic acid. Binding could be inhibited by the addition of the H2O2 degrading enzyme catalase or the peroxidase inhibitor thiourea. Results demonstrated that this binding was dependent upon the addition of, or endogenous production of hydrogen peroxide and its metabolism via airway eosinophil peroxidase. The relevance of this anomalous iodine binding phenomenon to radioiodinated ligand binding studies is discussed.

Localization of (-)-[125I]cyanopindolol binding in guinea-pig heart: characteristics of non-beta-adrenoceptor related binding in cardiac pacemaker and conducting regions

Receptor autoradiography was used in guinea-pig heart to locate binding sites for the beta-adrenoceptor ligand (-)[125I]cyanopindolol (CYP) resistant to blockade by the beta-adrenoceptor antagonist (-)-propranolol (1 microM). Highly localized binding was observed to regions closely associated with the sinoatrial node, atrioventricular node and bundle of His but was not observed on myocardial, pacemaker, conducting cells or adipose tissue. Free [125I] also bound to identical sites. Binding was enhanced in the presence of ascorbic acid but was completely inhibited by (-)-isoprenaline (100 microM), serotonin (5-HT) (10 microM) and phentolamine (10 microM).

Time dependent uptake of (-)125I-iodocyanopindolol but not (-)125I-iodopindolol by murine splenic lymphocytes

Time dependent uptake of (-)125I-iodocyanopindolol by intact murine splenocytes was investigated in an attempt to prevent its interference in receptor binding experiments. Four approaches were tried in an attempt to eliminate this uptake; including the use of the permeable amine chloroquine; the inclusion of catechol, chloroquine, and phentolamine into the binding assay; the addition of methylamine to the assay; and the elimination of ascorbate from the assay. None of these approaches completely eliminated the uptake of ICYP over time. Comparative studies with (-)125I-iodopindolol showed this ligand did not have time dependent uptake and may provide for less problematic intact cell receptor assays.

Characteristics and localisation of 125I ion binding in mammalian airways

We have examined some of the binding characteristics and the autoradiographic distribution of binding sites for Na125I (I-Na) in airway tissue from the guinea-pig, monkey, pig, rat, mouse and from man. Basal I-Na (100 pM) binding levels were extremely low. However, in the presence of ascorbic acid (10 microM) or dithiothreitol (10 microM), I-Na binding was markedly increased in guinea-pig trachea, with lesser increases detected in monkey and rat trachea and in monkey and human bronchus. In guinea-pig trachea, ascorbic acid-induced I-Na binding was not saturable within the concentration range 100-620 pM and could not be reduced by washout. Autoradiography revealed that in central airways, I-Na binding was localized at or near the interface of the airway epithelium and submucosa in small clusters, apparently involving one or two cells per focus. The physiological significance of these binding sites is yet to be established, although they may be involved in intracellular iodine storage.

Beta-adrenoceptor desensitization in guinea-pig isolated trachea

Exposure to (-)-isoprenaline (25 microM, 1 h) caused a stereoselective, time and concentration-related decrease in smooth muscle beta 2-adrenoceptor function in guinea-pig trachea. Furthermore, tracheal relaxant responsiveness to the beta-adrenoceptor agonists (+/-)-fenoterol and (-)-noradrenaline was reduced, while that to theophylline and nitroprusside was unaffected. Responsiveness to forskolin was marginally but significantly reduced. Indomethacin, a cyclooxygenase inhibitor and mepacrine, an inhibitor of phospholipid turnover, had no significant effect on the extent of isoprenaline-induced desensitization. Conversely, cortisol (25 microM) significantly reduced desensitization and enhanced the rate of spontaneous recovery of responsiveness to isoprenaline. Desensitization was not accompanied by a reduction in the density of beta-adrenoceptors in the trachea, as assessed by binding and light microscopic autoradiography using [125I]iodocyanopindolol [( 125I]CYP). Thus, desensitization was probably caused primarily by beta-adrenoceptor/adenyl cyclase uncoupling. This model may be useful in investigations of the effect of glucocorticoids on the beta-adrenoceptor dysfunction recognized in severe asthma.