Apparent capacity of cardiac muscarinic receptors for different radiolabeled antagonists

Citation
Cs. Sum et al., Apparent capacity of cardiac muscarinic receptors for different radiolabeled antagonists, BIOCH PHARM, 62(7), 2001, pp. 829-851
Citations number
70
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Pharmacology & Toxicology
Journal title
BIOCHEMICAL PHARMACOLOGY
ISSN journal
0006-2952 → ACNP
Volume
62
Issue
7
Year of publication
2001
Pages
829 - 851
Database
ISI
SICI code
0006-2952(20011001)62:7<829:ACOCMR>2.0.ZU;2-6
Abstract
Muscarinic receptors in sarcolemmal membranes, digitonin-solubilized extrac ts, and purified preparations from porcine atria have revealed a shortfall in the apparent capacity for N-[H-3]methylscopolamine, which was only about 75% of that for [H-3]quinuclidinylbenzilate. Since binding at near-saturat ing concentrations of [H-3]quinuelidinylbenzilate was inhibited fully at co mparatively low concentrations of unlabeled N-methylseopolamine, the data a re inconsistent with the notion that [3H]quinuclidinylbenzilate binds selec tively to a subclass of distinct, non-interconverting, and mutually indepen dent sites. The discrepancy is resolved by adjusting the specific activity of N-[H-3] methylscopolamine to account for unlabeled scopolamine that was identified in some batches of the radioligand. Also, there was no shortfall in capacity when N-[3H]methylscopolamine was devoid of scopolamine, and th e predicted effect was obtained when pure N-[H-3]methylscopolarnine was sup plemented with known amounts of scopolamine. A small discrepancy in the lev els of scopolamine estimated pharmacologically and by mass spectrometry can be attributed largely to a difference in the efficiency of ionization betw een scopolamine and N-methylseopolamine. Different capacities for different radioligands are not uncommon with muscarinic and other G protein-coupled receptors, and in some cases the effect may have been due wholly or in part to an unlabeled impurity. Binding data can be mechanistically ambiguous, p articularly when acquired only at graded concentrations of the radioligand. The predicted effects of an unlabeled impurity mimic or resemble those of alternative scenarios such as sequestration behind a hydrophobic barrier, a nucleotide-regulated interconversion from one state of affinity to another , and cooperativity between interacting sites. (C) 2001 Elsevier Science In c. All rights reserved.