/sup 3/H)pirenzepine and (-)-(/sup 3/H)quinuclidinyl benzilate binding to rat cerebral cortical and cardiac muscarinic cholinergic sites. II. Characterization and regulation of antagonist binding to putative muscarinic subtypes
Studies show (/sup 3/H)PZ identified selectively a subpopulation of muscarinic binding sites compared to classical antagonists like (-)-(/sup 3/H)QNB in many central and peripheral tissues. We characterized the binding and regulation of selected antagonists to high-affinity (/sup 3/H)PZ (putative M1) and low-affinity PZ (putative M2) sites in rat cerebral cortex (predominantly M1) and heart (predominantly M2). Saturation isotherms of (/sup 3/H)PZ and (-)-(/sup 3/H)QNB were performed under various conditions. Guanyl-5'-yl-imidodiphosphate (30 microM) showed little effect on Kd (dissociation constant) or total binding capacity (total receptor density) values. Higher ionic strength buffers yielded lower affinity values for (/sup 3/H)PZ and (-)-(/sup 3/H)QNB. Kinetic studies confirmed high affinity Kd values seen in steady-state assays. We conducted inhibition studies of selected muscarinic antagonists including the reportedly cardioselective (putative M2) drug, AF-DX 116 (11-((2-(diethylamino)methyl-1-piperidinyl)-acetyl)-5, 11-dihydro-6H-pyrido(2,3-b)(1,4)-benzodiazepine-6-one), the reportedly M1 selective compound, PZ, and the classical antagonist (-)QNB, using (/sup 3/H)PZ and (-)-(/sup 3/H)QNB-labeled cerebral cortical and cardiac homogenates. Assays were done with and without guanyl-5'-yl-imidophosphate at 25 degrees C in 10 mM Na-K-phosphate, 50 mM Na-K-phosphate and modified Krebs-phosphate buffer. Studies showed antagonists generally had higher affinity in 10 mM Na-K-phosphate buffer, were insensitive to guanyl-5'-yl imidodiphosphate and had Hill values (nH) nearly equal to one. Cardiac PZ/(/sup 3/H)QNB curves were steep.
- Research Organization:
- Univ. of Arizona Health Sciences Center, Tucson
- OSTI ID:
- 5578981
- Journal Information:
- J. Pharmacol. Exp. Ther.; (United States), Vol. 2
- Country of Publication:
- United States
- Language:
- English
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CHOLINE
RECEPTORS
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BIOCHEMICAL REACTION KINETICS
AFFINITY
BUFFERS
CEREBRAL CORTEX
IN VITRO
MYOCARDIUM
RATS
TRACER TECHNIQUES
TRITIUM COMPOUNDS
ALCOHOLS
AMINES
AMMONIUM COMPOUNDS
ANIMALS
AUTONOMIC NERVOUS SYSTEM AGENTS
BODY
BRAIN
CARDIOVASCULAR SYSTEM
CENTRAL NERVOUS SYSTEM
CEREBRUM
DRUGS
HEART
HYDROXY COMPOUNDS
ISOTOPE APPLICATIONS
KINETICS
LABELLED COMPOUNDS
LIPOTROPIC FACTORS
MAMMALS
MEMBRANE PROTEINS
MUSCLES
NERVOUS SYSTEM
ORGANIC COMPOUNDS
ORGANS
PROTEINS
QUATERNARY COMPOUNDS
REACTION KINETICS
RODENTS
VERTEBRATES
550201* - Biochemistry- Tracer Techniques