RT Journal Article
SR Electronic
T1 [3H]pirenzepine and (-)-[3H]quinuclidinyl benzilate binding to rat cerebral cortical and cardiac muscarinic cholinergic sites. II. Characterization and regulation of antagonist binding to putative muscarinic subtypes.
JF Journal of Pharmacology and Experimental Therapeutics
JO J Pharmacol Exp Ther
FD American Society for Pharmacology and Experimental Therapeutics
SP 419
OP 427
VO 237
IS 2
A1 M Watson
A1 W R Roeske
A1 H I Yamamura
YR 1986
UL http://jpet.aspetjournals.org/content/237/2/419.abstract
AB Studies show [3H]PZ identified selectively a subpopulation of muscarinic binding sites compared to classical antagonists like (-)-[3H]QNB in many central and peripheral tissues. We characterized the binding and regulation of selected antagonists to high-affinity [3H]PZ (putative M1) and low-affinity PZ (putative M2) sites in rat cerebral cortex (predominantly M1) and heart (predominantly M2). Saturation isotherms of [3H]PZ and (-)-[3H]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 [3H]PZ and (-)-[3H]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 [3H]PZ and (-)-[3H]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/[3H]QNB curves were steep.(ABSTRACT TRUNCATED AT 250 WORDS)