PHARMACOLOGICAL DIFFERENCES BETWEEN THE HIGH-AFFINITY MUSCARINIC AGONIST BINDING STATES OF THE RAT-HEART AND CEREBRAL-CORTEX LABELED WITH (+)-[H-3]CISMETHYLDIOXOLANE

  • 1 January 1984
    • journal article
    • research article
    • Vol. 229  (3) , 747-755
Abstract
The high-affinity agonist binding state of muscarinic receptors in the rat heart and cerebral cortex was pharmacologically characterized in parallel studies. Muscarinic sites were labeled and studied with the aid of a highly specific, rapid filtration binding assay using the potent muscarinic agonist (+)-[3H]CD [cismethyldioxolane]. Homogenates of both tissues contained a saturable high-affinity (Kd = 1-2 nM), low capacity (6-17% of (-)-[3H]QNB [quinuclidinyl benzilate] sites) (+)-[3H]CD binding state which demonstrated stereoselectivity and drug specificity typical of a muscarinic site. Comparative studies of drug potency profiles in competition for myocardial and cerebral cortical (+)-[3H]CD-labeled membranes revealed several major pharmacological differences between muscarinic sites in these tissues. Whereas the muscarinic agonists pilocarpine and McN-A-343 [[4-(m-chlorophenylcarbamoyloxy)-2-butynyl]trimethylammonium chloride], the nonclassical antagonist pirenzepine, and the acetylcholinesterase inhibitor physostigmine reduced (+)-[3H]CD binding in both tissues, their inhibitory effects were more potent (4-77-fold) in cerebral cortical membranes. Gallamine, a nicotinic cholinergic antagonist, demonstrated a 36-fold greater potency at the high-affinity (+)-[3H]CD binding state in myocardial membranes. Other classical muscarinic agonists and antagonists were nearly equipotent as inhibitors of high-affinity (+)-[3H]CD binding in these 2 tissues. Muscarinic receptors in the heart and cerebral cortex can evidently be distinguished pharmacologically by certain drugs which interfere with the high-affinity agonist binding state of the muscarinic recognition site. Support is provided for the subclassification of these receptors.