Agonist Regulation of the Muscarinic Cholinergic Receptor in Embryonic Chick Heart
J. B. Galper, L. C. Dziekan and T. W. Smith. The correlation between number of muscarinic cholinergic receptor sites as measured by binding of the muscarinic agonist (3H)methyl-scopolamine ((3H)MS) and the ability of muscarinic agonists to mediate a physiologic response was determined in intact heart cells cultured from chick embryos 10 days in ovo. The increase in K+ permeability and the decrease in beating rate mediated by the muscarinic agonist car-bamylcholine were the responses studied. In cells labelled to equilibrium with 42K+, exposure to 10–3 M carbamycholine caused a 33% increase in the rate of 42K+ efflux from the cell with a half-maximal effect at a carbamylcholine concentration of 8×10–5 M. Steady state Intracellular K+ content remained at control levels. A 15% decrease in beating rate occurred in cells exposed to 10–3 M carbamylcholine. Since muscarinic receptors on the surface of the Intact cell are presumed to mediate the physiologic response to the agonist, an assay for binding of (3H)MS to Intact cells was developed. (3H)MS bound specifically to intact heart cells (l85fmol/mg protein) with a Kd of 0.48 nM.
KeywordsMuscarinic Receptor Heart Cell Receptor Number Muscarinic Agonist Beat Rate
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