Corticosteroid modulation of muscarinic receptors in rat myocardial membranes.
Rat ventricular myocardial membranes contain muscarinic acetylcholine receptors which can be identified by binding of the muscarinic antagonist (-)-[3H]quinuclidinyl benzilate. Scatchard analysis of saturation binding data revealed binding to a single class of non-cooperative sites (0.693 pmol/mg protein) with high affinity (i.e. with an equilibrium dissociation constant of 0.24 nM). Competition binding curves of the agonist carbamylcholine were shallow (with a Hill coefficient, nH of 0.71) for membranes of untreated rats, suggesting the presence of two receptor subpopulations with different agonist affinity. These curves were steeper (nH = 0.86) for adrenalectomized animals and more shallow (nH = 0.62) for hydrocortisone-treated animals. In contrast, both treatments did not affect the total receptor number. This suggests that corticosteroids are required for the myocardial muscarinic receptors to adopt high agonist affinity. However, the inhibition of adenylate cyclase by muscarinic agonists disappeared after both corticosteroid treatment and adrenalectomy. But agonist receptor binding could still be modulated by guanine nucleotides. This indicates that both high and low affinity forms of muscarinic receptors induced by altered corticosteroid states retain functional coupling with the inhibitory nucleotide binding site, but are uncoupled from the adenylate cyclase catalytic subunit, C.