A model for the interaction of muscarinic receptors, agonists, and two distinct effector substances.

The binding of the agonist carbamylcholine to muscarinic receptors in rat heart myocytes from young and aged cultures and in rat atrial membranes has been measured in the absence and presence of GppNHp, pertussis toxin, and/or batrachotoxin. The effect of each of the added substances upon agonist binding was accounted for by a model according to which the receptor may form an equilibrium complex with agonist and either of two distinct effector substances, one of which is postulated to increase the affinity of receptor for agonist and the other of which is postulated to decrease the affinity of receptor for agonist.