1 The intrinsic properties of muscarinic ligands were studied through their
binding properties and their abilities to modulate the GTPase activity of
G proteins coupled to muscarinic M-2 receptors in pig atrial sarcolemma.
2 Competition binding experiments were performed with [H-3]-oxotremorine-M
to assess the affinity of receptors coupled to G proteins (R*), with [H-3]-
N-methylscopolamine ([H-3]-NMS) to estimate the affinities of coupled and u
ncoupled receptors (R*+R) and with [H-3]-NMS in the presence of GppNHp to a
ssess the affinity of uncoupled receptors (R).
3 The ranking of K-i values for the agonist carbachol was R*<<R*+R<<R (0.95
, 124 and 1017 nM). K-i values for atropine and AF-DX 116 were similar for
the three binding conditions (0.34, 0.42, 0.41 and 19, 22, 32 nM). The rank
ing of K-i values for pirenzepine was R*>R*+R>R (174, 155, 115 nM), suggest
ing inverse agonism.
4 The V-max of the basal high affinity GTPase activity of pig atrial sarcol
emma was increased by mastoparan and decreased by GPAnt-2 indicating the re
levance of this activity to G proteins coupled to receptors (R*). The K-M v
alue (0.26-0.33 mu M) was not modified by mastoparan or GPAnt-2.
5 Carbachol increased the V-max of GTP hydrolysis (EC50 8.1+/-0.3 mu M), wh
ereas atropine and AF-DX 116, up to 1 mM, did not modify it. Pirenzepine de
creased the V-max of GTP hydrolysis (EC50 77.5+/-10.3 mu M). This effect wa
s enhanced when KCl was substituted for NaCl (EC50 11.0+/-0.8 mu M) and was
antagonized by atropine and AF-DX 116 (IC50 0.91+/-0.71 and 197+/-85 nM).
6 Pirenzepine is proposed as an inverse agonist and atropine and AF-DX 116
as neutral antagonists at the muscarinic M-2 receptor.