MODULATION OF AGONIST BINDING BY GUANINE-NUCLEOTIDES IN CHO CELLS EXPRESSING MUSCARINIC M1-M5 RECEPTORS

In membranes prepared from CHO-m(2) cells, inhibition of [H-3]-N-methy lscopolamine ([H-3]NMS) binding by several muscarinic agonists resulte d in competition curves with Hill slopes significantly different from unity. Addition of 5'-guanylylimidodiphosphate (Gpp(NH)p) led to an in crease in the IC50 value of the agonists with significant steepening o f the inhibition curves. The shift in potency induced by Gpp(NH)p diff ered among the agonists with a rank order of oxotremorine-M = carbacho l > oxotremorine > McN-A-343 = pilocarpine. In CHO-m4 membranes, Gpp(N H)p was less efficacious than in CHO-m2 membranes whereas no effect of the guanine nucleotide was found in membranes prepared from CHO-m1, - m3, and -m5 cells. No major differences in the effect of Gpp(NH)p amon g agonists were found in CHO-m4 cells. Atropine binding was not affect ed by the guanine nucleotide. Together, these results indicate that co upling of G-proteins to muscarinic receptors linked to inhibition of c yclic adenosine monophosphate (cAMP) (m2 and m4) but not of those link ed to phosphoinositol turnover (m1, m3 and m5) can be perturbed by Gpp (NH)p. The differential effects observed with Gpp(NH)p between agonist binding to m2 and m4 receptors appear to be receptor-specific and may reflect differences in the G proteins activated by these receptors in CHO cells.