CARDIOSELECTIVE DERIVATIVES OF 2,2-DIPHENYL-2-ETHYLTHIOACETATE DO NOTDISCRIMINATE BETWEEN M2 AND M3-MUSCARINIC-RECEPTORS EXPRESSED IN CHO CELLS

Characteristics of interaction of two derivatives of 2,2-diphenyl-2-et hylthioacetate with muscarinic receptors were studied in Chinese hamst er ovary (CHO) cells stably transfected with the genes of human m2 and m3 muscarinic receptors. Data from radioligand-receptor binding assay s and measurements of m2 receptor-inhibited cyclic AMP formation and m 3 receptor-stimulated phosphoinositide (PI) hydrolysis showed that thi s new series of muscarinic receptor antagonists exhibited a middle ran ge of affinities in binding to muscarinic receptors (K-i = 0.2-0.7 mu mol/l), without being able to discriminate between m2 and m3 receptors . They completely displaced [H-3]N-methylscopolamine ([H-3]NMS) bindin g at equilibrium and inhibited receptor-mediated increase in PI turnov er in m3 CHO cells and decrease in cyclic AMP synthesis in m2 CHO cell s in an apparent competitive manner. However, higher concentrations of the compounds (> 10 mu mol/l) decelerated the kinetics of atropine-in duced dissociation of [H-3]NMS at m2 and m3 receptors, indicating an a llosteric interaction. Collectively, our results demonstrate that thes e derivatives of 2,2-diphenyl-2-ethylthioacetate display a mixed mecha nism of interaction with muscarinic receptors, being competitive at lo w concentrations and allosteric at higher concentrations. In contrast to previous reports of a significantly higher affinity at cardiac M(2) as compared to ileal M(3) receptor, these compounds do not exhibit su ch selectivity when the two receptor subtypes are expressed in the sam e type of cells.