MUSCARINIC CHOLINERGIC RECEPTORS OF 2 CELL-LINES THAT REGULATE CYCLIC-AMP METABOLISM BY DIFFERENT MOLECULAR MECHANISMS

  • 1 January 1984
    • journal article
    • research article
    • Vol. 26  (3) , 395-404
Abstract
The attenuation of cAMP accumulation occurs by different mechanisms in 1321N1 astrocytoma cells and NG108-15 neuroblastoma .times. glioma cells. In 1321N1 cells, cholinergic agonists reduce cAMP accumulation through a Ca2+-dependent activation of phosphodiesterase; in NG108-15 cells, muscarinic receptor-mediated effects on cAMP metabolism occur through inhibition of adenylate cyclase. Different pharmacological specificities may be expressed by the muscarinic receptor populations of these 2 cell lines. The affinity of muscarinic receptors for [3H]quinuclidinyl benzilate (6 pM), [3H]N-methylscopolamine (50 pM) and atropine (80 pM) was similar in membrane preparations from each cell line. The affinity of the antagonist, pirenzepine, which may be a selective ligand for a muscarinic receptor subtype, was 3-fold higher in competition binding assays carried out with membranes of 1321N1 cells, than with NG108-15 cells. The Hill coefficients of pirenzepine competition curves were not significantly different from unity in both cell lines. This selectivity of pirenzepine was also apparent in studies of the competitive inhibition of carbachol-induced attenuation of cAMP accumulation in intact cells. Differences in the relative affinities of agonsits were observed in competition binding analyses carried out with membranes in the presence of GTP and absence of Mg2+. The Ki values of bethanechol and carbachol were 5- and 12-fold lower for recpetors of NG108-15 cells than those of 1321N1 cells and the Ki of methacholine was 3.5-fold lower for 1321N1 cells than for NG108-15 cells. The affinities of oxotremorine and arecoline were similar between the 2 cell lines. These differences in agonist affinities between the 2 cell lines were much smaller in analyses of muscarinic receptor-mediated effects on cAMP metabolism in intact cells. Evidently, muscarinic receptors of differing pharmacological specificities regulate cAMP metabolism by different mechanisms in 1321N1 and NG108-15 cells.