OCCUPATION OF CHOLINERGIC RECEPTORS IN SMOOTH MUSCLE ESTIMATED WITH AID OF 14C-LABELLED ARECAIDINE-DERIVATIVES

1) The uptake and release of four 14C-labelled arecaidine-derivatives by longitudinal muscle strips of guinea-pig ileum were investigated. The tertiary arecaidine-ethyl-ester-HCI and arecaidine-isopropyl-ester-HCI were accumulated by the tissue (T/M ratios > 1.5), whereas the quarternized compounds arecaidine-ethyl-ester-methiodide and dihydro-arecaidine-isopropyl-ester-methiodide were adsorbed but to a small degree by the cell membranes (T/M ratios about 0.7. 2) The analysis of the data allows an estimation of the number of arecaidine molecules bound at the surface of a single muscle cell. This number gives an upper limit of the occupied acetylcholine receptors. At a concentration of 3 × 10-7 M arecaidine-ethyl-ester-HCL 1.2 × 106 molecules will be attached to the surface of one single muscle cell, covering 0.05% of the total cell surface. 3) The quarternized arecaidine-ethyl-ester-methiodide, which has antagonistic properties, will occupy 7 × 105 binding sites covering 0.033% of the surface, when applied in a concentration of 3 × 10-6 M. © 1993.