The affinity of [3H]quinuclidinyl benzilate and some other radioligands for glass microfiber and cellulose filters in some common liquid scintillation solvents.

The optimum conditions for measuring radioactivity in the filtration assay of muscarinic cholinoceptors with tritiated quinuclidinyl benzilate are to use Whatman GF/B filters and to add a simple toluene scintillant to them while they are still damp. Practically all the radioactive material is then slowly extracted into the scintillant and high counting efficiencies are obtained after 24 h. Dried filters, or dry filters in control experiments in the absence of receptor, adsorb much of the radioactivity with a 30% reduction in counting efficiency. Other scintillants were able to extract the radioactive material from dry filters, but were generally not preferable to toluene. The GF/B filters performed better than other glass microfiber and cellulose filters in terms of retention of receptor-bound ligand, rapid filtration rates, and low filter blanks. Toluene is unsuitable as a scintillant with GF/B filters for some other radioligands examined.