Trapping of labelled ligands in intact cells: a pitfall in binding studies

Binding on/in whole cells seems to be a more appropriate approach for studying receptor sites in physiological conditions. However, certain difficulties encountered throughout the characterization of [3H]spiperone binding in human lymphocytes led us to reconsider this problem. The IC50 values of [3H]spiperone binding to human lymphocytes did not correlate with those found in rat striatum; domperidone was inactive in lymphocytes whereas it is one of the most potent dopamine antagonists in rat striatal preparations in vitro. In contrast, chloroquine, a lysosomotropic drug, displaced [3H]spiperone at low concentration in intact lymphocytes but did not in the striatum. [3H]Spiperone binding was not displaceable in the membrane preparation of lymphocytes. Similar results were obtained with other intact cells, fibroblasts, hepatocytes and neuroblastoma cells using [3H]spiperone and other ligands, such as [3H]haloperidol, [3H]pyrilamine and [3H]ketanserin. Here again, displaceable binding was only present in intact cells but not in membrane fractions. Such a 'displaceable' binding was not related to receptor sites but may be regarded as non-specific binding which should correspond to a trapping phenomenon presumably in the lysosomes. Binding studies on intact cells need more caution than when performed on membrane preparations; indeed, permeation or trapping of ligands in the nanomolar range represents a serious drawback which, sometimes, can give the illusion of specific binding.