Calcium channel ligand binding to intact, concanavalin A and cyclic AMP-treated cells of the immune system

To examine whether the cells of immune system express calcium channel-forming proteins, we studied the binding of calcium channel ligands, known to detect certain types of the above channels in excitable tissues, to murine splenic and human peripheral blood mononuclear cells. Specific (i.e., displaceable by excess cold ligand) binding of the 3H-labelled dihydropyridine drugs PN200-110 and nitrendipine, was not detected in these cells. Specific binding of a phenylalkylamine drug, [3H]verapamil, was detected, but cannot be attributed to the existence of certain specialized receptors, since multiple [3H]verapamil binding sites (about 10(6) per cell) appeared to be occupied. [3H]Verapamil binding to murine splenic mononuclear cells was inhibited following exposure to either the polyclonal T-cell activator, concanavalin A, or a cell-permeable analogue of the second messenger, cyclic AMP, suggesting that processes of lymphocyte activation and/or intracellular signalling may down-modulate at least some of calcium channel ligand binding sites.