A muscarinic receptor type in human lymphocytes: A comparison of ³H-QNB binding to intact lymphocytes and lysed lymphocyte membranes

Human blood lymphocytes from normal blood donors exhibited specific binding of the muscarinic antagonist ³H-quinuclidinyl benzilate (³H-QNB). The ³H-QNB binding to intact viable lymphocytes as well as to lysed lymphocyte membranes "P2" was saturable and displaceable by both muscarinic agonists and antagonists. For the lysed lymphocyte membranes "P2" a single binding site with a B_max of 109 pmol/g protein and a K_d of 15 nM was obtained. Intact viable lymphocytes also showed one binding site with a K_d of 24 nM and a B_max of 1556 pmol/g protein. The higher B_max value might be explained in terms of uptake of the ligand when using intact cells or through loss of binding sites when using lysed lymphocyte membranes "P2". IC₅₀ values were lower by a factor of 10² for atropine and scopolamine and by 10⁴ for pirenzepine when lysed lymphocyte membranes "P2" were used instead of intact viable lymphocytes.