Muscarinic cholinergic binding sites on rat lymphocytes

Receptors for neurotransmitters in blood cells could serve as useful markers for the same receptors in solid tissues. Muscarinic receptors have been identified in human, rat and mouse lymphocytes by binding of [3H]quinuclidinyl benzilate (3H-QNB); however, the biochemical and pharmacological characterization of such binding sites has not been complete. Spleen lymphocytes were isolated on a histopaque gradient and incubated in Hank's buffer with 3H-QNB. Binding of 3H-QNB was linear with increasing protein concentrations and was saturable. Binding constants were Bmax = 111 +/- 10.5 fmol/10(6) cells, and Kd = 29.7 +/- 3.9 nM (n = 7). An extensive pharmacological analysis of these binding sites indicated that several cholinergic muscarinic drugs were capable of inhibiting 3H-QNB binding. Muscarinic antagonists were more potent than agonists, and lipophilic drugs were more potent than hydrophilic drugs. Several non-cholinergic drugs were also capable of inhibiting 3H-QNB binding; however, they did so also in brain membranes, while a third group of non-cholinergic drugs and neurotransmitters were inactive. Similar results were also obtained in circulating lymphocytes and in lymphocyte membranes. These results suggest that lymphocytes possess muscarinic receptors which share several, although not all, characteristics of the same receptors in brain and other tissues. Measurement of these binding sites could be useful to monitor the status of muscarinic receptors in solid tissues.