Muscarinic receptors on rat lymphocytes; internalization of receptor-ligand complex

In the light of the data presented, the previously described explanation of the unusual shape of the kinetic curve of the muscarinic antagonist 3H-QNB binding to rat lymphocytes, with apparent maximum of bound radioactivity after 5 min of incubation, followed by decrease of specific binding, is not satisfactory. The kinetics of this binding carried out at 4 degrees C, 20 degrees C and 37 degrees C, or in the presence of the metabolic poison sodium azide, suggests involvement of an energy-consuming mechanism in this process. The surface receptors, sensitive to bacterial protease degradation before 3H-QNB binding, become somehow resistant to this protease action after 5 min of incubation of the cells with 3H-QNB. A possible explanation for this phenomenon, based on internalization in cell endosomes, is presented.