RESUMEN
Although M1-M4 muscarinic acetylcholine receptors (mAChRs) in HEK-293 cells internalize on agonist stimulation, only M1, M3, and M4 but not M2 mAChRs recycle to the plasma membrane. To investigate the functional consequences of this phenomenon, we compared desensitization and resensitization of M2 versus M4 mAChRs. Treatment with 1 mM carbachol for 1 h at 37 degrees C reduced numbers of cell surface M2 and M4 mAChRs by 40-50% and M2 and M4 mAChR-mediated inhibition of adenylyl cyclase, intracellular Ca2+ concentration ([Ca2+]i) increases, and phospholipase C (PLC) activation by 60-70%. Receptor-mediated inhibition of adenylyl cyclase and [Ca2+]i increases significantly resensitized within 3 h. However, M4 but not M2 mAChR-mediated PLC activation resensitized. At 16 degrees C, M2 mAChR-mediated [Ca2+]i increases and PLC stimulation desensitized to a similar extent as at 37 degrees C. However, at 16 degrees C, where M2 mAChR internalization is negligible, both M2 mAChR responses resensitized, demonstrating that M2 mAChR resensitization proceeds at the plasma membrane. Examination of M2 mAChR responses following inactivation of cell surface mAChRs by quinuclidinyl benzilate revealed substantial receptor reserve for coupling to [Ca2+]i increases but not to PLC. We conclude that M2 mAChR internalization induces long-lasting PLC desensitization predominantly because receptor loss is not compensated for by receptor recycling or receptor reserve.