Differential sensitivity of phosphoinositide and cyclic gmp responses to short-term regulation by a muscarinic agonist in mouse neuroblastoma cells. Correlation with down-regulation of cell surface receptors

Short-term agonist-induced loss of cell surface muscarinic receptors and desensitization of receptor-mediated cyclic GMP (cGMP) formation and phosphoinositide hydrolysis were examined in mouse neuroblastoma cells (clone N1E-115) in suspension. This treatment resulted in a time-dependent reduction of approximately 40% of the specific binding of the hydrophilic antagonist [³H]N-methyl-scopolamine ([³H]NMS) with a T _{1 2} of down-regulation of 4.83 min. Scatchard analysis revealed that brief exposure to the agonist resulted in a significant reduction in the B_max with no change in the K_d. Agonist-induced cGMP formation decreased in a similar time-dependent manner with an average T _{1 2} of 4.79 min. However, desensitization of muscarinic receptor-stimulated accumulation of inositol phosphates demonstrated a much slower time-course and was accompanied by a reduction in the maximal response with no change in the EC₅₀. In addition, there was rapid partial recovery of cell surface receptors and desensitized cGMP response, with no apparent resensitization of phosphoinositide hydrolysis. Thus, there was a differential rate of short-term desensitization and resensitization of these two muscarinic receptor-mediated responses. Moreover, desensitization of cGMP formation, but not phosphoinositide hydrolysis, closely paralleled loss of cell surface muscarinic receptors.