Regulation of Muscarinic Receptor‐Mediated Cyclic GMP Synthesis by Cultured Mouse Neuroblastoma Cells

Mouse neuroblastoma clone N1E‐115 has muscarinic acetylcholine receptors that mediate cyclic GMP synthesis. This receptor‐mediated response is not significantly higher than background until the cells have been maintained in the stationary phase for at least 1 week. The basis of the influence of time in culture on the cyclic GMP response was investigated. The relative amount of cyclic GMP synthesized by intact cells was measured by radioactively labeling the GTP pool with [³H]guanine, incubating cells with agonists, and then chromatographically isolating [³H]cyclic GMP. Carbamylcholine‐, ionophore X‐537A‐, and sodium azide‐induced cyclic GMP formation increased with time in culture to a maximum of 13‐, 9‐, and 2.5‐fold above basal, respectively. There was no change in the number or the apparent affinity of the muscarinic receptors as measured by [³H]quinuclidinyl benzylate ([³H]QNB) binding. In addition, there was no change in the apparent affinity of the receptors for agonist as measured by the ability of carbamylcholine to displace the specific binding of [³H]QNB. Guanylate cyclase activity per milligram protein and per cell in‐creased six‐ and sevenfold, respectively, from day 0 to day 22. However, this increase in guanylate cyclase appeared to precede the marked increase in sensitivity of the cells to agonists. These data suggest that, in addition to guanylate cyclase and muscarinic receptors, there is another factor which is responsible for the development of this muscarinic receptor‐mediated response.