| Abstrakt: |
Activation of muscarinic cholinergic receptors on 1321N1 human astrocytoma cells results in a 40-70% inhibition of isoproterenol- or prostaglandin E1 (PGE1)-stimulated accumulation of cyclic AMP. Previous investigations have demonstrated that this effect is due to a Ca2+-dependent activation of phosphodiesterase in the presence of muscarinic receptor agonists. However, during prolonged exposure of 1321N1 cells to a cholinergic agonist, a series of adaptive changes occurs which culminates in a complete loss of the muscarinic receptor-mediated inhibition of cyclic AMP accumulation. These alterations include: (a) A 50-100% increase in the capacity of isoproterenol and PGE1 to stimulate cyclic AMP accumulation. This phenomenon was rapid in onset, reached a maximum in 15-20 min, and disappeared over the next 2 hr even in the continued presence of carbachol. (b) A loss of the effects of muscarinic receptor stimulation on cyclic AMP accumulation. This phenomenon was apparent within 15 min after addition of carbachol, and complete desensitization was observed after 75 min. The loss of muscarinic receptor-mediated effects on cyclic AMP levels was due to a loss of the Ca2+-dependent stimulation of phosphodiesterase activity by muscarinic receptor agonists. (c) A loss of muscarinic receptors as assessed by [3H]quinuclidinyl benzilate binding. This effect was apparent after 90 min in the presence of carbachol. More than 80% of the receptors were lost after 24 hr, with no change occurring in the KD of [3H]quinuclidinyl benzilate. The concentration-effect curve for carbachol-induced changes in agonist responsiveness of the cyclic AMP system was similar to that for carbachol-induced reductions in cyclic AMP levels. Coincubation of carbachol with a saturating concentration of atropine prevented these adaptive changes from occurring. Although incubation of cells in Ca2+-free buffer or in the presence of 20 mM Co2+ prevented the inhibitory effects of muscarinic receptor stimulation on cyclic AMP accumulation, carbachol preincubations under these conditions still produced the adaptive changes in agonist responsiveness. The divalent cation ionophore, A23187, mimics the effects of muscarinic receptor stimulation on cyclic AMP levels by activating phosphodiesterase. Following complete carbachol-induced loss of responsiveness to muscarinic receptor agonists, A23187 was still capable of inhibiting cyclic AMP accumulation. |
