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SM Simasko, JR Soares and GA Weiland
Loss of responsiveness of the neuronal-type nicotinic acetylcholine receptor (nAChR) on PC12 cells, a cell line derived from a rat pheochromocytoma, was induced by exposure to carbamylcholine (carbachol). Nicotinic receptor function was assessed by carbachol- induced 22Na+ uptake. We found that, in addition to classically described desensitization, a second process occurs which results in a nonrecoverable loss of nAChR activity. This second process, which we have labeled inactivation, has a slower onset than the classically described desensitization (t1/2 = 14.7 min for inactivation and 0.78 min for desensitization at 1 mM carbachol). Inactivation could not be explained by inadequate washing, a loss of electrochemical driving force, or a loss of cell viability. The onset of inactivation is dependent on the concentration of desensitizing ligand and is blocked by nicotinic antagonists. No recovery of the loss of activity from inactivation was observed even after 2 hr of incubation in recovery buffer. Inactivation does not appear to require formation of a desensitized state since desensitization was reduced in the absence of Ca2+ whereas inactivation was not affected by the absence of Ca2+. The mechanism which underlies inactivation remains to be determined; however, it is possible that inactivation is the first step in nAChR down-regulation and it may also explain previous observations of rapid and prolonged tolerance to the effects of nicotinic agonists.
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