Agonist regulation of rat alpha 3 beta 4 nicotinic acetylcholine receptorsstably expressed in human embryonic kidney 293 cells

Effects of agonists on rat alpha3 beta4 nicotinic acetylcholine receptors e xpressed in KX alpha3 beta 4R2 cells (human embryonic kidney 293-derived ce lls) were studied. The potencies of seven agonists varied over a 7000-fold range, with a rank order of epibatidine >> A85380 > cytisine approximate to 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) approximate to nicotine > acetylcholine > carbachol. The efficacies of all of the agonists studied h ere were similar except for DMPP, which seemed to be a partial agonist comp ared with nicotine and acetylcholine. Nicotine and carbachol desensitized t he receptors in a time- and concentration-dependent manner. The EC50 values for nicotine and carbachol to desensitize the receptors during a 60-min ex posure were 3 and 51 muM, respectively, indicating that these agonists are more potent at desensitizing the receptors than at activating them. The fun ction of the receptors recovered from agonist-induced desensitization rapid ly and almost completely. The half-time for recovery of function from desen sitization after a 60-min treatment with nicotine increased with the concen tration of nicotine used to desensitize the receptors. In contrast, no such concentration dependence for time to recovery of function was found when c arbachol was used to desensitize the receptors. We propose that this differ ence may be due to the cell permeability of nicotine, allowing it to enter and be sequestered inside of cells and then slowly diffuse out to maintain receptor desensitization. After a 5-day exposure to 100 muM nicotine, the r eceptors were completely desensitized, but receptor function recovered to 8 3% of control values with a half-time of about 10.5 min. Although the numbe r of nicotinic receptor binding sites measured with (+/-)-[H-3]epibatidine was increased during the chronic treatment with nicotine, no increase in fu nction was detected.