Human nicotinic acetylcholine receptor: the influence of second messengers on activation and desensitization.
The amplitude and time course of acetylcholine(ACh)-induced membrane current were determined in cells of the human medulloblastoma cell line TE 671. ACh was applied and washed out very rapidly (about 50 ms) by a shift of a cell between two streams of solution, one of which contained the transmitter. ACh-induced current was recorded in the whole-cell mode of patch clamping. The time course of activation of the ACh-induced current could not be resolved because the method of ACh application was still too slow. Desensitization started immediately with ACh application; it could be described by two time constants. With an ACh concentration of 3 microM, the fast time constant was about 0.5 s and the slow time constant was about 3.9 s. When the ACh concentration was raised in steps to 100 mM, the peak amplitude of the current increased, reached a maximum at 1 mM and decreased again. The rate of desensitization was directly correlated with increasing ACh concentration. Current amplitude and desensitization time constants were not affected by intracellular application of cAMP or of a catalytic subunit of a cAMP-dependent protein kinase. When the intracellular calcium concentration was raised at a constant magnesium concentration, desensitization time constants remained unaffected, but the current amplitude decreased. This decrease is not caused by a decrease in single-channel conductance, therefore it may represent a decrease in the number of activatable channels.
Siara
J
J
Abteilung für Allgemeine Physiologie, Universität Ulm, Federal Republic of Germany.
Ruppersberg
J P
JP
Rüdel
R
R
D000109:Acetylcholine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D000242:Cyclic AMP / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D009994:Osmolar Concentration; D011494:Protein Kinases / Q000494:pharmacology; D011978:Receptors, Nicotinic / Q000502:physiology*; D015290:Second Messenger Systems / Q000502:physiology*; D014407:Tumor Cells, Cultured
D011978:Receptors, Nicotinic; D000242:Cyclic AMP; D011494:Protein Kinases; D000109:Acetylcholine; D002118:Calcium