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.