ACTIVATION OF THE RECOMBINANT HUMAN ALPHA(7) NICOTINIC ACETYLCHOLINE-RECEPTOR SIGNIFICANTLY RAISES INTRACELLULAR FREE CALCIUM

The alpha(7) nicotinic acetylcholine receptor (nAChR) subtype, unlike other neuronal nicotinic receptors, exhibits a relatively high permeab ility to Ca++ ions. Although Ca++ entry through this receptor subtype has been implicated in various Ca++-dependent processes in the central nervous system, little is known about how this receptor modulates mam malian intracellular Ca++ dynamics. Intracellular Ca++ responses evoke d by activation of the human alpha(7) nAChRs stably expressed in HEK-2 93 (human embryonic kidney) cells were studied. Inward current and int racellular Ca++ transients were recorded simultaneously in response to a fast drug application system. Current recording under whole-cell vo ltage-clamp and fast ratiometric intracellular Ca++ imaging acquisitio n were synchronized to drug pulses. The mean peak [Ca++](i) observed w ith 100 mu M (-)-nicotine was 356 +/- 48 nM (n = 8). The magnitude of the intracellular Ca++ elevation corresponds to a 20% fractional curre nt carried by Ca++ ions. The EC(50) of the intracellular Ca++ response s for (-)-nicotine, (+/-)-epibatidine, 1,1 dimethyl-4-phenyl-piperazin ium and acetylcholine were 51, 3.5, 75 and 108 mu M, respectively. The se EC(50) values strongly correlate with those recorded for the cation ic inward current through alpha(7) nAChR. alpha-Bungarotoxin, methyllc aconitine or extracellular Ca++ chelation ablated (-)-nicotine-evoked increase in intracellular Ca++ concentration. This study provides evid ence that cation influx through the human alpha(7) nAChR is sufficient to mediate a significant, transient, rise in intracellular Ca++ conce ntration.