Human neuronal threonine-for-leucine-248 α7 mutant nicotinic acetylcholine receptors are highly Ca(2+) permeable

A cDNA coding for the human neuronal nicotinic α7 receptor subunit with Leu-248 mutated to threonine was expressed in Xenopus oocytes. When activated by acetylcholine (AcCho), the receptors expressed generated currents that had low desensitization, linear current–voltage relation, and high apparent affinity for both AcCho and nicotine. These characteristics are similar to those already described for the chick threonine-for-leucine-247 α7 nicotinic AcCho receptor (nAcChoR) mutant (L247Tα7). These properties were all substantially maintained when the human L248Tα7 mutant was transiently expressed in human Bosc 23 cells. Simultaneous whole-cell clamp and fluorescence measurements with the Ca(2+) indicator dye Fura-2 showed that nicotine induced a Ca(2+) influx in standard 2 mM Ca(2+) solution. The average fractional Ca(2+) current flowing through L248Tα7 nAcChoRs was 6.7%, which is larger than that flowing through muscle αβɛδ nAcChoRs (4.1%). The relative Ca(2+) permeability, determined in oocytes in the absence of Cl(−), was measured from the shift in reversal potential caused by increasing the external Ca(2+) concentration from 1 to 10 mM. The human wild-type α7 nAcChoR was found to be more permeable than the L248Tα7 mutant to Ca(2+). Our findings indicate that the Ca(2+) permeability of the homomeric α7 nAcChoR is larger than that of the heteromeric neuronal nicotinic receptors studied to date and is possibly similar to that of the N-methyl-d-aspartate subtype of brain glutamate receptors.