Human neuronal threonine-for-leucine-248 α7 mutant nicotinic acetylcholine receptors are highly Ca2+ permeable

A cDNA coding for the human neuronal nicotinic alpha 7 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 alpha 7 nicotinic AcCho receptor (nAcChoR) mutant (L247T alpha 7). These properties were all substantially maintained when the human L248T alpha 7 mutant was transiently expressed in human Bose 23 cells. Simultaneous whole-cell clamp and fluorescence measurements with the Ca2+ indicator dye Fura-2 showed that nicotine induced a Ca2+ influx in standard 2 mM Ca2+ solution. The average fractional Ca2+ current flowing through L248T alpha 7 nAcChoRs was 6.7%, which is larger than that flowing through muscle alpha beta epsilon delta nAcChoRs (4.1%). The relative Ca2+ permeability, determined in oocytes in the absence of Cl-, was measured from the shift in reversal potential caused by increasing the external Ca2+ concentration from 1 to 10 mM. The human wild-type alpha 7 nAcChoR was found to be more permeable than the L248T alpha 7 mutant to Ca2+. Our findings indicate that the Ca2+ permeability of the homomeric alpha 7 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.