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A cDNA coding for the human neuronal nicotinic alpha7 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 alpha7 nicotinic AcCho receptor (nAcChoR) mutant (L247Talpha7). These properties were all substantially maintained when the human L248Talpha7 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 L248Talpha7 nAcChoRs was 6.7%, which is larger than that flowing through muscle alpha(beta)epsilon(delta) 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 alpha7 nAcChoR was found to be more permeable than the L248Talpha7 mutant to Ca(2+). Our findings indicate that the Ca(2+) permeability of the homomeric alpha7 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.
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