Functional differences between ACh receptor channels containing gamma and epsilon subunits

The nicotinic acetylcholine (ACh) receptor exhibits functional heterogeneity to the extent that at least 5 different channel classes can be identified in skeletal muscle. We have explored the possibility that this heterogeneity results from differences in subunit composition. ACh receptors were expressed in Xenopus oocytes from various combinations of RNAs coding for the αβγδε subunits of mammalian skeletal muscle. Functional classification of the channel types was based on single channel measurements of conductance and open time from outside-out patch recordings. Injection of RNAs encoding αβγδ reconstitutes 3 classes of long open time receptor channels similar to those observed in embryonic and denervated muscle. Substitution of ε for γ resulted in three different classes of channels, all bearing brief mean channel open times. Our findings support the proposal by Sakmann and co-workers that substitution of the ε subunit for γ subunit imparts the developmental shortening in channel open time. In addition, our findings point to at least two other mechanisms underlying functional heterogeniety, one involving subunit deletion.