Development of ion channels in the flight muscles of Drosophila.

An entire picture of developing membrane electrical properties can be observed in the flight muscles (DLM) of Drosophila. The developmental history of membrane electrogenesis begins in the mid-pupal period and extends into the second day of adult life. Of five prominent extra-junctional ion currents which can be observed, only two are clearly mature before the adult ecloses from the pupal case. These are the two voltage-activated potassium currents, a fast transient current, and a slowly activating current. A fast transient calcium current rapidly develops around the time of adult eclosion. Suprisingly, two more ion currents develop in the adult stage: a fast transient Ca2+-activated potassium current develops during the first few hours of adult life, and a slow noninactivating inward current develops during the following two days. Both the earlier and later developing potassium currents of the transient type function in the role of fast spike repolarization in the adult. However, the later developing current appears to largely supplant the earlier developing current in this role. Thus, Shaker mutants which specifically lack the earlier developing K+ current, nevertheless, have normal appearing action potentials in mature muscle cells.