The roles of potassium currents in Drosophila flight muscles

The roles of different K+ currents in regulating the generation and waveform of action potentials in Drosophila dorsal longitudinal flight muscles (DLMs) were examined in current-clamp experiments. In response to depolarizing current, DLMs displayed an initial transient rectification of the electronic potential lasting for up to hundreds of milliseconds. This delay in excitation was followed by oscillations or graded spikes that finally gave way to sharply rising spikes. Previous voltage-clamp studies of DLMs have revealed an inward Ca2+ current and at least three K+ currents: IA and IK, which are voltage-dependent, and IC, which is Ca2+ dependent. IA and IC are early inactivating currents, while IK is a slow, noninactivating current. In mature adults, selective elimination of IA either with Shaker (Sh) mutations or with 4-aminopyridine (4-AP), had no effect on spike duration or on the delay in excitation. In contrast, when IC was specifically eliminated with the slowpoke (slo) mutation, there was no delay before excitation, the amplitude of the spikes was significantly increased, and the spike duration was increased by 10-fold. Similar results were obtained by reducing IC in normal muscle by intracellular injections of EGTA or by use of low Ca2+ saline. Furthermore, DLM spikes evoked in slo by stimulation of the motorneuron were also broadened, suggesting that IC functions in a similar manner during normal flight as in current-clamped muscles. Elimination of IK along with IA and IC in saline containing tetraethylammonium or Ba2+ resulted in further prolongation of the DLM spike. In Ba2+ saline, there was an additional increase in spike amplitude as well. We conclude that in mature adults, IC, rather than IA, plays the major role in repolarization of DLM spikes and in the delay before excitation.(ABSTRACT TRUNCATED AT 250 WORDS)