Journal of Neuroscience, Vol 8, 428-434, Copyright © 1988 by Society for Neuroscience
The roles of potassium currents in Drosophila flight muscles
T Elkins and B Ganetzky
Department of Zoology, University of Wisconsin, Madison 53706.
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)
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