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Journal of Neuroscience, Vol 11, 3047-3059, Copyright © 1991 by Society for Neuroscience
Characterization of single calcium channels in Drosophila embryonic nerve and muscle cells
HT Leung and L Byerly
Department of Biological Sciences, University of Southern California, Los Angeles 90089-2520.
Voltage-activated Ca channels play a central role in synaptic transmission,
control of cell excitability, and many other cellular processes. It is now
clear that there are multiple types of Ca channels with various modes of
modulation. Drosophila offers exceptional advantages for studying the
molecular basis of the diversity and modulation of Ca channels. As a step
in this study, we have characterized the single-channel Ba currents
recorded from cell- attached patches on cultured embryonic Drosophila nerve
and muscle cells. The voltage dependence and selectivity of the channels
carrying these Ba currents identify them as Ca channels. All Ca channels
found in Drosophila neurons appear to have the same voltage dependence of
activation and similar single-channel conductance, 12-17 pS (100 mM Ba2+).
However, the kinetic properties of individual Ca channels vary greatly. The
mean open time of individual channels ranges from 2 msec to less than 0.2
msec. Some channels completely inactivate during the first half of a 90
msec depolarization, while others are more active in the second half. Many
channels open during almost every depolarization, while others open in less
than 20% of the depolarizations. Channels with longer open times tend to
inactivate and open during a small fraction of depolarizations. When these
kinetic properties were quantified, a continuum of values was found,
instead of the clustering of values that might be expected for discrete
types of channels. Muscle Ca channels form a more homogeneous class than do
the neuronal Ca channels. The muscle Ca channel conductance is 18 pS. These
channels do not inactivate during 90 msec depolarizations and open during a
majority of depolarizations applied. Muscle Ca channels are similar to a
subset of neuronal Ca channels. When a purified toxin from the spider
Hololena curta is applied to neurons, the number of active Ca channels is
reduced, and those channels still active open in a small fraction of
depolarization. Since channels that open in a small fraction of
depolarizations tend to inactivate, these data support the hypothesis that
this toxin selectively blocks noninactivating neuronal Ca channels. This
differential toxin sensitivity and the much larger variability observed in
kinetic properties of neuronal, compared to muscle, Ca channels suggest
that there are at least two types of neuronal Ca channels in Drosophila.
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