Characterization of single calcium channels in Drosophila embryonic nerve and muscle cells.

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.