Drosophila mutants reveal two components of fast outward current

The gating of potassium ion channels has been shown to be dependent on voltage¹ or Ca²⁺ ions^2,3 or both^4,5. A fast transient potassium current (sometimes denoted I_A) is found in a wide variety of animals^6-14. The Ca²⁺-sensitivity of this early outward current has been a matter of dispute as reports from different systems have indicated complete insensitivity^6-8, marked sensitivity^10-12 or only partial sensitivity^13,14. It is possible that there are two distinct early outward current systems, one Ca ²⁺-sensitive and the other not. Thus, the reports of partial Ca ²⁺-sensitivity would indicate the presence of both systems in the membrane. I now report that in the adult Drosophila flight muscles, the transient outward current is also partially Ca²⁺-sensitive. The hypothesis that two separate currents are present is strengthened by the discovery that in several mutants of the X-linked Shaker locus (Sh ^KS133, Sh¹⁰² and Sh^K0120; refs^15-17) the Ca²⁺-independent component (I_Aci) is absent with only the Ca²⁺-dependent component (I_Acd) remaining. Hence, the mutations seem to delete one of two separate current systems. An alternative hypothesis is that only one channel type is present that can be modified by mutation to be totally Ca²⁺-dependent.