Two distinct calcium-activated potassium currents in larval muscle fibres ofDrosophila melanogaster

The non-synaptic membrane currents of muscle fibres have been studied in late embryogenesis ofDrosophila melanogaster using the voltage-clamp technique in wild-type andShaker mutant third instar larvae. Five currents were found in the wild type muscle membrane at this embryonic stage: one fast inward Ca current (I_Ca), two fast outward K currents (I_A and I_Acd) and two slow outward K currents (I_K and I_C). I_Acd and I_C are Ca-dependent.Several procedures were used to separate I_Acd from I_A: I_Acd is present inShaker mutants which are characterized by the absence of I_A (Salkoff and Wyman 1981); I_Acd, but not I_A, is suppressed by Co²⁺ (10 mM) or La³⁺ (1 mM); I_Acd shows steady-state inactivation at more positive potentials than I_A; I_Acd, unlike I_A, is 3,4-diaminopyridine (3,4-DAP) resistant. Furthermore, tetraethylammonium (TEA, 20 mM) which is known to be uneffective on I_A, blocks I_Acd. I_Acd could not be triggered by using strontium or barium as calcium substitutes. By partial substitution of Ca by Ba or Sr ions, it was found that Ba, but not Sr, blocks the I_Acd channel.A non-inactivating, TEA sensitive, Ca-dependent K current (I_C), which gave N-shaped I-V plots, could be separated from I_K by using Ca-channel blockers. I_C and I_K activate at membrane potentials of about –25 mV and –10 mV, respectively.The participation of I_Acd and I_C to membrane electrophysiology is discussed.