RESUMO
Voltage-dependent outward currents were studied in freshly dissociated somata of locust lamina cells. These currents were recorded in 142 somata using the whole-cell patch-clamp technique. By measuring the reversal potential at altered external [K+] and by replacing internal K+ with Cs+, we determined that the outward currents were carried by K+. The outward currents consist of a transient A-type K+ current (KA) and a delayed-rectifier-like K+ current (KD). Amongst the cells studied, we observed two distinct groups of cells. The most obvious difference between the two groups is that in group I cells the total outward current is dominated by KA (KA/KD=12.5), whereas in group II cells KA makes a smaller contribution (KA/KD=2.1). Furthermore, in cells of group I, the KA current shows a steeper voltage-dependence of activation, where VG50 is -29.9 mV and s is 11.9 (N=22), and inactivation, where VI50 is -84.5 mV and s is -6.3 (N=18), compared with the KA current in cells of group II: VG50=-7.9 mV; s=26.6 (N=36) and VI50=-68.4 mV; s=-7.5 (N=21) (VG50 is the voltage at which the whole-cell conductance G is half-maximally activated, VI50 is the voltage of half-maximal inactivation and s is the slope of the voltage-dependence). The transient KA current in group I cells decayed mono-exponentially. The decay of the KA current in group II cells was fitted with a double-exponential curve and was significantly faster than in group I cells. In contrast to the large differences in KA currents, the KD currents appeared to be quite similar in the two groups of cells.
