Potassium conductance and internal calcium accumulation in a molluscan neurone

1. The Aplysia neurone R-15 was injected with the Ca 2+ sensitive dye arsenazo III. Changes in dye absorbance were measured with a differential spectrophotometer to monitor changes in the free internal Ca 2+ concentration, [Ca] i , during membrane depolarization and during intracellular Ca 2+ ion injection under voltage clamp conditions. 2. The absorbance change, and thus [Ca] i , increases linearly with Ca 2+ injection intensity at constant duration. The absorbance change produced by a constant intensity Ca 2+ injection also increases with injection duration, but this increase is asymptotic. 3. The Ca 2+ activated K + current, I K, Ca , increases linearly with the increase in [Ca] i and its rise and decay follows closely the time course of the absorbance change produced by internal Ca 2+ injection. 4. The Ca 2+ activated K + conductance increases exponentially with membrane depolarization. The increase in K + conductance activated by a constant intensity and duration Ca 2+ injection is on average e-fold for a 25.3 mV change in membrane potential. 5. The difference in net outward K + current measured during depolarizing pulses to different membrane potentials in normal and in Ca 2+ free ASW was used as an index of I K, Ca . Its time course was approximately linear for the first 50-100 msec of depolarization, but for longer times the relation approached a maximum. Simultaneous measurements of the arsenazo III absorbance changes were broadly consistent with the activation of I K, Ca being brought about by the rise in [Ca] i during a pulse. 6. The relation between Ca 2+ activated K + conductance and membrane potential is bell shaped and resembles the absorbance vs . potential curve, but its maximum is displaced to more positive membrane potentials. The shift in the two curves on the voltage axis can be explained by the potential dependence of G K, Ca . 7. The net outward K + current measured with depolarizing voltage pulses in normal and in Ca 2+ free ASW is increased when [Ca] i is elevated by internal Ca 2+ injection. With large and prolonged Ca 2+ injections the net outward current is depressed following the decline of [Ca] i . 8. The time and frequency dependent depression of the net outward K + current which occurs during repetitive stimulation is shown to have no obvious temporal relation to the increase in [Ca] i . The depression is relieved by an increase in [Ca] i caused by internal Ca 2+ injection. 9. The net outward K + current measured with brief