ACTIVATION AND INACTIVATION OF THE BURSTING POTASSIUM CHANNEL FROM FUSED TORPEDO-SYNAPTOSOMES

1. The voltage dependence of the bursting potassium channel in fused s ynaptosomes from Torpedo electric organ was studied in vitro, using th e inside-out and the cell-attached configurations of the patch clamp t echnique. 2. The patch of membrane was held at various holding potenti als (-140 to -50 mV) and then stepped to test potentials (-50 to +40 m V) for periods ranging from 5 to 300 ms. Each potential step was repea ted 200-600 times. After subtraction of the capacitative transients an d the leakage currents, an ensemble- averaged current was obtained. Th is ensemble current showed a marked activation upon depolarization, fo llowed by an inactivation. 3. The activation of the bursting potassium channel is markedly dependent on the voltage step. Activation was det ected at voltages positive to -50 mV. The peak of the ensemble current increases with the degree of depolarization, while the time to the pe ak decreases. With progressively larger depolarization, there is a sho rtening in the delay between the onset of the voltage step and the ope ning of the bursting potassium channels. 4. The inactivation phase of the ensemble current could be described adequately in most of the expe riments, as a single exponential decay to a steady-state inactivation level. The time constant of inactivation was not markedly voltage depe ndent. 5. Single channel analysis of the inactivation reveals that it is due to a reduction in the number of channel openings and not due to changes in single channel current amplitude or channel mean open time along the pulse. 6. The holding potential has a marked effect on the peak amplitude of the ensemble current, indicating that hyperpolarizat ion removes inactivation and depolarization induces it. The peak ampli tude vs. voltage relation was fitted by the Boltzmann equation. The ha lf-maximal inactivation was -105.2 +/- 5.8 mV (mean +/- S.E.M.), sugge sting that at the resting potential a substantial fraction of the burs ting potassium channels is in an inactivated state. 7. Two-pulse exper iments show that the recovery from inactivation is a slow process whic h lasts well over 1 s. 8. High-frequency stimulation (20-66.7 Hz) by 5 ms pulses produces a progressive decline in the peak ensemble current amplitude. The decline is larger at higher stimulation frequencies. 9 . The voltage- and time-dependent activation and inactivation properti es of the bursting potassium channel make it a possible candidate for participating in frequency modulation of transmitter release and thus of synaptic transmission. We propose the potassium inactivation hypoth esis for frequency modulation, which states that the inactivation of t he potassium channel by previous stimulation could cause a broadening of the subsequent action potential and hence augmentation of calcium e ntry and transmitter release.