Onset and offset of calcium current in response to short, depolarizing voltage pulses were studied in giant synaptic terminals of goldfish bipolar neurons. Tail-current decays were extremely rapid (80/μs at -70 mV), and exponentially slowed at more positive repolarization potentials. The amplitude of tail current following voltage pulses varied with the pulse amplitude, according to a Bolzmann distribution with a V(1/2) of -27.6 mV and a slope factor of 13.8 mV. When interpreted with a Hodgkin-Huxley model, calcium-current onset was best described by m2 kinetics and a time constant (τ(m)) of 0.62 ms at -10 mV. The kinetics of calcium-current onset and offset are at least two- to threefold faster than those described in other native cells. Combined with other recent data, the results suggest that the rapid kinetics may participate in a fast signaling mode in the goldfish ON-center pathway.
- Calcium channel