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Variations in onset of action potential broadening: effects on calcium current studied in chick ciliary ganglion neurones

John M. Pattillo, Debra E. Artim, James E. Simples Jr and Stephen D. Meriney

The voltage dependence and kinetic properties of stage 40 ciliary ganglion calcium currents were determined using short (10 ms) voltage steps. These properties aided the interpretation of the action potential-evoked calcium current described below, and the comparison of our data with those observed in other preparations.

Three different natural action potential waveforms were modelled by a series of ramps to generate voltage clamp commands. Calcium currents evoked by these model action potentials were compared before and after alterations in the repolarization phase of each action potential.

Abrupt step repolarizations from various time points were used to estimate the time course of calcium current activation during each action potential. Calcium current evoked by fast action potentials (duration at half-amplitude, 0·5 or 1·0 ms) did not reach maximal activation until the action potential had repolarized by 40-50 %. In contrast, calcium current evoked by a slow action potential (duration at half-amplitude, 2·2 ms) was maximally activated near the peak of the action potential.

Slowing the rate of repolarization of the action potential (broadening) from different times was used to examine effects on peak and total calcium influx. With all three waveforms tested, broadening consistently increased total calcium influx (integral). However, peak calcium current was either increased or decreased depending on the duration of the control action potential tested and the specific timing of the initiation of broadening the repolarization phase.

The opposite effects on peak calcium current observed with action potential broadening beginning at different time points in repolarization may provide a mechanism for the variable effects of potassium channel blockers on transmitter release magnitude.

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