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Action potential waveform voltage clamp shows significance of different Ca²⁺ channel types in developing ascidian muscle

Julia E. Dallman, Jennie B. Dorman and William J. Moody

1. Early in development, ascidian muscle cells generate spontaneous, long-duration action potentials that are mediated by a high-threshold, inactivating Ca²⁺ current. This spontaneous activity is required for appropriate physiological development.

2. Mature muscle cells generate brief action potentials only in response to motor neuron input. The mature action potential is mediated by a high-threshold sustained Ca²⁺ current.

3. Action potentials recorded from these two stages were imposed as voltage-clamp commands on cells of the same and different stages from which they were recorded. This strategy allowed us to study how immature and mature Ca²⁺ currents are optimized to their particular functions.

4. Total Ca²⁺ entry during an action potential did not change during development. The developmental increase in Ca²⁺ current density exactly compensated for decreased spike duration. This compensation was a function purely of Ca²⁺ current density, not of the transition from immature to mature Ca²⁺ current types.

5. In immature cells, Ca²⁺ entry was spread out over the entire waveform of spontaneous activity, including the interspike voltage trajectory. This almost continuous Ca²⁺ entry may be important in triggering Ca²⁺-dependent developmental programmes, and is a function of the slightly more negative voltage dependence of the immature Ca²⁺ current.

6. In contrast, Ca²⁺ entry in mature cells was confined to the action potential itself, because of the slightly more positive voltage dependence of the mature Ca²⁺ current. This may be important in permitting rapid contraction-relaxation cycles during larval swimming.

7. The inactivation of the immature Ca²⁺ current serves to limit the frequency and burst duration of spontaneous activity. The sustained kinetics of the mature Ca²⁺ current permit high-frequency firing during larval swimming.

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