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

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.