Abstract
Early in development, ascidian muscle cells generate spontaneous, long-duration action potentials that are mediated by a high-threshold, inactivating Ca
2+
current. This spontaneous activity is required for appropriate physiological development.
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 Ca
2+
current.
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 Ca
2+
currents are optimized to their particular functions.
Total Ca
2+
entry during an action potential did not change during development. The developmental increase in Ca
2+
current density exactly compensated for decreased spike duration. This compensation was a function purely of Ca
2+
current density, not of the transition from immature to mature Ca
2+
current types.
In immature cells, Ca
2+
entry was spread out over the entire waveform of spontaneous activity, including the interspike voltage trajectory. This almost continuous Ca
2+
entry may be important in triggering Ca
2+
-dependent developmental programmes, and is a function of the slightly more negative voltage dependence of the immature Ca
2+
current.
In contrast, Ca
2+
entry in mature cells was confined to the action potential itself, because of the slightly more positive voltage dependence of the mature Ca
2+
current. This may be important in permitting rapid contraction-relaxation cycles during larval swimming.
The inactivation of the immature Ca
2+
current serves to limit the frequency and burst duration of spontaneous activity. The sustained kinetics of the mature Ca
2+
current permit high-frequency firing during larval swimming.
