Abstract
The development of electrical excitability involves complex coordinated changes in ion channel activity. Part of this coordination appears to be due to the fact that the expression of some channels is dependent on electrical activity mediated by other channel types. For example, we have previously shown that normal potassium current development in embryonic skeletal muscle cells of the frog Xenopus laevis is dependent on sodium channel activity. To examine the interrelationships between the development of different ionic currents, we have made a detailed study of electrical development in cultured Xenopus myocytes using whole-cell patch-clamp recording. The initial expression of potassium, sodium, and calcium currents is followed by a brief period during which the densities of potassium currents decrease, while at the same time sodium and calcium current densities continue to increase, which may increase electrical excitability during this time. The normal developmental increase in both potassium and sodium currents is inhibited by the sodium channel blocker tetrodotoxin, suggesting that electrical activity normally stimulates the expression of both these currents. These effects of electrical activity appear to be mediated via activation of voltage-gated calcium channels. We suggest that the developmental acquisition of sodium and calcium channels by these cells, possibly coupled with a transient decrease in potassium current density, lead to an increase in electrical excitability and calcium entry, and that this calcium entry provides a critical developmental cue controlling the subsequent development of mature electrical properties.