Na+ channel mis-expression accelerates K+ channel development in embryonic Xenopus laevis skeletal muscle.

1. The normal developmental pattern of voltage-gated ion channel expression in embryonic skeletal muscle cells of the frog Xenopus laevis was disrupted by introduction of cloned rat brain Na+ channels. 2. Following injection of channel mRNA into fertilized eggs, large Na+ currents were observed in muscle cells at the earliest developmental stage at which they could be uniquely identified. Muscle cells normally have no voltage-gated currents at this stage. 3. Muscle cells expressing exogenous Na+ channels showed increased expression of at least two classes of endogenous K+ currents. 4. This increase in K+ current expression was inhibited by the Na+ channel blocker tetrodotoxin, suggesting that increased electrical activity caused by Na+ channel mis-expression triggers a compensatory increase in K+ channel expression. 5. Block of endogenous Na+ channels in later control myocytes retards K+ current development, indicating that a similar compensatory mechanism to that triggered by Na+ channel mis-expression operates to balance Na+ and K+ current densities during normal muscle development.