Developmental sequence of expression of voltage-dependent currents in embryonic Xenopus laevis myocytes.

Although the development of several of the voltage-dependent currents in embryonic amphibian myocytes has been described, the overall muscle electrical development, particularly the relative times of expression of different voltage-dependent currents, has not been addressed in a single study under one set of conditions. We have found that, in mesoderm isolated and cultured from neurula stage embryos, myocytes are identifiable before they express voltage-gated currents. These ionic currents are absent from all Xenopus mesodermal cells during the late gastrula/early neurula stages of embryonic development. At about the time of first somite segregation an inward rectifier K+ current is expressed in some myocytes, followed within 2 hr by a delayed rectifier K+ current. The density of both currents increases fourfold over the next 24 hr in culture. A Na+ current is not expressed in large numbers of myocytes until late in this culture period, at about the time that a slow Ca2+ current appears. Under our culture conditions the myocytes have a very low chloride conductance. A fast inactivating component to the outward K+ current is expressed in all myocytes by 24 hr in culture. In some experiments we dissociated embryos at later times and made recordings when all previously isolated myocytes expressed currents. In the late dissociations, most myocytes did not express currents, but developed them after a short period in culture. Because we have evidence that in vivo development is more closely approximated by the early dissociations, these results suggest that dissociation causes some degree of dedifferentiation.