Dependence of Ca2+ and K+ current development on RNA and protein synthesis in muscle-lineage cells of the ascidian Boltenia villosa.

The early development of excitability of muscle-lineage cells of the ascidian Boltenia villosa is characterized by the appearance, just after gastrulation, of a Ca2+ current and a delayed outward K+ current, while an inwardly rectifying K+ current, present since fertilization, disappears. The muscle-lineage cells are the first cells in which we detect tissue-specific electrical properties after gastrulation. Here, we show that the development of electrical properties in these cells involves RNA and protein synthesis. If transcription or translation is blocked, the Ca2+ and outward K+ currents fail to appear, whereas the inward K+ current disappears normally. For the Ca2+ current, the sensitive period for transcription extends until just before gastrulation, while the sensitive period for translation extends until after gastrulation. The oocyte has a Ca2+ current present at about 5-10% the density of that in the muscle-lineage cells; this current disappears by gastrulation. A comparison of the oocyte and muscle Ca2+ currents indicates that they are similar in voltage dependence and inactivation mechanism. A small difference in permeability sequence can be attributed to different surface charge properties at the two stages of development.