Comparison of ionic currents expressed in immature and mature muscle cells of an ascidian larva.
We have compared the voltage-gated ion channels present in larval ascidian muscle at two developmental stages: muscle precursor cells just after the terminal cell division and mature contractile muscle, 7-11 hr later. All precursor cells express a high-threshold transient Ca current and a slowly activating delayed K current, and about half the cells express a low-threshold transient Ca current. An inwardly rectifying K current, which had been present from fertilization until just before the terminal cell division, is absent. Mature muscle retains two of the tailbud currents: the low-threshold transient Ca current and the slow delayed K current, although at larger densities, and also expresses a high-threshold Ca current that is similar in most respects to the precursor cell current but that lacks inactivation. In addition, mature muscle expresses two rapidly activating outward K currents, one voltage and one Ca dependent, that generate a composite outward K current that is eight times larger and activates eight times faster than the tailbud K current. Mature muscle also reexpresses the inward rectifier. We propose that the transient absence of the inward rectifier and the slow activation of the delayed K current early in development create a window of developmental time when spontaneous electrical activity is likely.
Davis
A K
AK
Department of Zoology, University of Washington, Seattle 98195, USA.
Greaves
A A
AA
Dallman
J E
JE
Moody
W J
WJ
D000200:Action Potentials; D000375:Aging / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology; D016922:Cellular Senescence; D004553:Electric Conductivity; D007477:Ions; D007814:Larva; D009119:Muscle Contraction; D024510:Muscle Development; D009132:Muscles / Q000166:cytology / Q000502:physiology*; D011188:Potassium / Q000502:physiology; D013234:Stem Cells / Q000502:physiology; D014561:Urochordata / Q000254:growth & development / Q000502:physiology*
D007477:Ions; D011188:Potassium; D002118:Calcium