Development of the fast sodium current in early embryonic chick heart cells

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Abstract
Single ventricle cells were dissociated from the hearts of two-, theree-, four-, or seven-day-old chick embryos, and were maintained in vitro for an additional 6 to 28 hr. Rounded 13 to 18 μm cells with input capacitance of 5 to 10 pF were selected for analysis of fast sodium current (I Na). Voltage dependence, and kinetics ofI Na were applied with patch electrodes in the wholecell clamp configuration.I Na was present in over half of the 2d, and all 3d, 4d and 7d cells selected. The current showed no systematic differences in activation kinetics, voltage dependence, or tetrodotoxin (TTX) sensitivity with age or culture condition, Between the 2d and 7d stages, the rate of current inactivation doubled an channel density increased about eighfold. At all stages tested,I Na was blocked by TTX at a half-effective concentration of 0.5 to 1.0 nM. We conclude that the lack of Na dependence of the action potential upstroke on the second day of development results from the relatively depolarized level of the diastolic potential, and failure to activate the small available excitatory na current. The change from Ca to Na dependence of the upstroke during the third to the seventh day of incubation results partly from the negative shift of the diastolic potential during this period, and in part from the increase in available Na conductance.