Intracellular potassium and sodium activities of chick ventricular muscle during embryonic development.

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Abstract
The basis of the resting potential of chick embryo ventricular muscle was studied by use of ion-selective micro-electrodes. Membrane resting potential hyperpolarized from -65.4 .+-. 1.1 mV (mean .+-. SE) at age 4 day to -75.8 .+-. 0.6 mV at age 18 day. Action potential overshoot increased from +19.8 .+-. 0.9 at age 4 day to +33.1 .+-. 0.6 mV at age 18 day. Intracellular K+ activity measured with ion-selective micro-electrodes increased from 71.3 .+-. 1.9 mM at age 4 day to 89.9 .+-. 1.1 mM at age 18 day. Intracellular Na+ activity decreased from 12.5 .+-. 0.4 to 7.0 .+-. 0.3 mM during the same period. The difference between membrane resting potential and the calculated K equilibrium potential decreased with development. PNa/PK estimated from the constant field equation decreased from 0.012 at age 4 day to 0.005 at age 18 day. The hyperpolarization of resting potential and the increased action potential overshoot during development could be explained by a rise in intracellular K+ activity and a fall in intracellular Na+ activity, as if the Na-K exchange pump became more active.