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

1. 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 +/- S.E.) 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. 2. 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 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 potassium 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. 3. 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.