Changes in membrane properties of chick embryonic hearts during development

The electrophysiological properties of embryonic chick hearts (ventricles) change during development; the largest changes occur between days 2 and 8. Resting potential (E(m)) and peak overshoot potential (+E(max)) increase, respectively, from -35 mv and +11 mv at day 2 to -70 mv and +28 mv at days 12-21. Action potential duration does not change significantly. Maximum rate of rise of the action potential (+V(max)) increases from about 20 v/sec at days 2-3 to 150 v/sec at days 18-21; + V(max) of young cells is not greatly increased by applied hyperpolarizing current pulses. In resting E(m) vs. log [K(+)](o) curves, the slope at high K(+) is lower in young hearts (e.g. 30 mv/decade) than the 50-60 mv/decade obtained in old hearts, but the extrapolated [K(+)](i) values (125-140 mM) are almost as high. Input resistance is much higher in young hearts (13 M ohm at day 2 vs. 4.5 M ohm at days 8-21), suggesting that the membrane resistivity (R(m)) is higher. The ratio of permeabilities, P(Na)/P(K), is high (about 0.2) in young hearts, due to a low P(K), and decreases during ontogeny (to about 0.05). The low K(+) conductance (g(K)) in young hearts accounts for the greater incidence of hyperpolarizing afterpotentials and pacemaker potentials, the lower sensitivity (with respect to loss of excitability) to elevation of [K(+)](o), and the higher chronaxie. Acetylcholine does not increase g(K) of young or old ventricular cells. The increase in (Na(+), K(+))-adenosine triphosphatase (ATPase) activity during development tends to compensate for the increase in g(K). +E(max) and + V(max) are dependent on [Na(+)](o) in both young and old hearts. However, the Na(+) channels in young hearts (2-4 days) are slow, tetrodotoxin (TTX)-insensitive, and activated-inactivated at lower E(m). In contrast, the Na(+) channels of cells in older hearts (> 8 days) are fast and TTX-sensitive, but they revert back to slow channels when placed in culture.