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 MΩ at day 2 vs. 4.5 MΩ 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.