Differential effects of hypoxia with age on the chick embryonic heart. Changes in membrane potential, intracellular K and Na, K efflux and glycogen.

The effects of hypoxia on different parameters of cell membrane function were studied in 7 and 19 day chick embryonic hearts. The following changes were observed: 1. Transmembrane potential: A depolarization of the cell membrane and a decrease in the duration and in the overshoot of the action potential. 2. Intracellular ion concentrations: A decrease in (K)i and an increase in (Na)i. Cellular Ca-content remained constant. 3. K efflux: An increase in the rate coefficient, which was larger in stimulated preparations. These changes were more pronounced in 19 day than in 7 day hearts. The effects of hypoxia were increased by simultaneous substrate depletion and counteracted by an excess external glucose. We conclude that: 1. The 19 day hearts are more sensitive to oxygen lack than the 7 day hearts. The difference can be correlated with the observation that the younger hearts are able to consume more glycogen during hypoxia. 2. The changes of the resting membrane potential and the overshoot of the action potential correlate with changes in respectively (K)i and (Na)i. 3. An increase in the background K current may be an important factor in explaining the shortening of the action potential during hypoxia.