Developmental aspects of potassium flux and permeability of the embryonic chick heart.

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Abstract
1. The rate coefficient of 42K efflux, the transmembrane potential, the intracellular concentrations of Na and K and the volume/surface area have been measured in embryonic chick hearts of different ages. 2. With respect to age, the rate coefficient for 42K efflux was minimal for preparations from 6‐8 day old embryos, and distinctly higher values were obtained for the hearts of 3‐5 and 18‐20 days. With respect to the effect of external K concentration (Ko), all age groups showed a five‐ to sevenfold increase in rate coefficient between 2‐5 and 140 mM‐Ko. The effect of Ko was found to be indepedent of extracellular Na, except in the 18‐20 day hearts bathed in K‐free solution. 3. Intracellular concentrations of K and Na were found to decrease, membrane potential to increase with age. The volume/surface area measured by stereologic and morphometric techniques did not change with age. 4. The permeability coefficient for K (PK), calculated from the absolute K flux and the measured membrane potentials, was fairly constant for a given age between 2‐5 and 20 mM‐Ko. In K‐free solution, PK was markedly reduced (factor 4). At a given Ko, PK increased twofold between 6‐8 and 18‐20 days while PNa remained relatively constant.