Substrate effect on functions of embryonic chick heart

¹ Laboratories of Biochemistry, Howard Hughes Medical Institute; and the University of Miami School of Medicine, Miami, Florida

The 72-hr chick embryo heart has a rhythm resulting from the regular discharge of a pacemaker center and the subsequent conduction of the electrical impulse through the tubular heart. This phenomenon is recordable and results in an electrocardiogram indistinguishable in its general form from the adult mammalian record. The excised embryonic heart tissue placed in Tyrode's solution and maintained at 28.5–29.5 C continues to perform its electrical and contractile functions. General properties, i.e., deterioration of contractility, were studied and the effect of substrate alterations on this deterioration as well as the electrocardiogram was observed. The electrocardiogram is resistant to alterations in the concentration of potassium and becomes obliterated whenever sodium or calcium is absent from the fluid environment. The persistence of cardiac contraction similar to the control state occurred with certain anion alterations (I > NO₃ > Br > SCN), by the addition of glucose, and by an alkaline environment (pH 7.6–8.6). "Injury potentials," seen on the electrocardiogram, developed and were associated with a rapid deterioration of speed and intensity of contraction. This abnormal electrical potential appears to be linked to the glycolytic cycle, was absent in an alkaline environment and in a potassium-free solution, and was accentuated when the heart was placed in Tyrode's solution containing no chloride ions.