CHANGES IN SENSITIVITY TO HYPOXIA AND GLUCOSE DEPRIVATION IN ISOLATED PERFUSED RABBIT HEART DURING PERINATAL DEVELOPMENT

The isometric contraction of the isolated rabbit myocardium was measured from 24 days post coitum (dpc) to young adulthood. Tension per gram of heart as developed by the isolated perfused hearts remained constant during late fetal life but increased during the 1st postnatal week. Sensitivity to hypoxia rapidly increased during fetal life 26-28 post coitum. In young fetal hearts (up to 28 days post coitum), contraction continued for several hours in the absence of glucose. In contrast, from 28 days post coitum onwards fetal hearts became increasingly dependent on external glucose to maintain their contractility. This change was concomitant with a decrease in myocardial glycogen content. Intracellular electrical activity recorded in the absence of glucose showed that during hypoxia the duration and amplitude of the action potential in the fetus at term were reduced, whereas normal activity continued in the same hypoxic glucose-free medium in hearts from fetuses 26 days post coitum. The relative role of glycolysis and oxidative metabolism is discussed and the importance of glycogenolytic metabolism in young isolated fetal hearts is pointed out.