Oxidative and glycogenolytic capacities within the developing chick heart

Cardiac morphogenesis and function are known to depend on both aerobic and anaerobic energy-producing pathways. However, the relative contribution of mitochondrial oxidation and glycogenolysis, as well as the determining fact ors of oxygen demand in the distinct chambers of the embryonic heart, remai ns to be investigated. Spontaneously beating hearts isolated from stage 11, 20, and 24HH chick embryos were maintained in vitro under controlled metab olic conditions. O-2 uptake and glycogenolytic rate were determined in atri um, ventricle, and conotruncus in the absence or presence of glucose. Oxida tive capacity ranged from 0.2 to 0.5 nmol O2/(h . mug protein), did not dep end on exogenous glucose, and was the highest in atria at stage 20HH. Howev er, the highest reserves of oxidative capacity, assessed by mitochondrial u ncoupling, were found at the youngest stage and in conotruncus, representin g 75 to 130% of the control values. At stage 24HH, glycogenolysis in glucos e-free medium was 0.22 0.17, and 0.04 nmol glucose U(h . mug protein) in at rium, ventricle, and conotruncus, respectively. Mechanical loading of the v entricle increased its oxidative capacity by 62% without altering glycogeno lysis or lactate production. Blockade of glycolysis by iodoacetate suppress ed lactate production but modified neither O-2 nor glycogen consumption in substrate-free medium. These findings indicate that atrium is the cardiac c hamber that best utilizes its oxidative and glycogenolytic capacities and t hat ventricular wall stretch represents an early and major determinant of t he O-2 uptake. Moreover, the fact that O-2 and glycogen consumptions were n ot affected by inhibition of glyceraldehyde-3-phosphate dehydrogenase provi des indirect evidence for an active glycerol-phosphate shuttle in the embry onic cardiomyocytes.