Energy Metabolism in Rat Fetal Hearts Just after the Initiation of Heartbeat is Altered via Increased Glycolytic Flux and Activated Mitochondrial Function

We previously demonstrated that the heartbeat in a rat embryonic heart initiates at around embryonic day 10.0 (E10.0) with a calcium transient via extracellular calcium influx. Although ATP demand during this phase of development is increased to maintain the excitation-contraction coupling, it remains unclear how energy metabolism is altered just after the initiation of heartbeat. The embryos at E10.0 in Wistar rats were divided into two groups by the hearts without (pre-) or with (post-) heartbeat. Microarray pathway analysis showed that among up-regulated genes in KEGG pathways in post-heartbeat group compared with pre-heartbeat group, the pathway of glycolysis/gluconeogenesis ranked next to the best matched pathway of striated muscle contraction, supporting the notion that changes in energy metabolism play pivotal roles in initiation of the heartbeat. According to the principal component analysis in the metabolomic analysis, fructose 6-phosphate, fructose 1,6-bisphosphate, which are intermediate metabolites in glycolytic pathway, were major factor loading components in pre-heartbeat group, whereas GTP, reduced glutathione, and ATP, which are associated with mitochondrial respiration, were major determinants in post-heartbeat group, suggesting increased mitochondrial respiration just after initiation of the heartbeat. Finally, in isolated embryonic myocytes, baseline oxygen consumption rate (OCR) assessed by an extracellular flux analyzer was mostly very low in both groups; however, increased response in OCR by adding mitochondrial uncoupler FCCP was observed in post-heartbeat group but not in pre-heartbeat group, suggesting increased substrate availability in post-heartbeat group. The findings suggest that increased aerobic glycolysis with increased gene expressions and increased mitochondrial respiration capacity contribute to energy source to maintain heartbeat just after the initiation of heartbeat.