Metabolite utilization by isolated embryonic rat hearts in vitro

Isolated hearts from 11-, 12- and 13-day rat embryos were incubated in a simple defined salt solution to which was added a variety of single substrates. Utilization of the added substrate was determined by comparing the contractile rates of the hearts in the presence and absence of the compound being tested. Of all the compounds tested only those involved in the Embden-Meyerhof glycolytic pathway were capable of maintaining cardiac contraction at a maximum rate in the 11-day heart. This was accomplished under both aerobic and anaerobic conditions. Although glycolysis remained important, the 12- and 13-day hearts exhibited a shift in dependence towards other metabolic pathways. This conclusion was based on the observations that anaerobic glycolysis could no longer maintain maximum heart rates and that a variety of non-glycolytic compounds could be utilized for contractile activity by the 12- and 13-day organs.