Hexose monophosphate shunt in isolated cardiac myocytes from normal rats

The activity of the hexose monophosphate shunt was studied in myocytes obtained from the ventricles of normal, adult, male rats. When myocytes were incubated in buffer containing either 1-14C- or 6-14C-labeled glucose the ratio of C-1/C-6 14CO2 evolved was essentially unity. The addition of plasma levels of amino acids did not alter this finding. If, however, a competitive substrate (pyruvate, octanoate, acetate, or lactate) was present, in sufficient quantity to lower the oxidation of glucose to approximately 20% of the control, the C-1/C-6 14CO2 ratio rose to values between 1.3 and 2.1. This ratio was dependent on the concentration of the competitive substrate, which was dependent on the buffer system. The data indicates that the hexose monophosphate shunt is active in the heart because it can be demonstrated when a substrate, which competes with glucose for oxidation, is present. The presence of competing substrates parallels the situation occuring in vivo.