OXIDATION OF ENERGY SUBSTRATES IN ENTEROCYTES OF HYBRID STRIPED BASS Morone chrysops × M. saxatilis

In most mammals, enterocytes are major users of glutamine and glutamate to product ATP, citrulline and arginine. It is unknown whether this is also true for fish species. The present study was conducted with enterocytes from hybrid striped bass (HSB) to determine oxidation of energy substrate. Isolated enterocytes were incubated at 26 °C for 30 min in in oxygenated Krebs-Henseleit bicarbonate buffer (pH 7.4, with 5 mM d-glucose) containing 2 mM L-[U-¹⁴C] glutamate, L-[U-¹⁴C] glutamine, L-[U-¹⁴C] alanine, L-[U-¹⁴C] aspartic acid, L-[U-¹⁴C] leucine, or L-[U-¹⁴C] palmitate, or a trace amount of D-[U-¹⁴C] glucose. In parallel experiments, the incubation medium contained in a mixture of unlabeled substrates [glutamate, glutamine, alanine, aspartic acid and leucine (2 mM each) plus 5 mM D-glucose] with one tracer. ¹⁴CO₂ was collected to calculate the rates of substrate oxidation and ATP production from those nutrients.

Results indicated that in the presence of glucose or a mixture of substrates, the rates of oxidation of glutamate and ATP production from glutamate by the enterocytes were much higher than those for other amino acids, palmitate and glucose. Compared with palmitate and glucose, fish enterocytes preferred to use amino acids to produce ATP. There was no synthesis of citrulline or arginine from glutamate or glutamine in these cells. We conclude that glutamate was the primary energy substrate in HSB enterocytes. Together, amino acids contributed to about 87% of ATP production in the presence of a mixture of substrates. Our novel findings aid in better understanding of protein nutrition in fish.