Evidence of extensive plasma glucose recycling following a glucose load in seabass

Seabass and other carnivorous fish are highly dependent on gluconeogenesis from dietary amino acids to maintain glycemia. Glucose recycling (glucose → C3-intermediate → glucose) may potentiate the effects of glucose administration in sparing amino acid gluconeogenesis. To date, very few measurements of glucose recycling have been reported in fish. Thus, to determine the extent of glucose recycling following a glycemic challenge, juvenile seabass were given an intraperitoneal glucose load (2 g kg− 1) enriched with [U-13C]glucose. 13C NMR analysis of plasma glucose 13C-isotopomers was used to determine the fractional contributions of glucose derived directly from the load versus that from glucose recycling at 48 h after the load. Both fed and 21-day fasted fish (20 per condition) were studied. In fasted fish, 18 ± 4% of plasma glucose was directly derived from the load while 13 ± 2% was derived from glucose recycling. In fed fish, the load accounted for 6 ± 1% of plasma glucose levels while glucose recycling contributed 16 ± 4%. 13C NMR analysis of plasma lactate revealed 13C-isotopomers corresponding to the expected C3-intermediates of peripheral [U-13C]glucose catabolism indicating that circulating lactate was a key intermediate in glucose carbon recycling under these conditions. In conclusion, glucose recycling was shown to contribute a significant portion of plasma glucose levels in both fed and fasted seabass 48 h after an intraperitoneal glucose challenge and circulating lactate was shown to be an intermediate of this pathway.