Plasma levels of glucose, alanine, lactate, and β‐hydroxybutyrate after mammalian insulin treatment in the American bullfrog (Rana catesbeiana)

Abstract

AbstractSerial blood samples were collected from the adult American bullfrog (Rana catesbeiana) via a non‐occlusive cannula chronically placed in the systemic subdivision (aortic) of the right truncus arteriosus. Plasma glucose, alanine, lactate, and β‐hydroxybutyrate levels were estimated by standard enzymatic procedures. The metabolites remained relatively stable in the control animals during the 8‐day experiments. A single infusion of mammalian insulin (5, 15, or 45 IU/kg body weight) via the cannula depressed plasma glucose and alanine to near or below detectable levels that required a minimum of 3 days to return to approximately the time‐zero levels. Plasma lactate also fell severely after the infusion, but subsequently displayed precipitous surges followed by sharp declines on days 1 and 3, and also in some animals on day 5. In contrast, plasma β‐hydroxybutyrate levels were not significantly altered by insulin. Despite the severe depressions of plasma glucose, alanine, and initially lactate, no symptoms were observed in the bullfrogs infused with 5 or 15 IU of insulin. The carnivorous adult bullfrog consumes abundant protein and lipid, but little carbohydrate, and is adapted to periods of food scarcity. The results are consistent with the hypothesis that the bullfrog may rely on non‐esterified fatty acids and ketone bodies as primary energy sources. Glucose, most of which must be synthesized, probably serves to maintain muscle glycogen reserves that are used for anaerobic glycolysis when the animal is engaged in predation, escape, or other vigorous activity. If other tissues and organs require glucose as a fuel, the amounts must be exceptionally small.