Differential effects of alanine on ketogenesis and triacylglycerol formation by isolated perfused livers from euthyroid and hyperthyroid rats.

We examined the effects of infusion of alanine on hepatic concentration of glycero-3-phosphate (glycero-3-P) and output of triacylglycerol (TG) by isolated perfused livers from triiodothyronine (T3)-treated rats. It was expected that because of its gluconeogenic and antiketogenic properties, alanine might stimulate accumulation of glycero-3-P, which in turn might result in enhanced TG production by the hyperthyroid livers. The hepatic concentration of glycero-3-P is lower in livers from T3-treated rats than in euthyroid rats. Infusion of 1.83 and 4.23 mmol alanine/4 h did not alter the hepatic concentration of glycero-3-P and output of triacylglycerol by livers from T3-treated rats. However in these livers, the two concentrations of infused alanine increased the output of glucose and decreased the output of ketone bodies. In livers from euthyroid animals, the infusion of 1.83 mmol alanine/4 h had no effect, whereas 4.23 mmol/4 h decreased hepatic concentration of glycero-3-P. These concentrations of alanine did not alter the output of ketone bodies or TG, but progressively decreased the output of glucose by the euthyroid livers. Our data suggested that the decreased availability of glycero-3-P in livers from T3-treated rats is rate-limiting for TG production and correlated with the diminished output of TG, whereas in livers from euthyroid rats, the glycero-3-P concentration is sufficient to maintain maximal synthesis and secretion of TG. Furthermore, the effects of alanine on the output of ketone bodies or glucose appear to be independent of its effects on hepatic glycero-3-P.