The effects of insulin and glucagon on the release of triacylglycerols by isolated rat hepatocytes are mere reflections of the hormonal effects on the rate of triacylglycerol synthesis.

1. Isolated hepatocytes from meal-fed donor rats secrete newly synthesized very-low-density lipoproteins (VLDL) when incubated in a simple bicarbonate buffer. When incubated with 3H2O for 2 h, 72-81% of the 3H-labelled triacylglycerols secreted by the hepatocytes were recovered in VLDL. The secretion of newly synthesized triacylglycerols shows a lag phase of about 30 min. 2. Insulin stimulates the secretion of newly synthesized VLDL triacylglycerols, whereas glucagon has an inhibitory effect on this process. 3. When hepatocytes triacylglycerols were labelled by preincubating the cells with 3H2O or [1-14C]oleate and the cells were subsequently washed and further incubated in radioisotope-free buffer containing hormones, it was observed that the release of the pre-labelled triacylglycerols is not hormone-sensitive. This suggests that insulin and glucagon do not affect the release of triacylglycerols per se. 4. It is concluded that the effects of insulin and glucagon on the overall process of triacylglycerol secretion are reflections of the hormone-determined rate of triacylglycerol synthesis.