Diet-induced alterations in progesterone clearance appear to be mediated by insulin signaling in hepatocytes

Factors that affect progesterone clearance from plasma and by hepatocytes in culture were examined in a series of experiments. In Exp. 1, the objective was to determine whether an increase in hepatic portal blood acetate or propionate could alter progesterone metabolism by the liver. For ewe lambs gavaged orally with sodium propionate compared with those gavaged orally with sodium acetate, serum progesterone concentrations began to diverge as early as 0.5 h after administration and were greater (P < 0.05) at 3 and 4 h after administration. The objective of Exp. 2 was to determine the effect of a single oral gavage of either sodium acetate or sodium propionate on peripheral insulin and glucagon concentrations. Ewes gavaged orally with sodium propionate had greater (P < 0.05) insulin concentrations at 0.5 and 1 h after gavage than ewes gavaged with sodium acetate. Furthermore, glucagon concentrations were greater (P < 0.05) at 0.5, 1, and 2 h for ewe lambs gavaged orally with sodium propionate compared with those receiving sodium acetate. The third experiment investigated the rate of in vitro progesterone clearance by cultured hepatocytes in response to treatment with different concentrations of insulin and glucagon. Progesterone clearance was reduced (P < 0.05) with the addition of 0.1 nM insulin compared with the control. Furthermore, there was a greater reduction (P < 0.05) in progesterone clearance in response to 1.0 and 10 nM insulin compared with the control and 0.1 nM insulin. No change was observed in progesterone clearance in hepatocytes treated with either physiological (0.01 and 0.1 nM) or supraphysiological (1.0 nM) glucagon. Supraphysiological concentrations of glucagon (1.0 nM) negated the effects of either 0.1 or 1.0 nM insulin on progesterone clearance by hepatocytes. However, with physiological concentrations of glucagon (0.1 nM) and 1.0 nM insulin, glucagon was not able to negate the reduction in progesterone clearance caused by insulin. These data are consistent with a paradigm in which elevated hepatic portal vein propionate increases plasma insulin in ruminants, which decreases progesterone clearance, thereby increasing serum progesterone concentrations.