Diet-induced alterations in hepatic progesterone (P4) catabolic enzyme activity and P4 clearance rate in lactating dairy cows.

Elevated rates of steroid clearance may lead to lower reproductive success in several mammalian species. Cytochrome P450 (EC 1.14.14.1) and aldo-keto reductases (AKR; EC 1.1.1.145-151) are involved in the first phase of steroid inactivation, before second phase conjugation and excretion of the steroid metabolite. The current objectives were to determine liver blood flow (LBF), hepatic enzyme activity, and metabolic clearance rate (MCR) of progesterone (P(4)) in dairy cows consuming isoenergetic and isonitrogenous diets formulated to cause divergent insulin secretion. Insulin concentrations increased by 22% in cows fed the high cornstarch diet, and both cytochrome P450 2C and cytochrome P450 3A activities were decreased (P<0.05) by approximately 50%, while AKR1C tended (P<0.10) to be lower in cows fed the high cornstarch diet. LBF was similar between the two diets (1891+/-91 l/h). MCR of P(4) tended (P<0.10) to be lower in cows fed the high cornstarch diet (25+/-5 l/hxBW(0.75)) versus the high fiber diet (40+/-6 l/hxBW(0.75)). The half-life of P(4) was increased (P<0.05) in cows fed the high cornstarch diet (73+/-10 min) versus the high fiber diet (24+/-10 min). In summary, cows with elevated insulin concentrations and lower enzyme activity showed a decrease in P(4) clearance without any changes in LBF. This dietary relationship with hepatic enzyme activity may explain some of the observed alterations in steroid profiles during the estrous cycle or gestation of the high producing dairy cow.