High feed intake increases liver blood flow and metabolism of progesterone and estradiol-17β in dairy cattle

Increased liver blood flow (LBF) resulting from elevated feed intake in lactating dairy cows may increase steroid metabolism. Continuous infusion of bromo-sulphthalein (BSP; specifically metabolized in liver) was used to measure LBF. Similarly, progesterone.(P-4) and estradiol-17beta (E-2) were administered by continuous infusion. Circulating concentrations at steady state were used to calculate the metabolic clearance rate (MCR) of BSP, P-4, and E-2. Experiment. 1: Variation in LBF was determined in thee nonlactating and four lactating cows over 3 d at 3 to 5 h after feeding. Coefficients of variation ranged from 14 to 31% among cows within day and from 4 to 8% within cows across days. Experiment 2: Six nonlactating cows were used in a 3 x 3 Latin-square design with three feed regimens: no feed, 0.5 maintenance diet (M), and 1.5 M. Experiment 3: Eight lactating cows were used in a 4 x 4 Latin-square design with four feed regimens: no feed, 0.5 M, 1.5 M, and 2.2 M. In experiments 2 and 3, LBF and MCR of P-4 increased immediately after feed consumption and increases, persisted longer at higher intakes. The LBF reached a maximum at 2 h after feeding and MCR of P-4 reached maximum at 3 h after feeding with a positive correlation (r = 0.92) between LBF and MCR for P-4. Experiment 4: A. crossover design was used to determine MCR of E-2 in unfed or full-fed lactating dairy cows. The MCR of E2 increased immediately after feeding and stayed elevated throughout the 4.5-h infusion period. Thus, LBF and steroid metabolism were acutely elevated by feed consumption in lactating and nonlactating cows. Higher rates of LBF and steroid metabolism in lactating than in nonlactating cows may indicate chronic effects of higher feed intakes as well.