Production, Management and the Environment: Heat Stress and Environment

The objective of these experiments was to determine if a change in the ratio of acetate to propionate (A:P ratio) in hepatic portal blood, draining the gastrointestinal tract, can alter the metabolism of progesterone by the liver. Experiment 1; 4 crossbred, ewe lambs (BW 45.5 + 2.5 kg) were fed for maintenance and given a once daily oral bolus (0.146 Mcal/d) of acetate (0.7 moles) or propionate (0.4 moles) for 11 d. A portal-vein catheter was inserted 14 d prior to collection. Portal and jugular venous blood were simultaneously collected (-0.17, 0.17, .33, 0.5, .66, .83, 1, 1.17, 1.33, 1.5, 1.66, 1.83, 2, 2.33, 2.66, 3, 4, 5, 6, 7 h with respect to feeding and VFA bolus) and serum was analyzed for concentrations of VFA by GLC. Experiment 2; 30 crossbred ewe-lambs, (BW 45.2 + 1.9 kg) were blocked by BW and fed for maintenance for 11 d. On d 12, each lamb was randomly assigned to one of two treatments, and given an oral bolus (0.146 Mcal/d) of acetate (0.7 moles) or propionate (0.4 moles). Animals received (i.m.) 20 mg progesterone (P4) in corn oil. Plasma samples were collected (-0.5, 0.5, 1, 2, 3, 4, 5, 6, 8 h relative to feeding, VFA bolus, and P4 injection) via jugular venipuncture and circulating concentrations of P4 were determined by P4-RIA. An oral bolus of acetate or propionate caused a mark change in the A:P ratio for at least 4 h. By 24 h after the oral bolus, the A:P ratio returned to baseline. Plasma concentrations of P4 after the oral bolus of either acetate or propionate began to diverge as early 0.5 h and were different (P < 0.05) by 3-h (1.09 + 0.09 ng/ml vs. 2.04 + 0.48 ng/ml) and 4-h (1.20 + 0.09 ng/ml vs. 1.95 + 0.41 ng/ml) for ewe lambs bolused with acetate and propionate, respectively. By 5 h, coincident with the return to baseline A:P ratios seen in Experiment 1, circulating concentrations of P4 were not different. Changes in the A:P ratio altered the circulating concentrations of P4 by reducing the metabolic clearance of P4 when the A:P ratio was decreased.