Effects of heat stress and dietary organic acid and pure botanical on hepatic one-carbon metabolism.

Heat stress (HS) increases plasma urea-nitrogen in cows. Urea reduces the activity of glycerophosphocholine (GPC) phosphodiesterase, the enzyme that regulates the degradation of phosphatidylcholine (PC) into GPC and choline. Our objective was to evaluate the effects of HS and dietary organic acid and pure botanical (OA/PB) supplementation on liver one-carbon and phospholipid metabolism. Following a 7 d accli mation in thermoneutrality (temperature-humidity index [THI] 68), 46 pregnant late-lactation Holstein cows were randomly assigned to 1 of 4 groups: thermoneutral conditions (TN-Con, n = 12), HS conditions (HS-Con, n = 12; diurnal THI 74 to 82), TN conditions pair-fed to match HS-Con (TN-PF, n = 12), or HS fed OA/PB (HS-OAPB, n = 10; 75 mg/kg of body weight; 25% citric acid, 16.7% sorbic acid, 1.7% thymol, 1.0% vanillin, and 55.6% triglyceride; AviPlus R; Vetagro S.p.A) for 14 d. Liver biopsies were performed on d 6 of acclimation (baseline) and d 12 of environmental conditioning. Hepatic choline metabolites were analyzed by LC/MS. Data were analyzed using a general linear mixed model including fixed effects of treatment and block, and the random effect of cow. Planned contrasts included HS-Con vs. TN-Con, HS-Con vs. TN-PF, and HS-Con vs. HS-OAPB. Liver choline concentrations were lower in HS-Con compared with TN-Con and TN-PF (P < 0.05). Liver betaine and total PC concentrations were greater in TN-PF compared with HS-Con (P < 0.01). HS-Con had greater hepatic GPC concentrations compared with TN-Con, TN-PF, and HS-OAPB (P < 0.05). Hepatic GPC:choline ratios were greater in HS-Con, relative to TN-Con and TN-PF (P < 0.01). Liver S-adenosyl methionine (SAM) tended to be lower in HS-Con compared with TN-Con (P < 0.10). Although treatment did not modify S-adenosyl homocysteine (SAH) concentrations, HS-Con had lower liver SAM:SAH ratios, relative to TN-Con (P = 0.05) and HS-OAPB (P = 0.06). We conclude that HS develops with methyl donor deficiency in parallel with impaired N metabolism and dietary supplementation of OA/PB appears to maintain remethylation capacity in bovine liver during heat stress.