Effects of short-term exposure to heat stress on splanchnic metabolism in sheep

Four wethers with chronic indwelling catheters in the portal, hepatic and mesenteric veins as well as in the abdominal aorta were housed in temperature-controlled rooms at 20oC or 35oC. The sheep consumed a diet at the level of maintenance requirements with both temperatures. Hepatic blood fl ow tended to be greater at 35oC than 20oC. The absorption of acetate and propionate as well as the oxygen uptake by the portal-drained viscera were signifi cantly greater at 35oC than at 20oC. The increased oxygen uptake may be one cause of the heat load under hot temperature.


INTRODUCTION
Animal exposed to excessive heat load reduces feed intake to maintain homeothermy.Ambient temperature, relative humidity and metabolic heat with maintenance and production are important factors relating to the heat load.The portal-drained viscera (PDV) and liver use much oxygen due to the high metabolic activity, and the proportion of oxygen uptake by the total splanchnic (TS) tissues accounts for over 40% of total oxygen use by the whole body (Huntington, 1999).McGuire et al. (1989) have reported that the net fl ux of alpha amino nitrogen across the PDV of lactating cows was reduced by heat stress.However, few data on net fl ux of the other metabolites and oxygen consumption by PDV and liver under heat stress are available.We investigated the infl uence of hot temperature on metabolite net fl ux and oxygen consumption across the splanchnic tissues of sheep.

MATERIAL AND METHODS
Four wethers (average body weight 64.9 kg) were surgically fi tted with chronic indwelling catheters in the hepatic and hepatic portal vein, mesenteric vein and caudal aorta.All sheep housed in metabolic crates were offered the diet consisting of 70% alfalfa hay cubes and 30% rolled barley in 12 equal daily meals using automatic feeders.The daily amount of the diet was restricted at the maintenance energy level to exclude any infl uence associated with different feed intake.The room temperature as treatment was controlled at 20ºC and 35ºC with a relative humidity of 64 and 48%, respectively.From the aspects of animal welfare, the confi nement in 35ºC room as heat stress treatment was restricted to three days.On day 3, thus, blood samples of the artery and the portal and hepatic vein at 35ºC were taken every 30 min for three hours.The blood samples at the comfortable room temperature (20ºC) were taken after 14 days for the preliminary period.p-Aminohippurate was continuously infused (2% w/v, 1 ml/min) into the mesenteric vein catheter for measuring blood fl ow.Treatment means were compared by paired-t test.

RESULTS AND DISCUSSION
There were no refusals of the offered diet with any of the temperature treatments.Table 1 summarizes the blood fl ow, oxygen consumption and metabolite net fl ux across the splanchnic tissues with both temperature treatments.Hepatic blood fl ow tended to be greater (P=0.15) at 35ºC compared with 20ºC.Oxygen consumption by PDV at 35ºC increased by 15% compared with 20ºC, which resulted in increased oxygen consumption by TS (P<0.05).Net absorption of acetate and propionate across PDV were greater (P<0.01) at 35ºC than at 20ºC.The PDV net fl ux of isobutyrate and isovalerate also tended to be greater at 35ºC (P<0.14).Consequently, the absorption of total short chain fatty acids increased by 42% at 35ºC compared with 20ºC.These results suggest that as heat stress promotes ruminal fermentation of ingested substrates, the increased short chain fatty acids as fermentation products need more oxygen for the absorption metabolism by the ruminal tissue.Heat stress reduces the gut motility of ruminants (Christopherson and Kennedy, 1983).In this study, reduced gut motility at 35ºC might cause the prolongation of digesta retention time in the rumen, resulting in the increase absorption of fermentation products from same dietary substrates.
Although hepatic uptake of propionate at 35ºC compared with 20ºC increased (P<.05), the net glucose production by the liver did not differ.Because the liver can regulate gluconeogenesis under rigid glucose homeostasis, the increased propionate uptake at 35ºC was used for other metabolic processes or fueled through the TCA cycle.Net release of propionate by TS was not different between the treatments.On the other hand, little uptake of acetate by the liver resulted in a tendency of the greater release by TS (P<0.15).Heat stress did not affect the net fl uxes of butyrate and βhydroxybutyrate across the splanchnic tissues.

CONCLUSIONS
Hot temperature compared with comfortable environment at a same level of feed intake increased short chain fatty acids absorption from the digestive tract and PDV oxygen consumption.The increased oxygen consumption by PDV could be one cause of heat load driven from metabolic process under heat stress.

Table 1 .
Blood fl ow, oxygen consumption and net fl ux of metabolites across portal, hepatic and total splanchnic tissues in sheep exposed to 20 and 35ºC