| The experiments were conducted to elucidate the influence of continuous cycling high environmental temperatures on the activity of digestion enzymes, growth performance, deposition of nitrogen in skeletal muscle, and expression level of HSP70 mRNA. The feasibility was analysed about supplement of acid protease to improve the performance under high temperature conditions. We also investigated the reason of heat stress on performance and deposition of nitrogen in skeletal muscle.Three hundred and eighty four male Acbor Acres broilers were allocated to four treatments randomly at 28 days of age, according to the following design: 23 C, ad lib. feeding (23AL); 27 C-33 C-27 C cyclic high temperature group, ad lib. feeding (HT AL); 23 C, pair-feeding with 27 C-33 C -27 C group (23PF), and HT+enzyme group, ad lib feeding a basal diet supplemented with 1500U/kg acid protease (HT ALP). Broilers were fed a standard corn-soybean meal diet.Heat exposure increased the mean level of amylase activity in the pancreas by 17%between 3 and 21 days (P<0.05) , while the level of intestinal amylase activity decreased by 27%, 23%, 24% in duodenum, jejunum and ileum respectively (P<0.05), pair-feeding trial showed that this decrease was not due to the lower intake.Heat exposure increased the mean level of trypsin activity in the pancreas by 17% (P < 0.05) in first two weeks, except for a sharp decrease on the third day (P < 0.05). The mean level of trypsin activity decreased by 20.3% and 40.6% (P<.05 ) in duodenum and jejunum respectively. Under heat ambient, addition of acid protease to the feed, the mean level of trypsin activity decreased by 20.25% in the pancreas on 1-21d (P<.05) , while in jejunum and ileum, the trypsin activity increased in the first two weeks (P<0.05) .The level of chrymotrypsin activity in the pancreas increased by 20% (P<0.05) under acute heat exposure(24h), while it reduced by 28.92% (PO.05) significantly on 3-11d under chronic heat exposure, pair-feed showed it is a result of lower intake. Chrymotrypsin activity increased by 35% in jejunum in first three weeks, pair-feed experiment due it to both lower intake and high temperature. Chrymotrypsin activity decreased by 19% in ileum in first week (P<0.05). The chrymotrypsin activity in the pancreas increased supplement of acid protease under heat exposure (P<0.05) .After 3 weeks of heat exposure, the average daily gain, average daily intake, and feed efficiency decreased by 38.2% , 26.8.6% and 15.8% respectively (P<0.01). Pair-feeding experiment showed that the decrease of growth cannot attribute to the lower feed intake simply. Under continuous heat exposure, growth performance did not improve with addition of acid protease.Serum glucose content and serum uric acid content increased (P<0.05) , but the serum total protein decreased significantly (P<0.05) after 1 week of heat exposure; Serum triglycerides concentration decreased significantly (P<0.05) in 1 and 3 weeks.The amount of skeleton muscle deposition and protein retention in skeleton muscle decreasedunder heat exposure, pair-feeding experiment showed it is a joint result of both hot environment and lower intake.Under heat exposure, the expression of HSP70 mRNA increased in liver and breast muscle in 1-21 day.These results clearly show that continuous cycling high environmental temperatures had adverse effects on performance in broilers.In the continuous heat environment, the intestinal amylase activity, trypsin activity decreased and the expression of HSP70 mRNA increased in liver and breast muscle, and average daily intake decreased, which may result in poor growth performance and a decrease in the amount of skeleton muscle and protein retention in skeleton muscle.
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