Synergism of cholecystokinin (CCK) and leptin on nutrient mediayed intake control in cattle
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Experiments using lactating cows and rumen cannulated steers were conducted to investigate the role of CCK and leptin in intake control of cattle fed high fat diets. Jugular blood samples and estimates of fat thickness were taken from 16 lactating dairy cows that had varied body condition scores and d in milk. Serum leptin positively correlated with backfat (r 2 0.64, P < 0.01), rumpfat (r 2 0.60, P < 0.01), and days in milk (r 2 0.48, P < 0.01). In another experiment, circulating levels of leptin were measured in 12 Holstein cows, fed anionic salts 2 wk pre-partum and fed one of three levels of supplemental fat (0, 3, or 6 % added fat using MEGALAC-R®) post-partum. Serum leptin decreased approximately 15% due to higher levels of fat in the diet. Milk production decreased (linear, P < 0.01) and dry matter intake increased (quadratic, P < 0.05) due to fat in the diet. Dry matter intake increased (linear, P < 0.01) and milk production increased (linear, P < 0.01) during the first 8 wk of lactation. Dry matter intake was correlated (r 2 0.23, P < 0.01) with serum leptin during first 8 weeks of lactation. Four growing and four mature ruminally cannulated steers were used in two simultaneously run 4X4 latin square design trials to investigate the role of CCK and leptin in intake regulation of cattle fed high fat diets. Steers were fed diets containing 0, 3, 6, or 9% supplemental fat. Rumen digesta disappearance, reticulorumen motility, and reticuloomasal orifice opening time were measured and serum glucose, leptin and insulin were assayed with or without injecting animals with a CCK receptor antagonist, devazepide. Dietary fat decreased DMI in a linear fashion (P<.05). Dietary fat had no effect on reticulorumen motility, but devazepide increased motility irrespective of dietary treatment. Dietary fat decreased reticuloomasal orifice opening time (Linear, P<.05) in growing steers and this effect was completely blocked by devazepide. In addition, dietary fat decreased disappearance of ruminal digesta in growing steers and this effect was also reversed by devazepide. Both leptin and CCK are involved in gastrointestinal function and dietary fat mediated intake regulation in ruminants.