OPIOID-MEDIATED RESPONSES OF DIETARY-PROTEIN ON RETICULAR MOTILITY AND PLASMA-INSULIN

Four ruminally and abomasally cannulated steers (603 +/- 22.7 kg of bo dy weight) were used to determine whether ruminally undegradable prote in (RUP) would exert opioid-mediated effects on reticuloruminal motili ty or circulating concentrations of insulin. Steers were fed isonitrog enous diets (16% crude protein) containing either 30 or 40% RUP. The l ow RUP diet was supplemented with urea and soybean meal, and the high RUP diet was supplemented with blood meal, fish meal, corn gluten meal , and meat and bone meal. Diets contained 57% wheat silage and were fe d twice daily at 0800 and 1600 h. Experimental periods were 10 d in le ngth. Blood samples were taken from jugular catheters, ana reticular m otility was measured at hourly intervals on d 10 over a 16-h period. E ither naltrexone (0.5 mg/kg of body weight) or saline was infused into the abomasum at the second feeding (9 h). Naltrexone reduced the freq uency of reticular contractions by 16.5% for steers fed the low RUP di et. Naltrexone decreased the duration of reticular contractions by 9.3 % for steers fed the low RUP diet and increased duration by 8.7% for s teers fed the high RUP diet. Naltrexone decreased the opening time of the reticuloomasal orifice, expressed as a percentage of predose measu rements, by 16.3% for steers fed the high RUP diet. Insulin was 21.3% higher with the high RUP diet. The postprandial rise in insulin decrea sed 36.7% with naltrexone. Dietary protein can exert effects mediated by opioids in ruminants.