Influence of intrajugular administration of insulin, glucagon and propionate on voluntary feed intake of sheep.

The effect of intrajugular injections of insulin, glucagon and propionate, administered singly and in combination, as possible peripheral feedbacks in the control of feed intake in wethers was studied. A complete mixed diet (25% chopped hay: 75% cracked corn) was fed ad libitum. The treatments were saline, 6 mU insulin/kg body weight (BW), 9 ng glucagon/kg BW and 1.3 mg propionate/kg BW. In Exp. 1, five wethers were given the treatments at the beginning of each spontaneous meal over a 24-hr period, and total daily feed intakes were measured. The average number of injections per sheep for a 24-hr period was eight. In Exp. 2, the effects of the treatments on plasma concentrations of insulin, glucagon, propionate and glucose at 15, 30, 60 and 120 min after injection were measured in six other wethers. In Exp. 1, insulin (P less than .01), glucagon (P less than .01), insulin plus propionate (P less than .05) and glucagon plus propionate (P less than .05) decreased 24-hr feed intake by 18.5, 15.8, 11.0 and 11.8%, respectively, compared to the saline control. In Exp. 2, plasma insulin concentrations were increased (P less than .05) at 15 min after administration of insulin and insulin plus propionate, to 2.0 and 2.1 times the preinjection levels, respectively. Glucagon concentrations in plasma were increased (P less than .01) at 15 min after the injection of glucagon, to 2.0 times the pretreatment values. Insulin and glucagon concentrations in plasma were increased only slightly (P less than .10) after administration of glucagon plus propionate. No treatments affected glucose or propionate concentrations in the plasma. Increases in plasma concentrations of insulin, glucagon and propionate may interact directly or initiate other mechanisms involved in the short-term control of feed intake by sheep on a concentrate diet.