Elevated blood glucose levels and satiety in the rat

Thirteen rats were placed on a feeding schedule of two 3 hr periods of food availability daily. blood glucose levels of the experimental animals were altered by intraperitoneal injection of 8 ml of 2, 5, 8, 12, 16, 20 and 25 percent glucose solutions and by intragastric loading of 8 ml of 25, 50 and 65 percent glucose solutions in 10 different feeding experiments. In 4 additional experiments, experimental animals received intraperitoneal injections of 8 ml of 12, 16, 20 and 25 percent solutions of mannitol, a non-metabolizable sugar-alcohol. Controls always received identical quantities of mammalian Ringer's solution administered via the same route as in the experimentals. Another set of 20 rats was used to determine glucose tolerance curves for each concentration of glucose and mannitol administered. No food intake depression occurred following intraperitoneal injections of 2, 5, 8 and 12 percent glucose, 12 percent mannitol and following intragastric loading of 25, 50 and 65 percent glucose. However, intraperitoneal injections of 16, 20 and 25 percent glucose and mannitol caused depression of food intake. No depression of food intake occurred following intragastric loading of 50 and 65 percent glucose solutions which raised blood glucose levels a minimum of 43 mg percent and 55 mg percent above basal, respectively, for the duration of the 3 hr feeding period. These minimum blood glucose levels greatly exceed normal values following a meal. It was concluded that blood glucose level per se is not an important feedback parameter in the long-term control of food intake, and the depression in food intake following intraperitoneal injections of 16, 20 and 25 percent glucose and mannitol solutions was due to an abnormal physiological condition.