Interaction of insulin and somatotropin on body weight gain, feed intake, and body composition in rats

Abstract

This study investigated the interaction of insulin and somatotropin on body weight gain and feed conversion in rats. Female rats (initial wt 215 g) were assigned to one of the following four treatments for a 2-wk period: 1) control; 2) 40 U protamine zinc insulin.kg-1.day-1; 3) 2 mg/day somatotropin; 4) insulin + somatotropin. Relative to the control group (gain, 1.4 g/day; intake, 16.7 g/day) insulin stimulated the rate of gain (250%), feed intake (73%), and fat pad weight (215%). Insulin caused a 270% increase in carcass fat and a 30% increase in carcass protein. Somatotropin also increased gain (178%) but did not have a significant effect on intake or fat pad weight. Somatotropin increased carcass protein 28% but had no effect on carcass fat. The greatest stimulation of body weight gain (392%) was observed with the insulin plus somatotropin combination treatment, indicating an additive effect. There were also additive effects on protein accretion and organ weights. However, feed intake and carcass fat in the combination group were intermediate between that of the control and insulin alone groups, indicating that somatotropin attenuated the ability of insulin to stimulate these parameters. These results indicate that certain effects of insulin and somatotropin, such as the promotion of lean tissue accretion, are additive, whereas other effects, such as those associated with lipid metabolism, oppose each other.