Successive resetting of the set point for target body size.

Previous studies support the existence of a central set point for target body size. The set point is a hypothetical fixed reference for body size. The present experiments were carried out to determine whether target body size and, by implication, the putative set point can be consecutively reset or whether, once set by an extraneous factor, it is refractory to another extraneous factor. Male and female rats were given 6.0 Gy whole head X-irradiation at two days of age or were sham-irradiated as controls. Significant permanent reduction of body weight, tibia length, and pituitary weight resulted from head-irradiation in both males and females; gonad weight was reduced in males but not significantly in females. At 41 days of age the irradiated animals were injected sc with cortisone acetate, 1 mg/25 g body weight/day, or saline. During cortisone treatment the head-irradiated rats had a decrease in body weight velocity; the effect was greater in males than in females. Following treatment growth velocity approached normal in treated males, and slightly exceeded normal in treated females. Treated males had significantly reduced body weight, tail length, and tibia length at the end of the study. Treated females showed a similar pattern of body weight growth to the treated males, but there was not a significant long term reduction of body weight or change in tail length or tibia length. Final pituitary and gonad size was not affected by cortisone treatment in either sex. The results indicate that the reset of target size after head-irradiation does not result from damage to putative control.(ABSTRACT TRUNCATED AT 250 WORDS)