Stimulation of intra-uterine growth in rat by alpha-melanocyte-stimulating hormone.

We have studied whether endogenous alpha-MSH has a function in stimulating intra-uterine growth in the rat. The approach used was to determine whether or not this hormone is present during the intra-uterine growth spurt, and if binding of endogenous foetal alpha-MSH by antibodies would inhibit this growth. Antibodies against alpha-MSH or ACTH 1-24, either purified or non-purified, induced immunofluorescence in the intermediate lobe of adult male control rats. Using purified anti-alpha-MSH, fluorescence appeared in the foetal intermediate lobe on day 18 of pregnancy, the day that biologically active MSH was first seen. A negative correlation was observed between the pituitary MSH content and foetal body weight only on day 19 of pregnancy. Injection of purified anti-alpha-MSH induced a drop in foetal body weight, but no effect on placental weight was observed. Purified anti-acth 1-24 had no effect upon body weight but caused an increase in placental weight. These results support our previous findings and indicate that endogenous MSH has a function in the stimulation of foetal growth.