Serum insulin-like growth factor-I, insulin-like growth factor binding protein-3, sex steroids, osteocalcin and bone mineral density in male and female rats

Although it has been reported that the race of weight gain and linear growt h increases markedly during puberty in rats, little is Known about the rela tionship between endocrine changes and bone mineral density (BMD) changes u pon sexual maturation in these animals. The aim of this study was to examin e the levels of serum insulin-like growth factor-I (ICF-I), IGF binding pro tein (IGFBP)-3, sex steroids and osteocalcin, and the changes in BMD in nor mal aging male and female rats. Male rats exhibited increases in serum IGF-I and IGFBP-3 concentrations bef ore increases in serum testosterone levels. IGF-land testosterone peaked at 9 weeks of age, and thereafter remained in a steady state, whereas IGFBP-3 reached a peak at 7 weeks of age, and then gradually declined. A strong co rrelation between serum IGF-I and IGFBP-3 levels was found in subjects 3-9 weeks old. A highly significant correlation between serum IGF-I and testost erone levels was also found. In females, serum 17 beta-estradiol, IGF-I and IGFBP-3 levels increased gra dually from 3 to 5 weeks old, peaked at 9 weeks, and then decreased slowly thereafter. The correlation coefficient between serum IGF-I and IGFBP-3 was highly significant. The correlation coefficient between serum IGF-I and 17 beta-estradiol levels was weak, although it war strongest when the subject s were 3-9 weeks old. Serum osteocalcin is a marker of bone formation; its level remained relativ ely high from 3 to 9 and from 3 to 7 weeks of age in males and females, res pectively, although osteocalcin in both sexes declined gradually with age. As for bone mass, sharp increases in BMD in. the tibia, femur and lumbar ve rtebrae appeared earlier in female than in male rats, and the BMD in female s tended to be higher than in males between 5 and 9 weeks old. After 9 week s of age, BMD in males was higher than that in females, as BMD in males con tinued to increase whereas females tended to remain in a steady state after this stage. The correlation coefficients between tibial BMD and serum IGF- I or IGFBP-3 levels were highly significant when the subjects were from 3 t o 9 weeks old. Taken together, these results suggest that BMD development occurs earlier i n female than in male rats. This sex-related difference in changer in the B MD pattern may result from the earlier onset of puberty in females, and fro m sex-specific differences in concentrations of IGF-I, IGFBP-3 and sex ster oids during maturation.