Growth hormone pulsatility and the endocrine milieu during sexual maturation in male and female rats

Although a sexually dimorphic pattern of rat growth hormone (rGH) secretion is well recognized in adult rats, episodic rGH release has been incompletely characterized in younger rats. In the present study, 28 to 113-day-old rats received jugular catheters and were housed in chambers designed for stress-free blood sampling. Three to seven days after surgery, 300 µl of blood was withdrawn every 10 min between 16.00 and 22.00 h. Pulsatile rGH secretion before 33 days of age was similar in males and females. Low baseline rGH secretion was interspersed with infrequent, low amplitude pulses. In early puberty, between 33 and 40 days of age, rGH pulse amplitudes increased more than tenfold in both sexes. The duration of rGH pulses was significantly greater at this time in males versus females, a pattern that continued though adulthood. By late puberty, between 41 and 54 days of age, rGH pulse amplitudes similarly increased twofold in both sexes. Baseline rGH secretion increased at this time in both sexes, with females having a higher baseline compared to males. Only in adult rats over 54 days of age, was the typical pattern of low basal rGH secretion, combined with high amplitude, low frequency rGH pulses in males versus females evident. A sex difference in body weight, i.e. males weighing more than females, became evident after day 33 of age. This sex difference was preceded by an increase in testosterone in males versus females at all ages examined. Serum 17β-estradioI concentrations were higher in female rats compared to male rats between 33 and 41 days of age, reflecting the extended first proestrus endocrine phase. Plasma insulin-like growth factor-1 (IGF-1) concentrations increased threefold in both sexes from days 28 through 54 of age. However, only in adults was a significant sex difference, males greater than females, noted. In addition, serum corticosterone levels were higher in females than in males at all times after 33 days of age. These studies suggest that sexually dimorphic episodic rGH release develops gradually during the peripubertal period in rats. Because a sex difference in body weight develops bevore this pattern is fully established, other factors, such as gonadal and adrenal steroids, or their interaction with rGH, may contribute to the sex difference in body weight.