Sex differences in the effects of surgical isolation of the medial basal hypothalamus on linear growth and plasma growth hormone levels in the rat.

Abstract

The effects of surgical isolation of the medial basal hypothalamus (MBH) on linear growth and plasma growth hormone (GH) levels were studied in adult male and female rats. Whereas MBH isolation resulted in reduced linear growth in males, increased growth resulted in females. These effects were apparent in males 4 weeks after surgery and in females at 10 weeks. The altered growth rates persisted throughout the 28 weeks of the study. MBH isolation in both sexes led to increased food intake, obesity, and atrophy of reproductive tract organs. Non-stress plasma GH levels were unchanged at 4 weeks in both sexes but were elevated at 10 and 26 weeks in females and at 19 and 26 weeks in males. Serial samples of tail vein blood were obtained at 4-h intervals for 44 h from MBH-isolated and shamoperated females at 17 weeks postoperatively, and from similar groups of males at 19 weeks. GH levels were elevated in MBH-isolated rats of both sexes during the afternoons and evenings. Only sham-operated females showed evidence of a 24-h rhythmicity in circulating GH levels. The amplitudes of trough to peak excursions were similar in MBH-isolated and control females, but these excursions were reduced in MBH-isolated males. Although opposite effects on linear growth were noted in the two sexes, the resulting growth rates of operated males and females were similar, suggesting that MBH isolation disrupts mechanisms that normally regulate sex-specific growth patterns. Despite the sex difference in growth responses, MBH isolation caused an eventual increase in plasma GH levels in both sexes. Therefore, it is unlikely that the sex difference in growth responses is attributable exclusively to corresponding changes in circulating GH levels. A reduction in levels of circulating gonadal steroids may be an important factor. The elevations in plasma GH levels observed in both sexes suggest that MBH isolation interferes with mechanisms which inhibit GH secretion.