Hypothalamic Control Mechanisms of Sexually Dimorphic Growth Hormone Secretory Patterns in the Rat

Abstract

In most mammalian species, including humans, the secretion of growth hormone (GH) is characterized by multiple episodic bursts. There is, however, a marked sex difference both in the pattern of GH secretion and in the rate of somatic growth [see (1) for review]. The male GH secretory profile, particularly in the rat, is characterized by high-amplitude GH bursts at regular 3.3-h intervals, separated by low or undetectable basal plasma GH levels (2); in contrast, females exhibit irregular, more frequent, lower-amplitude GH pulses superimposed on an elevated GH baseline (3). These gender-related differences in GH secretory patterns are of biological importance in the determination of sex-specific patterns of growth (4) and metabolism (5) in this species. Although the sexual dimorphism in GH secretion has been the subject of intense investigation in recent years (6–21), the underlying mechanisms are still far from clear. This chapter focuses on the roles of the hypothalamic neuropeptides, somatostatin and GH-releasing hormone, and on the modulatory influence of the sex steroids, in genesis of the sexually dimorphic GH secretory patterns in the rat.