Induction of growth hormone (GH) mRNA by pulsatile GH-releasing hormone inrats is pattern specific

Growth hormone-releasing hormone (GHRH) is a main inducer of growth hormone (GH) pulses in most species studied to date. There is no information regar ding the pattern of GHRH secretion as a regulator of GH gene expression. We investigated the roles of the parameters of exogenous GHRH administration (frequency, amplitude, and total amount) upon induction of pituitary GH mRN A, GH content, and somatic growth in the female rat. Continuous GHRH infusi ons were ineffective in altering GH mRNA levels, GH stores, or weight gain. Changing GHRH pulse amplitude between 4, 8, and 16 mu g/kg at a constant f requency (Q3.0 h) was only moderately effective in augmenting GH mRNA level s, whereas the 8 mu g/kg and 16 mu g/kg dosages stimulated weight gain by a s much as 60%. When given at a 1.5-h frequency, GHRH doubled the amount of GH mRNA, elevated pituitary GH stores, and stimulated body weight gain. In the rat model, pulsatile but not continuous GHRH administration is effectiv e in inducing pituitary GH mRNA and GH content as well as somatic growth. T hese studies suggest that the greater growth rate, pituitary mRNA levels, a nd GH stores seen in male compared with female rats are likely mediated, in part, by the endogenous episodic GHRH secretory pattern present in males.