Regulation of somatotroph cell function by the adipose tissue

Obese subjects exhibit a marked decrease in plasma growth hormone (GH) leve ls. However, the mechanisms by which increased adiposity leads to an impair ment of GH secretion are poorly understood. Recent evidence suggests that t he adipose tissue can markedly influence GH secretion via two different sig nals, namely free fatty acids (FFA) and leptin, FFA appear to inhibit GH se cretion mainly by acting directly at pituitary level, Interestingly, reduct ion in circulating FFA levels in obese subjects led to a marked increase in GH responses to different GH secretagogues. This indicates that FFA exert a tonic inhibitory effect that contributes to blunted GH secretion in obese subjects. Recent data have shown that leptin is a metabolic signal that re gulates GH secretion, since the administration of leptin antiserum to adult rats led to a marked decrease in spontaneous GH secretion. However, leptin prevents,the inhibitory effect exerted by fasting on plasma GH levels. The effect of leptin in adult rats appears to be exerted at hypothalamic level by regulating growth hormone releasing hormone (GHRH), somatostatin and ne uropeptide Y (NPY)-producing neurones. In addition, during fetal life or fo llowing the development of pituitary tumors, leptin can also act directly a t the anterior pituitary.