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post ' puberty

adult

aged

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FIGURE 8 (A) Relation between the integrated concentration of growth hormone and age in 173 normal male and female subjects. (From Zadik Z, Chalew SA, McCarter RJ, Jr, Meistas M, Kowarski AA, J Clin Endocrinol Metab 1985; 60:513-516. With permission.) (B) Changing patterns of GH secretion with age. (Modified from Robinson ICAF, Hindmarsh PC. In: Kostyo JL, Ed., Handbook of physiology, Section 7: The endocrine system, Vol. V: Hormonal Control of Growth, New York: Oxford University Press, 1999, pp. 329-396.)

physiological significance of these changes in GH secretion is not understood, but provocative tests using these signals are helpful for judging the competence of the GH secretory apparatus (Fig. 9). Traumatic and psychogenic stresses are also powerful inducers of GH secretion in humans and monkeys, but whether increased secretion of GH is beneficial for coping with stress is not established and is not universally seen in mammals. In rats, for example, GH secretion is inhibited by the same signals that increase it in humans. However, regardless of their significance, these observations indicate that GH secretion is under minute-to-minute control by the nervous system. That control is expressed through the hypothalamo-hypophysial portal circulation, which delivers two hypothalamic neuropeptides to the somatotropes: GH-releasing hormone (GHRH) and somatostatin. It is possible that a third hormone, ghrelin (see Chapter 38), also plays a role in this regard, but data to support this premise are not yet in hand. GHRH provides the primary drive for GH synthesis and secretion. In its absence, or when a lesion interrupts hypophysial portal blood flow, secretion of GH ceases. Somatostatin reduces or blocks the response of the pituitary to GHRH on GH secretion but has little or no influence on GH synthesis. Somatostatin and GHRH also exert reciprocal inhibitory influences on GHRH and somatostatin neurons (Fig. 10).

In addition to neuroendocrine mechanisms that adjust secretion in response to changes in the internal or external environment, secretion of GH is also under negative feedback control. As for other negative feedback systems, products of GH action, principally IGF-I, act as inhibitory signals (Fig. 11). IGF-I acts primarily at the pituitary level, where it decreases GH secretion in response to GHRH. IGF-I may also increase somatostatin secretion. Increased concentrations of FFA or glucose, which are also related to GH action (Chapter 42), also exert inhibitory effects, probably through increased somatostatin secretion, but fasting, which also leads to increased FFA, appears to inhibit somatostatin secretion. Growth hormone exerts a short loop feedback effect by inhibiting the secretion of GHRH and increasing the secretion of somatostatin.

Negative feedback control sets the overall level of GH secretion by regulating the amounts of GH secreted in each pulse. The phenomenon of pulsatility and the circadian variation that increases the magnitude of the secretory pulses at night are entrained by neural mechanisms. Pulsatility appears to be the result of reciprocal intermittent secretion of both GHRH and somatostatin. It appears that bursts of GHRH secretion are timed to coincide with interruptions in somatostatin secretion. Experimental evidence obtained in rodents indicates that GHRH-secreting neurons in the arcuate nuclei

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