Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 24

The Pituitary Gland 385

secretion are summarized in Table 24–3. Most of them fall into
three general categories: (1) conditions such as hypoglycemia
and fasting in which there is an actual or threatened decrease in
the substrate for energy production in cells, (2) conditions in
which the amounts of certain amino acids are increased in the
plasma, and (3) stressful stimuli. The response to glucagon has
been used as a test of growth hormone reserve. Growth hor-
mone secretion is also increased in subjects deprived of rapid
eye movement (REM) sleep (see Chapter 15) and inhibited
during normal REM sleep.
Glucose infusions lower plasma growth hormone levels and
inhibit the response to exercise. The increase produced by 2-
deoxyglucose is presumably due to intracellular glucose defi-
ciency, since this compound blocks the catabolism of glucose
6-phosphate. Sex hormones induce growth hormone secre-
tion, increase growth hormone responses to provocative stim-
uli such as arginine and insulin, and also serve as permissive
factors for the action of growth hormone in the periphery.
This likely contributes to the relatively high levels of circulat-
ing growth hormone and associated growth spurt in puberty.
Growth hormone secretion is also induced by thyroid hor-
mones. Growth hormone secretion is inhibited, on the other
hand, by cortisol, FFA, and medroxyprogesterone.
Growth hormone secretion is increased by
L
-dopa, which
increases the release of dopamine and norepinephrine in the
brain, and by the dopamine receptor agonist apomorphine.

PHYSIOLOGY OF GROWTH

Growth hormone, while being essentially unimportant for fetal
development, is the most important hormone for postnatal
growth. However, growth overall is a complex phenomenon
that is affected not only by growth hormone and somatome-
dins, but also, as would be predicted by the previous discussion,

by thyroid hormones, androgens, estrogens, glucocorticoids,

and insulin. It is also affected, of course, by genetic factors, and

it depends on adequate nutrition. It is normally accompanied

by an orderly sequence of maturational changes, and it involves

accretion of protein and increase in length and size, not just an

increase in weight (which could reflect the formation of fat or

retention of salt and water rather than growth per se).

ROLE OF NUTRITION

The food supply is the most important extrinsic factor affect-

ing growth. The diet must be adequate not only in protein

content but also in essential vitamins and minerals (see Chap-

ter 27) and in calories, so that ingested protein is not burned

for energy. However, the age at which a dietary deficiency

FIGURE 24–7
Feedback control of growth hormone secretion.
Solid arrows represent positive effects and dashed arrows represent
inhibition.

SS

Anterior

pituitary

Liver

(and other

organs)

GH

Tissues

IGF-I

GHRH Hypothalamus

Ghrelin

TABLE 24–3

Stimuli that affect growth hormone

secretion in humans.

Stimuli that increase secretion

Hypoglycemia

2-Deoxyglucose

Exercise

Fasting

Increase in circulating levels of certain amino acids

Protein meal

Infusion of arginine and some other amino acids

Glucagon

Stressful stimuli

Pyrogen

Lysine vasopressin

Various psychologic stresses

Going to sleep

L

-Dopa and

α

-adrenergic agonists that penetrate the brain

Apomorphine and other dopamine receptor agonists

Estrogens and androgens

Stimuli that decrease secretion

REM sleep

Glucose

Cortisol

FFA

Medroxyprogesterone

Growth hormone and IGF-I