Anterior Lobe Hormones

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Follicle-Stimulating Hormone (FSH) FSH, one of the gonadotropins, is secreted by pituitary cells called gonadotropes. Its target organs are the ovaries and testes. In the ovaries, it stimulates the development of eggs and the follicles that contain them. In the testes, it stimulates sperm production.

Luteinizing Hormone (LH) LH, the other gonadotropin, is also secreted by the gonadotropes. In females, it stimu lates ovulation (the release of an egg). LH is named for the fact that after ovulation, the remainder of a follicle is called the corpus luteum ("yellow body"). LH stimulates the corpus luteum to secrete estrogen and progesterone, hormones important to pregnancy. In males, LH stimulates interstitial cells of the testes to secrete testosterone.

Thyroid-Stimulating Hormone (TSH), or Thyrotropin

TSH is secreted by pituitary cells called thyrotropes. It stimulates growth of the thyroid gland and the secretion of thyroid hormone, which has widespread effects on the body's metabolism considered later in this chapter.

Adrenocorticotropic Hormone (ACTH), or Corticotropin

ACTH is secreted by pituitary cells called corticotropes. ACTH stimulates the adrenal cortex to secrete its hormones (corticosteroids), especially cortisol, which regulates glucose, fat, and protein metabolism. ACTH plays a central role in the body's response to stress, which we will examine more fully later in this chapter.

Prolactin10 (PRL) PRL is secreted by lactotropes (mam-motropes), which increase greatly in size and number during pregnancy. PRL level rises during pregnancy, but it has no effect until after a woman gives birth. Then, it stimulates the mammary glands to synthesize milk. In males, PRL has a gonadotropic effect that makes the testes more sensitive to LH. Thus, it indirectly enhances their secretion of testosterone.

Growth Hormone (GH), or Somatotropin GH is secreted by somatotropes, the most numerous cells in the anterior pituitary. The pituitary produces at least a thousand times as much GH as any other hormone. The general effect of GH is to promote mitosis and cellular differentiation and thus to promote widespread tissue growth. Unlike the foregoing hormones, GH is not targeted to any one or few organs, but has widespread effects on the body, especially

Saladin: Anatomy & I 17. The Endocrine System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 17 The Endocrine System 643






Figure 17.5 Principal Hormones of the Anterior Pituitary Gland and Their Target Organs. The three axes physiologically link pituitary function to the function of other endocrine glands.

on cartilage, bone, muscle, and fat. It exerts these effects both directly and indirectly. GH itself directly stimulates these tissues, but it also induces the liver and other tissues to produce growth stimulants called insulin-like growth factors (IGF-I and II), or somatomedins,11 which then stimulate target cells in diverse tissues. Most of these effects are caused by IGF-I, but IGF-II is important in fetal growth.

Hormones have a half-life, the time required for half of the hormone to be cleared from the blood. GH is shortlived; it has a half-life of 6 to 20 minutes. IGFs, by contrast,

"Acronym for somatotropin mediating protein have half-lives of about 20 hours, so they greatly prolong the effect of GH. The mechanisms of GH-IGF action include:

• Protein synthesis. Tissue growth requires protein synthesis, and protein synthesis needs two things: amino acids for building material, and messenger RNA (mRNA) for instructions. Within minutes of GH secretion, preexisting mRNA is translated and proteins synthesized; within a few hours, DNA is transcribed and more mRNA is produced. GH enhances amino acid transport into cells, and to ensure that protein synthesis outpaces breakdown, it suppresses protein catabolism.

• Lipid metabolism. To provide energy for growing tissues, GH stimulates adipocytes to catabolize fat and

Saladin: Anatomy & I 17. The Endocrine System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

644 Part Three Integration and Control release free fatty acids (FFAs) and glycerol into the blood. GH has a protein-sparing effect—by liberating FFAs and glycerol for energy, it makes it unnecessary for cells to consume their proteins.

• Carbohydrate metabolism. GH also has a glucose-sparing effect. Its role in mobilizing FFAs reduces the body's dependence on glucose, which is used instead for glycogen synthesis and storage.

• Electrolyte balance. GH promotes Na+, K+, and Cl_ retention by the kidneys, enhances Ca2+ absorption by the small intestine, and makes these electrolytes available to the growing tissues.

The most conspicuous effects of GH are on bone, cartilage, and muscle growth, especially during childhood and adolescence. IGF-I stimulates bone growth at the epiphyseal plates. It promotes the multiplication of chon-drocytes and osteogenic cells and stimulates protein deposition in the cartilage and bone matrix. In adulthood, it stimulates osteoblast activity and the appositional growth of bone; thus, it continues to influence bone thickening and remodeling.

The blood GH concentration declines gradually with age—averaging about 6 ng/mL (ng = nanograms) in adolescence and one-quarter of that in very old age. The resulting decline in protein synthesis may contribute to aging of the tissues, including wrinkling of the skin and decreasing muscular mass and strength. At age 30, the average adult body is 10% bone, 30% muscle, and 20% fat; at age 75, it averages 8% bone, 15% muscle, and 40% fat.

GH concentration fluctuates greatly over the course of a day. It rises to 20 ng/mL or higher during the first 2 hours of deep sleep and may reach 30 ng/mL in response to vigorous exercise. Smaller peaks occur after highprotein meals, but high-carbohydrate meals tend to suppress GH secretion. Trauma, hypoglycemia (low blood sugar), and other conditions also stimulate GH secretion.

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