Hormones of the adenohypophysis

The gonadotropins, follicle-stimulating hormone and luteinizing hormone, exert their effects on the gonads (ovaries in the female and testes in the male). Taken together, the gonadotropins stimulate the gonads to:

• Produce gametes (ova and sperm)

• Secrete sex hormones (estrogen, progesterone, and testosterone)

Follicle-stimulating hormone (FSH), as its name indicates, stimulates the development of the ovarian follicles in females. It is within the follicles that the ova, or eggs, develop. This hormone also induces the secretion of estrogen from the follicle. In males, FSH acts on the Sertoli cells of the testes, which are involved with production of sperm. Luteinizing hormone (LH) is also named for its effects in the female, which are to cause the rupture of the follicle and the release of the ovum and to cause conversion of the ovarian follicle into a corpus luteum (Latin, yellow body). This hormone also induces secretion of estrogen and progesterone from the corpus luteum. In males, LH acts on the Leydig cells of the testes to stimulate secretion of testosterone. FSH and LH are produced by the same cell type in the adenohypophysis: the gonadotrope. The release of FSH and LH is regulated by the hypothalamic releasing hormone, gonadotropin-releasing hormone (GnRH).

Thyroid-stimulating hormone (TSH, thyrotropin) regulates the growth and metabolism of the thyroid gland. Furthermore, it stimulates synthesis and release of the thyroid hormones, T3 and T4. The release of TSH from the thyrotrope cells of the adenohypophysis is induced by thyrotropin-releasing hormone (TRH). Adrenocorticotropic hormone (ACTH, adrenocorticotropic stimulates growth and steroid production in the adrenal cortex. Specifically, it stimulates secretion of cortisol and other glucocorticoids involved with carbohydrate metabolism. The release of ACTH from the adenohypophysis is influenced by more than one factor. Corticotropin-releasing hormone (CRH) from the hypothalamus stimulates the secretion of ACTH. In addition, ACTH secretion follows a diurnal pattern, with a peak in the early morning and a valley in the late afternoon.

Prolactin (PRL), produced by the lactotrope cells of the adenohypophysis, is involved with the initiation and maintenance of lactation in females. Its function in males is uncertain. Lactation involves three processes:

• Mammogenesis

• Lactogenesis

• Galactopoeisis

Mammogenesis is the growth and development of the mammary glands that produce the milk. This process requires the actions of many hormones, including estrogens and progestins, in addition to PRL. Lactogenesis is the initiation of lactation. During pregnancy, lactation is inhibited by high levels of estrogens and progestins. At delivery, the levels of these two hormones fall, allowing PRL to initiate lactation. Galactopoeisis is the maintenance of milk production. This process requires PRL and oxytocin.

The release of prolactin from the adenohypophysis is normally inhibited by prolactin-inhibiting hormone (PIH, dopamine) from the hypothalamus. Pro-lactin secretion is also controlled by prolactin-releasing factor (PRF). The release of PRF from the hypothalamus is mediated by reflexes elicited by suckling and breast stimulation.

Growth hormone (GH, somatotropin) is one of the few hormones that exerts its effects on organs and tissues throughout the body. This hormone is essential for normal growth and development of the skeleton as well as visceral, or soft, tissues from birth until young adulthood. Growth of the skeleton involves an increase in bone thickness and an increase in bone length. The mechanism of this growth involves stimulation of osteoblast (bone-forming cell) activity and proliferation of the epiphyseal cartilage in the ends of the long bones. The growth of visceral tissues occurs by hyperplasia (increasing the number of cells) and hypertrophy (increasing the size of cells). Growth hormone causes hyperplasia by stimulating cell division and by inhibiting apoptosis (programmed cell death) and cellular hypertrophy by promoting protein synthesis and inhibiting protein degradation.

The growth-promoting effects of GH are carried out by somatomedins, which are peptides found in the blood. Two somatomedins have been identified and described. Structurally and functionally similar to insulin, these peptides are referred to as insulin-like growth factors I and II (IGF-I and IGF-II). Growth hormone stimulates the production of IGF-I in the liver, which is the predominant source of that found in the circulation. Local production of IGF-I also occurs in many target tissues. IGF-I is thought to mediate the growth-promoting effects of GH throughout life. Levels of GH and IGF-I increase in parallel during puberty and other periods of growth in children. In contrast, IGF-II production does not depend on GH. Instead, IGF-II is thought to be important during fetal growth and development and is secreted in response to prolactin. The role of IGF-II in the adult is unclear.

Growth hormone also has many metabolic actions in the body:

• Protein metabolism

• Increase in tissue amino acid uptake

• Stimulation of protein synthesis

• Lipid metabolism

• Increase in blood fatty acids

• Stimulation of lipolysis

• Inhibition of lipogenesis

• Carbohydrate metabolism

• Increase in blood glucose

• Decrease in glucose uptake by muscle

• Increase in the hepatic output of glucose (glycogenolysis)

The net effects of these actions include enhanced growth due to protein synthesis; enhanced availability of fatty acids for use by skeletal muscle as an energy source; and glucose sparing for the brain, which can use only this nutrient molecule as a source of energy.

The release of GH from the adenohypophysis is regulated by two hypo-thalamic hormones: growth hormone-releasing hormone (GHRH) and growth hormone-inhibiting hormone (GHIH, somatostatin). Any factor or condition that enhances the secretion of GH could do so by stimulating or inhibiting GHRH

release or by inhibiting GHIH release. The secretion of GH follows a diurnal rhythm with GH levels low and constant throughout the day and with a marked burst of GH secretion approximately one hour following the onset of sleep (deep or stage III and IV sleep). Other factors that stimulate GH secretion include exercise, stress, hypoglycemia, and increased serum amino acids, particularly arginine and leucine. Factors that inhibit GH secretion include hyperglycemia and aging. In most individuals, production of GH decreases after 30 years of age. This decrease in GH production is likely a critical factor in the loss of lean muscle mass at a rate of 5% per decade and gain of body fat at the same rate after 40 years of age.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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