CM Q8

The mechanisms of hormone action differ for nonsteroid and steroid hormones.

Nonsteroid hormones (also known as first messengers) bind to receptors on the target cell membrane, triggering intracellular second messengers such as cyclic AMP to affect the cell’s activities.

The primary effects produced by a steroid hormone occur by its binding to receptors within the target cell nucleus and influencing cell activity by acting on DNA—a slower process than nonsteroid action.

Secondary effects may occur when steroid hormones bind to membrane receptors to rapidly trigger functional changes in the target cell.

Hormone secretion is controlled by homeostatic feedback.

Negative feedback mechanisms reverse the direction of a change in a physiological system.

Positive feedback mechanisms amplify physiological changes; positive feedback is uncommon.

Endocrine regulation of body function usually operates at multiple levels of control at the same time for better efficiency and precision.

Prostaglandins, often called tissue hormones or paracrine agents, are powerful lipid substances found in a wide variety of body tissues; prostaglandins are modified fatty acids.

Prostaglandins are typically produced in a tissue and diffuse only a short distance to act on cells in that tissue.

Several classes of prostaglandins include prostaglandin A (PGA), prostaglandin E (PGE), and prostaglandin F (PGF).

Prostaglandins influence many body functions, including respiration, blood pressure, gastrointestinal secretions, and reproduction.

Now we’ll review specific endocrine glands.

The pituitary gland is located in a bony depression called the sella turcica of the sphenoid bone in the skull; it is connected to the hypothalamus by a pituitary stalk. The pituitary gland is divided into anterior and posterior regions. The anterior pituitary—also called the adenohypophysis; is made up of glandular epithelium. The posterior pituitary—also called the neurohypophysis; is made up of nervous tissue.

The anterior pituitary gland secretes these major hormones:

Thyroid-stimulating hormone (TSH);

Adrenocorticotropic hormone (ACTH);

Follicle-stimulating hormone (FSH);

Luteinizing hormone (LH);

Growth hormone (GH); and

Prolactin, also called lactogenic hormone, abbreviated PRL.

Now we’ll review the functions of the major hormones of the adenohypophysis:

Thyroid-stimulating hormone stimulates growth of the thyroid gland and it also stimulates the thyroid gland to secrete thyroid hormone

Adrenocorticotropic hormone stimulates growth of the adrenal cortex and stimulates it to secrete glucocorticoids (mainly cortisol).

Follicle-stimulating hormone initiates growth of ovarian follicles each month in the ovary and stimulates one or more follicles to develop to the stage of maturity and ovulation; FSH also stimulates estrogen secretion by developing follicles; and stimulates sperm production in the male.

Luteinizing hormone acts with follicle-stimulating hormone to stimulate estrogen secretion and follicle growth to maturity; LH causes ovulation, causes luteinization of the ruptured follicle; stimulates progesterone secretion by the corpus luteum; and causes interstitial cells in the testes to secrete testosterone in the male.

Growth hormone stimulates growth by accelerating protein anabolism; it also accelerates fat catabolism and slows glucose catabolism; by slowing glucose catabolism, it tends to increase blood glucose to a higher than typical level, called hyperglycemia.

Prolactin, or lactogenic hormone, stimulates breast development during pregnancy and secretion of milk after delivery of the baby.

The posterior pituitary gland (also called the neurohypophysis) secretes just two hormones: antidiuretic hormone (ADH) and oxytocin (abbreviated OT).

Antidiuretic hormone accelerates water reabsorption from urine in the kidney tubules into the blood, thereby decreasing urine secretion.

Oxytocin stimulates the pregnant uterus to contract; may initiate labor; causes glandular cells of the breast to release milk into ducts; and enhances social bonding.

The hypothalamus produces the posterior pituitary hormones ADH and oxytocin

After production in the hypothalamus, hormones pass along axons into the pituitary gland.

The secretion and release of posterior pituitary hormones is controlled by nervous stimulation; hence the name “neurohypophysis.”

The hypothalamus also regulates anterior pituitary secretion.

Releasing hormones and inhibiting hormones from the hypothalamus control secretion by the anterior pituitary; they reach the anterior pituitary through a direct capillary connection.

The hypothalamus controls many body functions related to homeostasis, including temperature, appetite, and thirst.

The thyroid gland is located in the neck, just inferior to the larynx.

Its tissue is made up of thyroid follicles filled with colloid.

Hormones produced by the thyroid gland include thyroxine (T4) and triiodothyronine (T3); they are produced by follicle cells and stored in the colloid of follicles.

The hormone calcitonin is made by cells outside the follicle walls.

Thyroid hormones accelerate catabolism and energy production, thereby increasing the body’s metabolic rate.

Calcitonin decreases the blood calcium concentration by inhibiting breakdown of bone, which would otherwise release calcium into the blood.

The parathyroid glands are small lumps of glandular tissue located on the posterior surface of thyroid.

They release parathyroid hormone, abbreviated PTH.

PTH increases blood calcium concentration by increasing the breakdown of bone with the release of calcium into the blood.

Parathyroid hormone and calcitonin have antagonistic effects that help maintain stable blood calcium concentrations needed for good health.

The adrenal glands are located on the superior surface of each kidney; their outer region—the cortex—is glandular and their inner region—the medulla—is secretory nervous tissue. Hormones produced by the adrenal cortex include mineralocorticoids—chiefly aldosterone—and glucocorticoids—chiefly cortisol (also called hydrocortisone).

The adrenal cortex also secretes small amounts of male hormones (or androgens) in both sexes.

The adrenal cortex has three cell layers or zones: an outer layer, an inner layer, and a middle layer.

The outer layer secretes mineralocorticoids. These hormones increase blood sodium and decrease blood potassium concentrations by accelerating kidney tubule reabsorption of sodium and excretion of potassium.

The inner layer of both men and women’s adrenal glands secretes male androgens, which are sex hormones similar to testosterone; they have a role in reproductive development.

The middle layer secretes glucocorticoids. Cortisol is the main glucocorticoid. Glucocorticoids have many functions. They help maintain normal blood glucose concentration by increasing gluconeogenesis—the formation of “new” glucose from amino acids produced by the breakdown of proteins, mainly those in muscle tissue cells; they also affect the conversion to glucose of fatty acids produced by the breakdown of fats stored in adipose tissue cells.

Glucocorticoids play an essential part in maintaining normal blood pressure. They make it possible for epinephrine and norepinephrine to maintain a normal degree of vasoconstriction, a condition necessary for maintaining normal blood pressure.

They act with epinephrine and norepinephrine to produce an anti-inflammatory effect, to bring about normal recovery from inflammations of various kinds, and they produce an anti-immunity, antiallergy effect; bringing about a decrease in the number of lymphocytes and plasma cells and therefore a decrease in the number of antibodies formed.

The secretion of glucocorticoid quickly increases when the body is thrown into a condition of stress; high blood concentration of glucocorticoids, in turn, brings about many other stress responses.

Note that chronic stress can disturb the body’s balance of metabolic and immune functions.

Now that we’ve reviewed the hormones of the adrenal cortex, the next hormones to review are those of the adrenal medulla: epinephrine, or adrenaline, and norepinephrine.

Hormones of the adrenal medulla help the body resist stress by intensifying and prolonging the effects of sympathetic stimulation. Increased epinephrine secretion is the first endocrine response to stress.

Next turn your attention to the pancreas, which is actually both an endocrine and exocrine gland.

Islands of endocrine tissue are scattered within the exocrine tissue of the pancreas, a digestive gland near the junction of the stomach and small intestine.

Hormones secreted by the pancreas include glucagon, secreted by alpha cells, and insulin, secreted by beta cells.

Glucagon increases the blood glucose level by accelerating liver glycogenolysis, the conversion of glycogen to glucose.

Insulin decreases the blood glucose by accelerating the movement of glucose out of the blood into cells, which increases glucose metabolism by cells.

Next we’ll discuss hormones of the sex glands.

The female sex glands, the ovaries, contain two structures that secrete hormones—the ovarian follicles and the corpus luteum.

Estrogen (the feminizing hormone) affects the development and maturation of breasts and external genitals, as well as the development of adult female body contours.

Estrogen also initiates the menstrual cycle.

The interstitial cells of the male sex glands, the testes, secrete the male hormone testosterone.

Testosterone (the masculinizing hormone) affects maturation of the external genitals, beard growth, voice changes at puberty, and development of musculature and body contours typical of the male.

Just three more endocrine glands to review: the thymus, placenta, and pineal gland.

The thymus secretes the hormone thymosin, which is actually a group of related hormones. Thymosin plays an important role in the development and function of T cells, which are agents of the body’s immune system.

The placenta functions as a temporary endocrine gland during pregnancy. Hormones of the placenta include chorionic gonadotropins, estrogens, and progesterone. Placental hormones help establish and maintain pregnancy.

The pineal gland is a small gland near the roof of the third ventricle of the brain. Glandular tissue predominates in pineal glands of children and young adults. It becomes fibrous and calcified with age. It is sometimes called the third eye because its influence on secretory activity is related to the amount of light entering the eyes. The pineal gland secretes melatonin, which inhibits ovarian activity and regulates the body’s internal clock.

Now that you’ve reviewed the individual endocrine glands, consider that there are additional endocrine functions throughout the body.

Many organs, such as the stomach, intestines, and kidney, produce endocrine hormones.

The stomach lining produces ghrelin, which affects appetite and metabolism.

The atrial wall of the heart secretes atrial natriuretic hormone, which stimulates sodium loss from the kidneys.

Fat-storing cells secrete leptin, which controls how full or hungry we feel.

Hormone actions occur in every organ of the body and will be discussed in later chapters as well.

This concludes the audio review of chapter 11.