1. A hormone is a regulating chemical made at one place in the body that exerts influence at another part. Hormones are manufactured in ductless glands, and travel to their target organs or cells through the bloodstream. Only certain organs or tissues respond because only they have the proper receptors.
2. Hormones can affect cells at a great distance from the endocrine gland that secretes them. In paracrine regulation, only an immediately local area is affected, and in autocrine regulation, the secretion only affects the cell that secreted it in the first place.
3. All steroid hormones are derived from cholesterol. Steroid hormones enter the nucleus of the target cell, alter gene expression, and thereby initiate or repress transcription. Thyroxine is a nonsteroid hormone that penetrates cells the same way as steroid hormones.
4. Peptide hormones and catecholamines interact with the receptor on the target cell surface, initiating a chain of events.
5. When epinephrine binds to _-adrenergic receptors on the surface of the target cell, G proteins activate adenylyl cyclase, which catalyzes the formation of cAMP from ATP. cAMP then binds to and activates protein kinase A, which can phosphorylate any number of important cellular proteins and effect a change in the cell.
6. The posterior pituitary gland secretes antidiuretic hormone (ADH), or vasopressin, which regulates water reabsorption in the kidneys and intestines. It also secretes oxytocin, which contracts muscles around the ducts of the mammary glands, initiating milk release, and causes uterine contractions during childbirth. These hormones are actually produced in the hypothalamus and are transported via nerve axons connecting the hypothalamus and the posterior pituitary gland.
7. The hormones of the anterior pituitary gland are called tropic hormones because they stimulate other endocrine glands to secrete their hormones, thereby creating a response. The products of the anterior pituitary stimulate the growth and development of other endocrine glands.
8. Generally, the hormones secreted by the hypothalamus are releasing hormones; they initiate the production of specific anterior pituitary hormones. They reach the anterior pituitary via the very short blood vessels that are between the hypothalamus and the anterior pituitary.
9. Parathyroid hormone is secreted by the parathyroid glands and serves as an antagonist to calcitonin. It elevates blood calcium by stimulating osteoclasts to break down bone for release of calcium into the blood, stimulates the kidneys to resorb calcium ions from the urine to increase the calcium concentration in the blood, and activates vitamin D, which facilitates the absorption of calcium by the intestine.
10. Norepinephrine and epinephrine are produced by the adrenal medulla; they serve as neurotransmitters in the sympathetic nervous system, which is related to their effect as hormones because it is similar to nervous function but longer lasting. The hypothalamus does not regulate their production; they are instead triggered by the sympathetic division of the nervous system. The adrenal cortex produces cortisol and aldosterone, which are homeostatically regulated by blood glucose and sodium ion levels, respectively.
11. Insulin is produced when glucose levels become elevated, and it mediates the uptake of glucose by the liver cells, stimulating formation of glycogen in the liver. Glucagon is produced when glucose levels drop, and acts within the liver to increase the conversion of glycogen to glucose.