Hormone Action page 504

Endocrine glands secrete hormones that affect target cells possessing specific receptors.

1. Chemistry of hormones a. Each kind of hormone has a special molecular structure and is very potent.

b. Chemically, hormones are steroids, amines, peptides, proteins, or glycoproteins.

2. Actions of hormones a. Steroid hormones

(1) Steroid hormones enter target cells and combine with receptors to form complexes.

(2) These complexes activate specific genes in the nucleus, which direct synthesis of specific proteins.

(3) The degree of cellular response is proportional to the number of hormone-receptor complexes formed.

b. Nonsteroid hormones

(1) Nonsteroid hormones combine with receptors in the target cell membrane.

(2) A hormone-receptor complex stimulates membrane proteins, such as adenylate cyclase, to induce the formation of second messenger molecules.

(3) A second messenger, such as cAMP, activates protein kinases.

(4) Protein kinases activate certain protein substrate molecules, which, in turn, change cellular processes.

(5) The cellular response to a nonsteroid hormone is amplified because the enzymes induced by a small number of hormone-receptor complexes can catalyze formation of a large number of second messenger molecules.

3. Prostaglandins a. Prostaglandins are paracrine substances present in small quantities that have powerful hormonelike effects.

b. Prostaglandins modulate hormones that regulate formation of cyclic AMP.

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