Map out and differentiate a simple endocrine negative feedback loop and one involving the hypothalamus, anterior pituitary and peripheral endocrine gland, and list the major endocrine glands under each type of feedback loop.
3.
Define a releasing hormone and a tropic hormone.
4.
Explain the chemical nature and the characteristics of protein/peptide hormones, catecholamine hormones, steroid hormones, and iodothyronines (thyroid hormones). Include such characteristics as site of regulation (synthesis or secretion), circulating form of hormone, subcellular localization of hormone receptor, and metabolic clearance.
5.
Integrate the concept of peripheral conversion with the function/action of a secreted hormone.
6.
Integrate the intracellular steps associated with a hormone response in a target cell.
he ability of cells to communicate with each other is an underpinning of human biology. As discussed in , cell-to-cell communication exists at various levels of complexity and distance. Endocrine signaling involves (1) the regulated secretion of an extracellular signaling molecule, called a hormone, into the extracellular fluid; (2) diffusion of the hormone into the vasculature and its circulation throughout the body; and (3) diffusion of the hormone out of the vascular compartment into the extracellular space and binding to a specific receptor within cells of a target organ. Because of the spread of hormones throughout the body, one hormone often regulates the activity of several target organs. Conversely, cells frequently express receptors for multiple hormones.
The endocrine system is a collection of glands whose function is to regulate multiple organs within the body to (1) meet the growth and reproductive needs of the organism and (2) respond to fluctuations within the internal environment, including various types of stress. The endocrine system comprises the following major glands (
Fig. 38.1
Endocrine tissues of the pancreas Parathyroid glands
Pituitary gland (in association with hypothalamic nuclei)
Gonads (testes or ovaries) These endocrine glands synthesize and secrete bioactive hormones and, with the exception of gonads, which perform both endocrine and gametogenic functions, are dedicated to hormone production (
Table 38.1 ). A transitory organ, the placenta, also performs a major endocrine function.
In addition to dedicated endocrine glands, there are endocrine cells within organs whose primary function is not endocrine (see
Table 38.1 ). These include cells within the heart that produce atrial natriuretic peptide, liver cells that produce insulin-like growth factor type 1 (IGF-1), cells within the kidney that produce erythropoietin, and numerous cell types within the gastrointestinal tract that produce gastrointestinal hormones. There also exist collections of cell bodies (called nuclei) within the hypothalamus that secrete peptides, called neurohormones, into capillaries associated with the pituitary gland.
A third subset of the endocrine system is represented by numerous cell types that express intracellular enzymes, ectoenzymes, or secreted enzymes that modify inactive precursors or less active hormones into highly active hormones (see
Table 38.1). An example is the generation of angiotensin II from the inactive polypeptide angiotensinogen by two subsequent proteolytic cleavages (see ). Another example is activation of vitamin D by two subsequent hydroxylation reactions in the liver and kidneys to produce the highly bioactive hormone 1,25-dihydroxyvitamin D (vitamin D).
Configuration of Feedback Loops Within the Endocrine System