Chapter 34 Endocrine Control 34 2 The Vertebrate

Chapter 34 Endocrine Control

34. 2 The Vertebrate Endocrine System • Animal cells communicate with one another by way of a variety of short-range and long-range chemical signals • Animal cells communicate with adjacent cells through gap junctions and by releasing molecules that bind to receptors in or on other cells

Mechanisms of Intercellular Signaling • Many cells secrete local signaling molecules – such as prostaglandins released by injured cells – which affect only neighboring cells • Animal hormones are secreted into interstitial fluid, enter the blood, and are distributed throughout the body

Hormones and the Endocrine System • Internal secretions carried by the blood that influence the activities of specific body organs are called hormones • Endocrine glands and other structures that secrete hormones make up an animal’s endocrine system • Some of the major endocrine glands also have functions unrelated to hormone secretion

Hypothalamus Pineal gland Parathyroid glands Pituitary gland Thyroid gland Thymus gland Adrenal glands Pancreas Gonads Figure 34 -2 p 587

ANIMATED FIGURE: Major human endocrine glands

34. 3 The Nature of Hormone Action • For a hormone to have an effect, it must bind to protein receptors on or inside a target cell • Hormone action involves three steps: 1. A hormone activates a target cell receptor 2. The signal is transduced (changed into a form that affects target cell behavior) 3. The cell makes a response

From Signal Reception to Response Signal Reception Signal Transduction Cellular Response

Table 34 -1 p 588

Intracellular Receptors • Steroid hormones are made from cholesterol and can diffuse across the plasma membrane • Most steroid hormones form a hormone-receptor complex that binds to a promoter inside the nucleus and alters the expression of specific genes

1 A steroid hormone molecule is moved from blood into interstitial fluid bathing a target cell. 2 Being lipid soluble, the hormone easily diffuses across the cell’s plasma membrane. 3 The hormone diffuses through the cytoplasm and nuclear envelope. It binds with its receptor in the nucleus. 5 The resulting m. RNA moves into the cytoplasm and is transcribed into a protein. gene product receptor 4 The hormone– receptor complex triggers transcription of a specific gene. Figure 34 -3 a p 589

Receptors at the Plasma Membrane • Large amine, peptide and protein hormones bind to a receptor at the plasma membrane • Binding triggers formation of a second messenger (molecule that relays signal into cell) • Enzyme converts ATP to c. AMP • c. AMP activates a cascading series of reactions

A peptide hormone molecule, glucagon, diffuses from blood into interstitial fluid bathing the plasma membrane of a liver cell. 1 unoccupied glucagon receptor at target cell’s plasma membrane ATP cyclic AMP + Pi 2 Glucagon binds with a receptor. 3 Cyclic AMP Binding activates another enzyme that catalyzes enzyme in the cell. the formation of cyclic AMP from ATP inside the cell. 4 The enzyme activated by cyclic AMP activates another enzyme, which in turn activates another kind that catalyzes the breakdown of glycogen to its glucose monomers. 5 The enzyme activated by cyclic AMP also inhibits glycogen synthesis. Figure 34 -3 b p 589

Receptor Function and Diversity • Only cells with appropriate and functional receptor proteins can respond to a hormone • Gene mutations that alter receptor structure can prevent or change cell response to a hormone • Examples: • Androgen insensitivity syndrome • Variations in ADH receptors

Take-Home Message: How do hormones exert their effects on target cells? • Hormones exert their effects by binding to protein receptors, either inside a cell or at the plasma membrane. • Steroid hormones often enter a cell and act by altering the expression of specific genes. • Peptide and protein hormones usually bind to a receptor at the plasma membrane. They trigger formation of a second messenger, a molecule that relays a signal into the cell. • Variations in receptor structure affect how a cell responds to a hormone.

ANIMATION: Hormones and target cell receptors

34. 4 The Hypothalamus and Pituitary Gland • The hypothalamus is the main center for control of the internal environment – it connects structurally and functionally with the pituitary gland • The pituitary gland has two parts: • The posterior lobe secretes hormones made in the hypothalamus • The anterior lobe makes its own hormones • The hypothalamus signals the pituitary by way of secretory neurons that make hormones

Control of Anterior Pituitary • Hormones from the hypothalamus control the release of anterior pituitary hormones • Releasing hormones encourage secretion of hormones by target cells • Inhibiting hormones reduce secretion of hormones by target cells • Releasing and inhibiting hormones are secreted into the stalk that connects the hypothalamus to the pituitary

Feedback Controls of Hormone Secretion • Positive feedback mechanisms • Response increases the intensity of the stimulus • Example: Oxytocin and childbirth contractions • Negative feedback mechanisms • Response decreases the stimulus

Take-Home Message: How do the hypothalamus and pituitary gland interact? • Some secretory neurons of the hypothalamus make hormones (ADH, OT) that move through axons into the posterior pituitary, which releases them. • Other hypothalamic neurons produce releasers and inhibitors that are carried by the blood into the anterior pituitary. These hormones regulate the secretion of anterior pituitary hormones (ACTH, TSH, LH, FSH, PRL, and GH).

34. 6 Sources and Effects of Other Vertebrate Hormones • In addition to the hypothalamus and pituitary gland, endocrine glands and endocrine cells secrete hormones • The gut, kidneys, and heart are among the organs that are not glands, but include hormone-secreting cells

Table 34 -3 p 593

Multiple Hormone Receptors • Most cells have receptors for multiple hormones, and the effect of one hormone can be enhanced or opposed by another one • Example: Skeletal muscle hormone receptors • Glucagon, insulin, cortisol, epinephrine, estrogen testosterone, growth hormone, somatostatin, thyroid hormone and others

Take-Home Message: What are the sources and effects of vertebrate hormones? • In addition to the pituitary gland hypothalamus, endocrine glands and endocrine cells secrete hormones. • The gut, kidneys, and heart are among the organs that are not considered glands, but do include cells that secrete hormones. • Most cells have receptors for multiple hormones, and the effect of one hormone can be enhanced or opposed by that of another.