Chapter 20 (2)

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Mechanisms of Hormonal RegulationChapter 20

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Hormones General characteristics

Specific rates and rhythms of secretion Diurnal, pulsatile and cyclic, and patterns depending

on circulating substances

Operate within feedback systems Affect only cells with appropriate receptors The liver inactivates hormones, rendering the

hormones more water soluble for renal excretion

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The Endocrine System

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Regulation of Hormone Release Hormones are released:

In response to an alteration in the cellular environment

To maintain a regulated level of certain substances or other hormones

Hormones are regulated by chemical, hormonal, or neural factors

Negative feedback Positive feedback

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Feedback

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Hormone Transport Hormones are released into the circulatory

system by endocrine glands Water-soluble hormones circulate in free,

unbound forms Lipid soluble hormones are primarily circulating

bound to a carrier

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Cellular Mechanism of Hormone Action Target cell Up-regulation Down-regulation Hormone effects

Direct effects Permissive effects

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Cellular Mechanism of Hormone Action Hormone receptors

Located in or on the plasma membrane or in the intracellular compartment of the target cell

Water-soluble hormones High molecular weight Cannot diffuse across the plasma membrane

Lipid-soluble hormones Easily diffuse across the plasma membrane and

bind to cytosolic or nuclear receptors

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Cellular Mechanism of Hormone Action Water-soluble hormones

First messenger Signal transduction Second-messenger molecules

Calcium Cyclic adenosine monophosphate (cAMP) Cyclic guanosine monophosphate (cGMP)

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Cellular Mechanism of Hormone Action

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Cellular Mechanism of Hormone Action Lipid-soluble hormones

Steroid hormones Androgens, estrogens, progestins, glucocorticoids,

mineralocorticoids, and thyroid hormones

Diffuse across the plasma membrane Bind to cytoplasmic or nuclear receptors

Activate RNA polymerase DNA transcription and translation

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Lipid-Soluble Hormones

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Structure and Function of the Endocrine Glands Hypothalamic-pituitary axis

Hypothalamus Pituitary gland

Anterior pituitary (adenohypophysis) Posterior pituitary (neurohypophysis)

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The Pituitary Gland

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Posterior Pituitary Hormones Synthesized with their binding proteins in the

supraoptic and paraventricular nuclei of the hypothalamus

Secreted by the posterior pituitary Antidiuretic hormone (ADH)

Controls plasma osmolality Oxytocin

Uterine contractions and milk ejection in lactating women

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Posterior Pituitary Hormones

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Anterior Pituitary Hormones Adrenocorticotropic hormone Melanocyte-stimulating hormone Growth hormone Prolactin Thyroid-stimulating hormone Luteinizing hormone Follicle-stimulating hormone β-lipotropin β-endorphins

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Anterior Pituitary Hormones

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Thyroid and Parathyroid Glands Thyroid gland

Two lobes lateral to the trachea Isthmus Follicles (follicle cells surrounding colloid) Parafollicular cells (C cells)

Secrete calcitonin Regulation of thyroid hormone secretion

Thyrotropin-releasing hormone and thyroid stimulating hormone

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Thyroid and Parathyroid Glands Thyroid hormones

90% T4 and 10% T3

Bound to thyroxine-binding globulin, thyroxine-binding prealbumin, or albumin

Affect growth and maturation of tissues, cell metabolism, heat production, and oxygen consumption

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Thyroid and Parathyroid Glands

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Thyroid and Parathyroid Glands Parathyroid glands

Small glands located behind the upper and lower poles of the thyroid gland

Produce parathyroid hormone Regulator of serum calcium Antagonist of calcitonin

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Parathyroid Glands

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Parathyroid Glands

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Endocrine Pancreas The pancreas is both an endocrine and an

exocrine gland Houses the islets of Langerhans

Secretion of glucagon and insulin Cells

Alpha—glucagon Beta—insulin Delta—somatostatin and gastrin F cells—pancreatic polypeptide

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Endocrine Pancreas

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Endocrine Pancreas Insulin

Synthesized from proinsulin Secretion is promoted by increased blood glucose

levels Facilitates the rate of glucose uptake into the cells

of the body Anabolic hormone

Synthesis of proteins, lipids, and nucleic acids

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Endocrine Pancreas Glucagon

Secretion is promoted by decreased blood glucose levels

Stimulates glycogenolysis, gluconeogenesis, and lipolysis

Somatostatin Possible involvement in regulating alpha and beta

cell secretions

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Endocrine Pancreas

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Adrenal Glands Adrenal cortex

80% of an adrenal gland’s total weight Zona glomerulosa Zona fasciculata Zona reticularis

Adrenal medulla Innervated by the sympathetic and

parasympathetic nervous systems

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Endocrine Pancreas

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Adrenal Glands Adrenal cortex

Stimulated by adrenocorticotropic hormone (ACTH)

Glucocorticoid hormones Direct effects on carbohydrate metabolism Anti-inflammatory and growth-suppressing effects Influence awareness and sleep habits Most potent naturally occurring glucocorticoid is

cortisol

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Adrenal Glands Adrenal cortex

Mineralocorticoid hormones Affect ion transport by epithelial cells

Increase the activity of the sodium pump of the epithelial cells

Cause sodium retention and potassium and hydrogen loss

Most potent naturally occurring mineralocorticoid is aldosterone Regulated by the renin-angiotensin system

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Aldosterone

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Adrenal Glands Adrenal cortex

Adrenal estrogens and androgens Estrogen secretion by the adrenal cortex is minimal The adrenal cortex secretes weak androgens

Androgens are converted by peripheral tissues to stronger androgens such as testosterone

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Adrenal Glands Adrenal medulla

Chromaffin cells (pheochromocytes) Chromaffin cells secrete the catecholamines

epinephrine (majority) and norepinephrine

Release of catecholamines has been characterized as a “fight or flight” response

Catecholamines promote hyperglycemia

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Catecholamines

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Neuroendocrine Response to Stress The endocrine system reacts with the nervous

system to respond to stressors The stress response also involves the immune

system Influenced by corticotropin-releasing

hormone from the hypothalamus

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Tests of Endocrine Function Radioimmunoassay Enzyme-linked immunosorbent assay

(ELISA) Bioassay

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Aging and the Endocrine System Thyroid gland

Glandular atrophy, fibrosis, nodularity, and increased inflammatory infiltrates

Parathyroid glands Related to alterations in calcium balance

Inadequate intake, malabsorption, or renal changes

Adrenal glands Decreased clearance of cortisol