Chapter 017 Hormonal

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Mechanisms of Hormonal Regulation

Chapter 17


The Endocrine System

• Functions Differentiation of reproductive and CNS in fetus Stimulation of growth and development Coordination of the male and female reproductive

system Maintenance of internal environment Adaptation to emergency demands of body


The Endocrine System


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 target cells with appropriate receptors The liver inactivates hormones, rendering the

hormones more water soluble for renal excretion


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


Feedback


Hormone Transport

• Hormones are released into the circulatory system by endocrine glands Water-soluble hormones circulate in free, unbound

forms• Short-acting response• Bind to surface receptors

Lipid-soluble hormones are primarily circulating bound to a carrier

• Rapid and long-lasting response• Bind to plasma membrane


Cellular Mechanism of Hormone Action

• Target cell

• Up-regulation

• Down-regulation

• Hormone effects Direct effects Permissive effects



• Hormone receptors Located in 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



• Water-soluble hormones First messenger

• Signal transduction Second-messenger molecules

• Calcium• Cyclic adenosine monophosphate (cAMP)• Cyclic guanosine monophosphate (cGMP)• Tyrosine kinase system



• Lipid-soluble hormones Steroid hormones

• Androgens, estrogens, progestins, glucocorticoids, mineralocorticoids, thyroid hormones, vitamin D, retinoid

Diffuse across the plasma membrane• Bind to cytoplasmic or nuclear receptors

Activate• DNA transcription and translation


Lipid-Soluble Hormones


Structure and Function of the Endocrine Glands

• Hypothalamic-pituitary axis Hypothalamus Pituitary gland

• Anterior pituitary (adenohypophysis)– Pars distalis– Pars tuberalis– Intermedia


Structure and Function of the Endocrine Glands

Pituitary gland • Posterior pituitary (neurohypophysis)

– Median eminence– Pituitary stalk– Pars nervosa


The Pituitary Gland


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


Anterior Pituitary Hormones

• Adrenocorticotropic hormone (ACTH)

• Melanocyte-stimulating hormone (MSH)

• Somatotropic hormone

• Prolactin

• Thyroid-stimulating hormone



• Luteinizing hormone

• Follicle-stimulating hormone

• Beta-lipotropin

• Beta-endorphins


Thyroid and Parathyroid Glands

• Thyroid gland Two lobes lie on either side of 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



• 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



• 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


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


Endocrine Pancreas


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 Amylin

• Peptide hormone co-secreted with insulin


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

• Gastrin and pancreatic peptides


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


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


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


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


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


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


Tests of Endocrine Function

• Radioimmunoassay (RIA)

• Enzyme-linked immunosorbent assay (ELISA)

• Bioassay


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

Chapter 017 Hormonal

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