Hormones and Exercise

I. Endocrine Overview

A. Endocrine and nervous system work together to achieve homeostasis

B. Different modes of action: Nervous System versus Endocrine System

C. Overview of hormone action

1. Endocrine glands release hormones directly into the blood

2. Hormones bind to tissues that possess receptors for that hormone

3. The plasma hormone concentration and the tissue receptor concentration are the primary determinants of the magnitude of the response to the hormone

D. Major Endocrine Glands

E. Types of Hormones – Non-steriod and Steriod

1. Non-steriod: amines (eg catecholamines), peptides, prostaglandins

2. Steriod – lipid soluble, cytosolic or nuclear receptor

F. Prostaglandins

1. Technically not hormones

2. Derived from arachidonic acid (a fatty acid)

3. Most act locally

4. Formation is blocked by NSAID’s

5. Some are involved in inflammatory response

II. Hormone-Receptor Interactions

A. Mechanism: Each hormone can bind to only one or a few receptors (specificity)

B. Magnitude of effect dependent on:

1. Concentration of the hormone

2. Number of receptors on the cell

3. Affinity of the receptor for the hormone

C. Blood Hormone Concentration determined by

1. Rate of secretion of hormone from endocrine gland

2. Rate of metabolism or excretion of hormone

3. Quantity of transport protein

4. Changes in plasma volume

5. Secretion generally regulated by negative feedback (hormone product/effect inhibits further hormone secretion)

D. Control of receptor number

1. Receptor downregulation (desensitization) – diabetes mellitus

2. Regulation of receptors for one hormone by another (e.g., estrogen and progesterone)

E. Mechanism of hormone action

1. Steroid: Stimulating DNA transcription and RNA translation to increase protein

2. Non-Steriod: Activating second messengers

III. The Endocrine Glands

A. Hypothalamus

1. Major products: Somatostatin (growth hormone inhibiting hormone), GHRH (growth hormone releasing hormone), TRH (thyrotropin releasing hormone), CRH (corticotropin releasing hormone), GnRH (gonadotropin releasing hormone)

2. Controls activity of the anterior and posterior pituitary glands

3. Influenced by positive and negative input

B. Anterior Pituitary Gland

1. Six major hormones, all increased by exercise

2. Growth Hormone - Somatotrophin

3. ACTH – stimulates adrenal cortex

4. TSH – stimulates thyriod

5. Prolactin

6. FSH/LH

7. 5. MSH – stimulates melanocytes

C. Posterior Pituitary Gland

1. Secretes antidiuretic hormone (ADH or vasopressin) and oxytocin

2. Responds to neural signals from the hypothalamus

3. ADH reduces water loss from the body to maintain plasma volume

4. ADH acts on the kidneys & blood vessels

5. ADH release stimulated by high plasma osmolarity and low plasma volume (sensors in hypothalamus) due to sweating

6. Exercise effect on ADH

D. Thyroid Gland

1. Triiodothyronine (T₃) & thyroxine (T₄)

- Increase basal metabolic rate – by up to 60 to 100%

- Increase protein synthesis

- Increase size and # of mitochondria in most cells

- Promote glucose uptake by cells

- Increase glycolysis and gluconeogenesis

- Increase FFA availability for metabolism

- Regulation

- Effect of exercise

2. Calcitonin

- Regulation of plasma Ca⁺⁺

E. Parathyroid Gland – Parathyriod Hormone

1. PTH is primary regulatory of plasma Ca⁺² concentration

2. Increases release of Ca⁺² from bone (stimulates osteoclasts)

3. Increases Ca⁺² absorption in intestines

4. Increases Ca⁺² retention in kidneys

5. Chronic exercise increases PTH level, builds bone

IV. Growth Hormone in more detail

A. Essential for normal growth (anabolic)

1. Deficiency stunts growth

2. Excess known as acromegaly

3. Stimulates protein synthesis, long bone growth, organ growth

B. Secretion

1. Stimulated by GHRH (from hypothalamus) (via cAMP and Ca++)

2. Inhibited by somatostatin (from hypothal) (decreases cAMP and Ca++)

3. GHRH pulses, GH secreted in pulses

4. Secretion peaks at puberty

5. Exercise stimulates secretion in proportion to intensity

6. Fasting, stress, androgens and estrogens also stimulate secretion

7. GH, somatomedins, old age, obesity, and pregnancy also inhibit release

C. Metabolic effects

1. Lypolysis and mobilization of fatty acids from adipose tissue

2. Decreases glucose uptake by fat and muscle cells

3. Increases cellular amino acid uptake

D. Action

1. Acts primarily through IGF-1 (insulin-like growth factor 1)

- a Somatomedin

- IGF-1 mainly from liver

- Strong anabolic action

2. Stimulates glucose uptake by muscle and fat

3. Abuse can cause diabetes, cardiomegally

4. Stimulates renin-angiotensin-aldosterone axis, inhibits secretion of ANP