C H A P T E R 5 HORMONAL REGULATION OF EXERCISE
Learning Objectives w Learn the role of your endocrine system in maintaining homeostasis in the body during rest and during acute physical activity. w Learn the difference between steroid and nonsteroid hormones and their actions within the body. w Discover the roles of hormones in regulating fat and carbohydrate metabolism. w Find out how hormones help keep your fluid levels in check and prevent dehydration.
Endocrine and Exocrine Functions w Endocrine system composed of endocrine glands—ductless glands that secrete hormones into the blood to affect distant cells and tissues w Exocrine glands secrete their products into ducts (e.g. sweat glands, salivary glands) w Pancreas has both functions: exocrine—digestive enzymes; endocrine—insulin and glucagon
Hormones w Chemical messengers from endocrine glands that travel in the blood placing them in contact with all cells w Non-endocrine tissues can release chemicals that act as hormones (e.g., sympathetic nerve endings release NE) w Hormones travel in the blood to their specific target organs wReceptors are specific to hormones so that only the correct hormone will “fit” the correct receptor—each cell has 2,000 to 10,000 specific receptors
Steroid Hormones w Lipid soluble (hydrophobic, or “water fearing”) w Diffuse easily through cell membranes; receptors located within cell w Chemical structure is derived from or is similar to cholesterol w Secreted by adrenal cortex (e.g., cortisol), ovaries (e.g., estrogen), testes (e.g., testosterone), placenta (e.g., estrogen)
Action of a Steroid Hormone Powers and Howley, Exercise Physiology, 2004
Nonsteroid Hormones w Not lipid soluble (hydrophilic, or “water loving”) w Cannot normally diffuse through cell membranes; receptors located on cell membrane that activate secondary intracellular pathways w Amino acid derivatives (e.g., epinephrine) and protein or peptide hormones (e.g., insulin)
Action of a Non-steroid Hormone Powers and Howley, Exercise Physiology, 2004
Control of Hormone Release w Plasma levels of specific hormones fluctuate. w Secretion is regulated by a negative feedback system. w Target cells can also alter their number of hormone receptors via down-regulation or up-regulation. Stimulus Gland Hormone
Alteration in Number of Receptors Down-regulation—Decrease in number of cell receptors; less hormone can bind to the cell to affect cell function Up-regulation—Increase in number of cell receptors; more hormone can bind to the cell to affect cell function Receptors are proteins, so up-regulation involves increased receptor protein synthesis, and down-regulation involves decreased receptor protein synthesis
Thus, there are two things that determine a cell’s response to a hormone 1. Amount of hormone in the blood 2. The number of receptors for the hormone on the cell
Thought Question Assume a weight lifter has taken high doses of anabolic steroids over a period of time. Would you expect the receptors in the muscle cells that bind the anabolic steroids to up-regulate or down-regulate? What does this say about the “sensitivity” of the muscle fibers to the steroids?
Growth Hormone (Anterior Pituitary Gland) w Promotes long bone growth and muscle growth and hypertrophy by facilitating amino acid transport into muscle fibers w Cloned human growth hormone taken with anabolic steroids to stimulate muscle growth (illegal, of course) w Levels are elevated during aerobic exercise in proportion to exercise intensity – directly stimulates lipolysis (FFA release from triglycerides) Giantism – oversecretion during development Dwarfism – undersecretion Acromegaly – oversecretion in adulthood
Antidiuretic Hormone Response to Exercise Powers and Howley, Exercise Physiology,2004
ENDOCRINE ORGANS Thyroid Gland Parathyroid Gland
Triiodothyronine (T3) and Thyroxine (T4) w Increase protein and enzyme synthesis w Increase size and number of mitochondria in cells w Promote rapid cellular uptake of glucose w Enhance glycolysis and glycogenesis w Increase FFA availability for oxidation Calcitonin w Decreases plasma calcium concentration w Acts primarily on bones and kidneys Hormones of the Thyroid Gland (Hypothyroidism; hyperthyroidism)
Parathyroid Hormone (Parathyroid Gland) The parathyroid gland produces parathyroid hormone (PTH), which regulates plasma calcium and plasma phosphate concentrations by targeting the bones, intestines, and kidneys.
ENDOCRINE ORGANS Adrenal (Suprarenal) Glands 1. Cortex 2. Medulla
Mineralocorticoids wAldosterone maintains sodium balance in extracellular fluids Glucocorticoids • Maintain consistent plasma glucose levels between meals; anti-inflammatory and decreases sensation of pain Gonadocorticoids w Released in addition to those released by reproductive organs but in lesser amounts w Include androgens, estrogens, and progesterones Steroid Hormones of the Adrenal Cortex
Plasma Volume during Exercise Following the initial drop due to contracting muscles “squeezing” water out of the muscle vessels (usually <10%), plasma volume remains relatively constant throughout exercise in thermo-neutral environments even with sweating and loss of water in the respiration due to: 1. The actions of aldosterone and ADH in prevention of water loss in the urine, 2. Water returning from the exercising muscles to the blood, and 3. The increase in amount of water produced by metabolic oxidation within muscles.
Postexercise Fluid Balance Fluid loss from the blood results in hemoconcentration—a concentration of the particles of the blood, which results in an increase in viscosity (or “thickness”), which increases the work of the heart. Hemodilution, on the other hand, is a dilution of the constituents of the blood caused by gains in fluid to the blood.
Hormones of the Adrenal Medulla wCatecholamines — epinephrine and norepinephrine w Stimulated by sympathetic nervous system to prepare you for “fight or flight” – extremely important during exercise ►Increase rate and force of heart contraction, blood pressure, and respiration ►Increase metabolic rate, glycogenolysis, and release of glucose and FFA into blood ►Increase blood flow to the active skeletal muscles through redistribution of cardiac output, i.e., vasodilation of vessels active muscles and vasoconstriction of vessels in inactive tissues (including muscles)
Catecholamine Receptors wAlpha- and beta-adrenergic receptors bind the catecholamines to exert the sympathetic effects in cells ►An important example of an alpha-adrenergic effect is the contraction of smooth muscle in the arterioles, i.e., vasoconstriction, which reduces blood flow to the tissue ►An example of a beta-adrenergic effect is the increased rate and increased force of heart contraction ►Another example of a beta-adrenergic effect is the stimulation of glycogenolysis in skeletal muscle fibers during exercise
Exercise plasma catecholamine concentrations at a fixed exercise intensity after training Powers and Howley, Exercise Physiology, 2004
ENDOCRINE ORGANS Pancreas 1. Beta cells 2. Alpha cells
Hormones of the Pancreas Insulin—secreted from beta cells when plasma glucose levels are elevated (hyperglycemia), e.g., after eating Glucagon—secreted from alpha cells when plasma glucose concentrations are below normal (hypoglycemia), e.g., during prolonged exercise
Two “Rules of Thumb” • Insulin is the only major hormone that decreases in the blood during exercise; the others increase • Blood levels of most hormones are “blunted” with • training
ENDOCRINE ORGANS Reproductive Glands 1. Ovaries (female) 2. Testes (male)
Reproductive Hormones w Androgens (i.e., testosterone) - male w Estrogens - female w Progesterone - female
ENDOCRINE ORGANS Kidneys 1. Erythropoietin 2. (Renin)
Erythropoietin and (Renin) Though not a major endocrine organ, the kidneys produce a hormone called erythropoietin, which regulates red blood cell (erythrocyte) production by stimulating bone marrow cells. This hormone is important in our adaptation to training and to altitude by increasing the number of RBCs and thus, the oxygen-carrying capacity of the blood. (The kidneys also release renin, an enzyme that initiates the cascade resulting in aldosterone release, for blood pressure control and fluid and electrolyte balance.)
Hormonal Changes that Increase Blood Glucose during Exercise 1. Increases in epinephrine 2. Increases in norepinephrine 3. Increases in glucagon 4. Decreases in insulin 5. Increases in cortisol Remember: the concentration of glucose in the blood is a result of the balance between the rate of glucose appearance and the rate of glucose disappearance
PLASMA LEVELS OF GLUCOSE AND INSULIN DURING CYCLING AT 65% TO 75% VO2MAX .
Thought Question The large number of hormones involved in maintaining blood glucose levels during exercise tells us something about the importance of this physiological function. Why is it so important biologically to prevent a drop in blood glucose?
Hormonal Control of Fat Breakdown When carbohydrate reserves are low, hormones accelerate the oxidation of fats to ensure your muscles get the energy they need. The rate of triglyceride breakdown into FFA and glycerol partly determines the rate at which muscles use fat as a fuel source during exercise.
Hormones Increasing Triglyceride Breakdown into FFA w Increases in cortisol w Increases in epinephrine w Increases in norepinephrine w Decreases in insulin
PLASMA LEVELS OF EPINEPHRINE, NOREPINEPHRINE, GH, AND FFA DURING CYCLING AT 65% TO 75% VO2MAX .