Endocrine System: Organization, Physiology, and Response to Exercise PDF

Endocrine System: Organization, Physiology, and Response to Exercise Chapter 20 Objectives Know the body’s major endocrine gland locations List the sequence of events to show how hormones affect specific “target cell” functions Explain how hormones affect enzyme activity and...

Endocrine System: Organization, Physiology, and Response to Exercise Chapter 20 Objectives Know the body’s major endocrine gland locations List the sequence of events to show how hormones affect specific “target cell” functions Explain how hormones affect enzyme activity and enzyme-mediated membrane transport Describe the hormonal, humoral, and neural stimulation influences on endocrine gland activity Know the hormones, function, and effect of acute and chronic exercise for: Anterior and posterior pituitary glands Thyroid and parathyroid glands Adrenal medulla and adrenal cortex Pancreas Endocrine system Endocrine glands: have no ducts, secrete hormone directly into extracellular space Exocrine glands: have secretory ducts to release hormones in a specific location Example: sweat glands Endocrine organs can have endocrine and exocrine functions. Endocrine system overview The endocrine system includes: Endocrine organs/glands Hormones Target/receptor organ Hormones Hormones: chemical messengers produced by endocrine glands that act on a target cell Used to communicate changes in bodily function There are two main categories of hormones: Steroid-derived hormones Amine- and peptide-derived hormones (3rd category: Biologically active lipids) Steroid hormones + lipids are fat-soluble, amine + peptide hormones are not. → Which are soluble in blood? → Which can pass through a plasma membrane? Hormone signaling Actions caused by hormones: Stimulate nuclear DNA to modify intracellular protein synthesis Alter enzymatic activity Activate second messenger system to change plasma membrane transport mechanisms Induce secretory activity Hormones can have local or systemic effects. Hormone signaling Hormones alter cellular activity – the effect is specific to the receptor that is activated. Extent of target cell’s activation by hormone depends on: Hormone concentration Number of receptors Affinity of the hormone for its receptor Hormone receptors Receptors are located either: On plasma membrane (*requires 2 nd messenger) Inside cell Up- and down-regulation of receptors can alter a hormone’s effectiveness. cAMP as a second messenger Cyclic AMP (3′5′-Cyclic Adenosine Monophosphate; aka “cAMP”) is a universal second messenger that activates a specific intracellular protein kinase to produce a response in the target cell. The sequence of reactions set into motion by cAMP depend on: Type of target cell Specific enzymes in the target cell Specific hormones that act as first messenger *cAMP is for non-steroid hormones that cannot enter a cell cAMP as a second messenger Hormones alter enzyme activity Stimulate enzyme production Allosteric modulation: binds hormone and changes shape, which either increases or decreases the enzyme’s effectiveness Activate inactive enzyme forms Also interact with enzymes involved in membrane transport to alter rate of substance uptake into cells. Hormone secretion Hormone secretion adjusts rapidly Factors that determine plasma to meet demands of changing concentration of a hormone: physiological conditions. Synthesis Goal is to maintain homeostasis Catabolism Regulated by negative feedback Transport protein availability loops (*for steroid hormones) Hormone secretion is pulsatile Plasma volume changes Plasma concentration ≠ hormone activity Hormone secretion A. Hormonal stimulation: hormones influence secretion of other hormones B. Humoral stimulation: Changing levels of ions and nutrients in blood, bile, and other body fluids stimulate hormone release C. Neural stimulation: Neural activity controls hormone release  The pituitary also secretes proopiomelanocortin, Anterior pituitary gland an important precursor of some neurotransmitters and other hormones. Secreted by anterior pituitary Target tissue Hormones secreted by target tissue Hypothalamic- Pituitary-Organ Axes Each pituitary hormone has a hypothalamic releasing factor that controls hormone release from the anterior pituitary. Example: growth hormone releasing hormone (GHRH) Releasing factors can also be inhibitory. Example: growth hormone inhibiting hormone (GHIH), also called somatostatin Growth hormone Promotes cell division and cellular proliferation throughout the body. Facilitates protein synthesis by: ↑ amino acid transport through plasma membrane Stimulating RNA formation Activating cellular ribosomes that increase protein synthesis Other metabolic effects: Slows carbohydrate breakdown Initiates mobilization and use of fat for energy Insulinlike growth factors (IGFs) IGF-I and IGF-II synthesized by Factors that influence IGF liver in response to GH release transport: Takes 8–30 hours Binding proteins within muscle Nutritional status IGFs mediate/perpetuate effects Plasma insulin levels of GH: ↑ protein synthesis ↑ cartilage formation Promotes skeletal growth Growth hormone and exercise: Acute ↑ GH pulse amplitude and pulse frequency ↑ plasma concentrations of GH Growth hormone and exercise: Acute GH isoforms released during exercise have longer half-life → can act on target tissues for longer. Optimizes fuel mixture during physical activity: ↓ glucose uptake by cells via inhibiting insulin action ↑ triglyceride breakdown in adipose cells → FFA mobilization Promotes liver gluconeogenesis Preserves plasma glucose for CNS and muscle functions Promotes muscle, bone, and connective tissue growth and remodeling Via IGF signaling → muscle hypertrophy and ↑ muscle, tendon, bone integrity Growth hormone and exercise: Training Maximal intensity exercise results in a similar increase in GH concentration for both trained and sedentary individuals. No change in resting values, lesser rise during exercise after training. Females maintain higher GH concentration at rest, but no difference during prolonged exercise. Posterior pituitary hormones Synthesized in hypothalamus and stored in the posterior pituitary: Antidiuretic hormone (ADH, aka vasopressin) promotes water reabsorption by the kidneys. Oxytocin initiates muscle contraction in uterus, stimulates milk production during lactation. Posterior pituitary hormones and exercise: Acute Sweating decreases plasma volume Osmoreceptors in hypothalamus signal posterior pituitary to release ADH ADH acts on kidneys to promote water reabsorption Preserves plasma volume and body fluids Posterior pituitary hormones and exercise: Chronic After training, concentrations of ADH are slightly lower at the same absolute intensity. No differences at the same relative intensity. Oxytocin response can adapt to aid with athlete performance anxiety. Thyroid hormones Thyroid stimulating hormone (thyrotropin; TSH) controls thyroid gland secretion of T3 and T4 Thyroxine (T4) secretion raises metabolic rate Except in brain, spleen, testes, uterus, and thyroid gland Triiodothyronine (T3) release facilitates neural reflex activity Mostly synthesized from T4 Most T4 and T3 are bound to carrier proteins in the blood T4 and T3: Regulate tissue growth and development, skeletal and nervous system formation, and maturation and reproduction Help maintain blood pressure by increasing adrenergic receptors in blood vessels Thyroid hormones The thyroid gland also releases calcitonin, which regulates high Ca2+ levels. Inhibits Ca2+ absorption in the small intestine ↓ Ca2+ retention in kidneys Promotes Ca2+ deposition into bone: inhibits osteoclasts and stimulates osteoblasts Thyroid hormone regulation Negative feedback regulates thyroid hormone production Thyroid hormones control basal metabolic rate TRH: thyrotropin (aka TSH) releasing hormone calcitonin Parathyroid hormone (PTH) PTH responds to low plasma Ca2+ levels. ↓ calcium levels stimulate PTH release ↑ calcium concentrations inhibit PTH release PTH has 3 main effects: Activation of osteoclasts, release Ca2+ and phosphate into to blood Enhanced Ca2+ reabsorption, ↓ retention of phosphate at kidneys ↑ Ca2+ absorption in small intestines Calcium homeostasis Maintaining plasma Ca2+ concentration is essential for: Nerve impulse conduction Muscle contraction Blood clotting If plasma Ca2+ is low → PTH If plasma Ca2+ is high → calcitonin Thyroid, parathyroid hormones and exercise: Acute Free T4 plasma levels increase by ~35% during physical activity Elevated core temperature likely alters binding characteristics of T4 with carrier protein Dysregulation of thyroid hormone secretion can alter responses to exercise and impact quality of life. Hyper- or hypothyroidism Some evidence suggests that exercise increases PTH release, promoting bone remodeling Thyroid, parathyroid hormones and exercise: Chronic Training results in coordinated pituitary-thyroid response and increased thyroid hormone turnover without incidence of hyperthyroidism. At rest: ↓ total T3 and free T4 concentration after training During exercise: ↑ T3 and T4 turnover No change in TSH concentration after training.