Endocrinology: Metabolism Lecture Notes PDF

Endocrinology Learning objectives Refresh key concepts of endocrinology Understand the hormonal control of body metabolism Content: Lecture 1 A brief refresher on endocrinology: functions of the endocrine system regulation of hormones classes of hormones hydrophi...

Endocrinology Learning objectives Refresh key concepts of endocrinology Understand the hormonal control of body metabolism Content: Lecture 1 A brief refresher on endocrinology: functions of the endocrine system regulation of hormones classes of hormones hydrophilic vs lipophilic hormones Metabolism: the thyroid gland & its hormones structure of the gland & synthesis of thyroid hormones function of thyroid hormones control of thyroid function  Endocrine system functions Integrates function across multiple physiological systems regulate metabolism, energy, water and electrolyte balance allow body to cope with stress regulate growth and bone structure control reproduction regulate circulation and red blood cell production control digestion & absorption of food Background reading: Sherwood - Chapters 18, 19 & parts of 20  Regulation of Hormone activity Two control pathways of hormone secretion  Central regulation controlled by the brain affected by negative-feedback loops, neuroendocrine reflexes, rhythms (e.g. diurnal) Can be fast, slow or long-term responses  Direct regulation endocrine cells respond directly to changes in extra- cellular fluid (especially plasma) levels of substances (e.g. glucose, calcium) very rapid response to critical needs  From BIOL1050/1: Central regulation Hormone cascade pathways: from endocrine glands in the brain to the body Direct regulation: Cells respond directly to changes in extracellular fluid (plasma) levels Fig. 19-20, p 699  Blood-borne chemical messengers: Hormones vs Neurotransmitters Fig. 4-20d, p. 114 Fig. 4-20e, p. 114  Classes of hormones Peptides: chains of amino acids, e.g. ADH, growth hormone, insulin. Hydrophilic (i.e. water soluble). Stored prior to release. Amines: derived from the amino acid ‘tyrosine’. All are stored. Catecholamines (adrenaline, noradrenaline & dopamine) - Hydrophilic; or Thyroid hormones - Lipophilic (i.e. fat soluble). Steroids: derived from cholesterol where appropriate enzymes for conversion are present e.g. cortisol, testosterone, oestrogens. Lipophilic. Not stored, released by diffusion.  Hydrophilic vs Lipophilic hormones Hydrophilic – Likes water – Can’t get through plasma membrane – Fast onset, short-acting Lipophilic (hydrophobic) – Hates water – Can diffuse across plasma membrane – Slower onset, longer- acting Saladin: Fig 17.20 Endocrinology: Metabolism  Metabolism ‘Metabolism’ refers to all the chemical reactions occurring in the body (‘metabolism’ is also for specific processes, eg breakdown of glucose) Includes ‘external work’ (energy used by skeletal muscles to move) & ‘internal work’ (energy used to sustain life) Rate of energy expenditure referred to as ‘metabolic rate’ (usually Calories/hr or kJ/hr) Two components of metabolic rate: energy used at rest - ‘basal metabolic rate’ (BMR); and additional energy used for activities BMR determined primarily by thyroid hormones  Follicular cells Colloid Thyroid gland Right lobe Trachea Isthmus Left lobe Sherwood Figure 19.1, Pg 684  Thyroid function Thyroid gland lies over the trachea in the neck Contains ‘follicles’, with follicular cells and colloid, produces the thyroid hormones T3 & T4 from tyrosine and iodine TH are amines, lipophilic, transported in plasma bound to carrier proteins, with a balance between bound and free hormone Most TH secreted as T4 which is converted to T3 in tissues – T3 is four times more potent Virtually every tissue in the body affected by TH TH = thyroid hormone  TH: synthesis & release Blood Colloid Thyroid follicular cell *Endoplasmic reticulum/Golgi complex Lysosome Sherwood TGB = Thyroglobulin DIT = Di-iodotyrosine Figure 19.2 I = Iodine T3 = Tri-iodothyronine Page 685 MIT = Monoiodotyrosine T4 = Tetraiodothyronine (thyroxine)  Thyroid function TH increases basal metabolic rate & increases heat production Increases metabolism by influencing fuel metabolism (synthesis & breakdown of protein, fat and carbohydrates) ‘Sympathomimetic’ effect – increases heart responsiveness to adrenaline & noradrenaline (increasing heart rate and contractility) TH is permissive for growth and development (esp. CNS) – promotes effects of growth hormone  Stress Cold in infants TH: control of synthesis and Hypothalamus release Thyrotropin- releasing hormone (TRH) Anterior pituitary Thyroid-stimulating hormone (TSH) Thyroid gland Thyroid hormone (T3 and T4) Metabolic rate and heat production; Figure 19.3 enhancement of growth and CNS development; enhancement of Page 687 sympathetic activity  Summary Endocrinology The endocrine system integrates physiological functions Hormonal activity is regulated centrally or directly The class a hormone belongs to (peptide, amine, steroid) will affect their characteristics Metabolism & thyroid hormone Metabolism primarily controlled by TH. Made in thyroid gland, stored in colloid; mostly T4 then converted to T3 in tissues TH increases basal metabolism, fuel metabolism & heat production, has sympathomimetic effects, permissive for growth