Endocrine Physiology PDF

Endocrine Physiology Autocrine (within cell) Paracrine (between local cells) Endocrine (between remote cells) Hormones General characteristics: ○ Specific rates and rhythms of secretion ○ Operate within feedback systems; either positive or negative to maintain optimal internal environment ○ Affect o...

Endocrine Physiology Autocrine (within cell) Paracrine (between local cells) Endocrine (between remote cells) Hormones General characteristics: ○ Specific rates and rhythms of secretion ○ Operate within feedback systems; either positive or negative to maintain optimal internal environment ○ Affect only target cells with appropriate receptors and then act on those cells to initiate specific cell functions or activities ○ Are excreted directly by the kidneys or deactivated by the liver, which renders the hormone more water soluble for renal excretion There are basically 3 types of hormones: ○ Steroids – cortisol from the adrenal cortex; estrogen, progesterone from the ovaries; testosterone from the testes ○ Amino acids – tyrosine and thyroxine from the thyroids; catecholamines from the adrenal medulla ○ Proteins – peptides like insulin from the pancreas 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 factors (blood sugar or calcium levels) ○ Endocrine factors (a hormone from one endocrine gland controlling another endocrine gland) ○ Neural control Endocrine regulation by way of feedback circuits is the most important way hormonal secretion is maintained within a physiologic range Negative feedback is the most common type ○ Plasma levels of one type of hormone influence the level of other types of hormones ○ Lack of negative feedback inhibition on hormonal release leads to pathologic conditions Water-soluble hormones circulate in free, unbound forms (insulin, pituitary, hypothalamic, parathyroid, norepinephrine, and epinephrine) ○ Short-acting response because catabolized by circulating enzymes ○ Bind to cell surface receptors Lipid-soluble hormones are primarily circulating bound to a carrier (cortisol and adrenal androgens, aldosterone, estrogens/progesterone, testosterone) ○ Small amt circulate in free or active form ○ Rapid and long-lasting response Target cells – recognize and bind with a high affinity to hormones; initiate a signal; the more receptors, the more sensitive the cell. Up-regulation – low concentrations of hormones increase the number of receptors per cell. Down-regulation – high concentrations of hormones decrease the number of receptors. Hypothalamus Connected to the anterior pituitary by portal blood vessels. Connected to the posterior pituitary by a nerve tract (supraopticohypophsial tract). Produces hormones ○ Prolactin-inhibiting factor (PIF) (Dopamine) – inhibition of the secretion of prolactin ○ Thyrotropin-releasing hormone (TRH) – release of TSH ○ Gonadotropin-releasing hormone (GnRH) – release of LH/FSH ○ Somatostatin – inhibits release of GH and TSH ○ Growth hormone–releasing factor (GRF) – release of GH ○ Corticotropin-releasing hormone (CRH) – release of ACTH ○ Substance P – inhibition of ACTH Hormones of the Posterior Pituitary Antidiuretic hormone (ADH, arginine vasopressin) ○ Controls plasma osmolality. ○ Causes water reabsorption in the kidneys. ○ Is released when plasma osmolality is increased or intravascular volume is decreased. Oxytocin ○ Causes uterine contractions and milk ejection (let-down) in lactating women. ○ Reduces the brain’s responsiveness to stressful stimuli, especially in pregnant and postpartum states Hormones of the Anterior Pituitary Regulation of anterior pituitary hormones is achieved by: ○ Feedback of hypothalamic releasing-inhibitory hormones and factors ○ Feedback from target gland hormones (cortisol, estrogen) ○ Direct effects of neurotransmitters 7 major stimulatory hormones released by the anterior pituitary: ○ Adrenocorticotropic (ACTH) Target organ is the adrenal cortex (gland) ○ ○ ○ ○ ○ ○ Functions to increase steroidogenesis (cortisol and androgenic hormones) Melanocyte-stimulating hormone (MSH) Target organ are the pigment cells Promotes secretion of melanin and lipotropin by anterior pituitary; makes skin darker Thyroid-stimulating hormone (TSH) Target organ is thyroid Functions to increase production and secretion of thyroid hormone, increased iodine uptake, promotes hypertrophy and hyperplasia of thymocytes Growth hormone (GH) Target organ is muscle, bone, liver Is essential to normal tissue growth and maturation. Affects aging, sleep, nutritional status, stress, and reproductive hormones. Is controlled by two hormones from the hypothalamus. Growth hormone–releasing hormone (GHRH): Increases growth hormone secretion. Somatostatin: Inhibits growth hormone. Prolactin Target organ is breast Functions is milk production Has effects on reproductive and immune functions Luteinizing hormone (LH) Target organ in women is granulosa cells; in men is Leydig cells Function in women: ovulation, progesterone production; in men testicular growth, testosterone production Follicle-stimulating hormone (FSH) Target organ in women granulosa cells; in men Sertoli cells Function in women is follicle maturation, estrogen production, in men spermatogenesis Thyroid Gland Produces hormones that control the rates of metabolic processes throughout the body Composed of follicles (follicle cells) – synthesize and secrete some of the thyroid hormones Parafollicular cells (C cells) – secrete calcitonin ○ Calcitonin (thyrocalcitonin) acts to lower serum calcium levels by inhibition of bone-resorbing osteoclasts Thyroid hormone (TH) Thyroxine = (T4) – precursor to Triiodothyronine Triiodothyronine = (T3) – regulates metabolic rate of all cells and processes cell growth When TSH is too high a decrease in or insufficient level of circulating thyroid hormone has occurred= hypothyroidism When TSH is too low then a increase in or excess level of circulating thyroid hormone has occurred = hyperthyroidism Parathyroid Gland Are small glands located behind the thyroid gland. Produce parathyroid hormone (PTH) ○ Regulates serum calcium. ○ Increases serum calcium concentration. ○ Decreases serum phosphate level. ○ Serves as co-factor with vitamin D to increase calcium absorption. ○ Is an antagonist of calcitonin. Endocrine Pancreas The pancreas is both an endocrine and an exocrine gland ○ Endocrine – produces hormones glucagon and insulin ○ Exocrine – produces digestive enzymes Houses the islets of Langerhans ○ Secretion of glucagon and insulin ○ These help to regulate much of the carbohydrate metabolism within the body Types of hormone secreting cells: ○ Alpha— secrete glucagon ○ Beta— secretes insulin and amylin ○ Delta— secrete somatostatin and gastrin ○ F cells— secrete pancreatic polypeptide Nerves from both divisions of the ANS innervate the pancreatic islets ○ Parasympathetic stimulate release of insulin and pancreatic juice by pancreas ○ Sympathetic stimulates alpha cells of pancreas to release glucagon Insulin ○ Synthesized from proinsulin in the beta cells ○ Is an anabolic hormone Promotes synthesis of proteins, lipids, and nucleic acids ○ Facilitates the rate of glucose uptake into the cells of the body therefore controlling blood glucose levels (GLUT4) ○ Facilitates the intracellular transport of potassium, phosphate, and magnesium into the cells ○ ○ Secretion is regulated by chemical, hormonal, and neural control Secretion is promoted by increased blood levels of glucose, increase amino acids, by parasympathetic (vagal) stimulation of the beta cells Secretion diminishes in response to low blood levels of glucose (hypoglycemia), high levels of insulin (through negative feedback to the beta cells), and sympathetic stimulation (catecholamines) of the alpha cells in the islets, hypokalemia Target effects: In the liver – inhibits glycogenolysis (catabolism of glucose; hormones glucagon and epinephrine stimulate glycogenolysis), inhibits gluconeogenesis (formation of glucose from noncarbohydrate sources; such as amino acids and glycerol protein or fats), and inhibits ketogenesis (process by which ketone bodies are produced as a result of fatty acid breakdown). Effects on the muscle – promotes protein synthesis, increases amino acid transport into muscle cells, promotes glycogenesis (conversion of glucose to glycogen) Effects on fat – increases fatty acid synthesis, promotes triglyceride storage into fat cells, decrease lipolysis (breakdown of fat stored in fat cells) Brain and red blood cells do not require insulin for glucose transport Amylin ○ Peptide hormone co-secreted with insulin by beta cells ○ Regulates blood glucose by: Delaying nutrient uptake Suppressing glucagon secretion after meals ○ Has satiety effect ○ Ultimately has a anti-hyperglycemic effect Glucagon ○ Produced by alpha cells and by cells lining the GI tract ○ High glucose levels cause glucagon release to be inhibited; low glucose levels and sympathetic stimulation promote glucagon release ○ Acts on the liver and increases blood glucose by stimulating glycogenolysis and gluconeogenesis Promotes conversion of glycogen to glucose; glucose is then secreted into the circulation = increasing blood sugar ○ Acts as antagonist to insulin Pancreatic somatostatin ○ Produced by delta cells ○ Essential in carbohydrate, fat, and protein metabolism Gastrin – presumably controls secretion of glucagon Ghrelin – stimulates GH secretion, controls appetite, and plays a role in regulation of insulin sensitivity Pancreatic polypeptides – released by F cells (or PP cells) in response to low blood glucose and protein rich meals and signals satiety Adrenal Glands Each adrenal gland consists of 2 separate portions and each which have different hormonal functions – inner medulla and outer cortex Adrenal cortex ○ 80% of an adrenal gland’s total weight ○ Subdivided into 3 zones Zona glomerulosa – outer layer Primarily produces mineralocorticoid aldosterone Zona fasciculata – middle layer – secretes glucocorticoids cortisol, cortisone, and corticosterone Zona reticularis – inner layer – secretes mineralocorticoids the adrenal androgens and estrogens, and glucocorticoids ○ Not directly innervated by the parasympathetic or sympathetic cholinergic fibers ○ Stimulated by adrenocorticotropic hormone (ACTH) ○ Glucocorticoid hormones: Cortisol, cortisone, and corticosterone Increase blood glucose and cause protein breakdown. Have anti-inflammatory, growth-suppressing effects. Decrease immune response, resulting in increased likelihood for infection and poor wound healing. Cortisol: Is the most potent naturally-occurring glucocorticoid hormone. ○ Mineralocorticoid hormones: Aldosterone ○ 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 medulla ○ Chromaffin cells (pheochromocytes) – cells of the medulla ○ Chromaffin cells secrete the catecholamines epinephrine or adrenaline (majority) and norepinephrine