Chapter 11 Endocrinology Flashcards Flashcards

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Endocrine glands

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Are ductless organs that secrete hormones directly into the bloodstream. Hormones travel to target cells that express specific receptor proteins to alter cellular metabolism.

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Hormone classes

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Include amines, polypeptides/proteins, glycoproteins, and steroids, which differ in structure and solubility. Steroid and thyroid hormones are lipophilic and can diffuse through cell membranes, while peptide hormones are hydrophilic and act via cell-surface receptors. Examples are insulin (peptide), $T_4$ (thyroid amine), and cortisol (steroid).

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Prohormone

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Is a precursor molecule that is processed to form an active hormone, such as proinsulin being cleaved to insulin. Prohormones allow regulated activation and storage prior to secretion.

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Synergistic effect

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Occurs when two hormones act together to produce an effect greater than the sum of their individual effects. Synergy often allows fine-tuning of physiological responses.

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Permissive effect

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Happens when one hormone increases the responsiveness of a target organ to a second hormone. The first hormone does not produce the full effect alone but enables or enhances the action of the second.

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Antagonistic effect

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Takes place when one hormone opposes or inhibits the action of another hormone. Antagonism helps maintain homeostasis by balancing opposing physiological processes.

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Half-life

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Is the time required for the blood level of a hormone to be reduced by half and ranges from minutes to days for different hormones. Hormone half-life influences duration of action and steady-state blood levels.

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Upregulation

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Also called the priming effect, it occurs when exposure to a hormone induces more of its receptors on target cells. This increases target cell sensitivity and amplifies the subsequent response.

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Downregulation

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Is desensitization caused by prolonged exposure to high hormone levels, resulting in decreased receptor numbers on target cells. This reduces responsiveness and prevents overstimulation by persistent hormonal signals.

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Nuclear receptors

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Are intracellular receptors for lipophilic hormones that act as transcription factors when bound by ligands. They include steroid and thyroid hormone receptor families and regulate gene transcription in a genomic action that typically takes at least 30 minutes. Examples include receptors for cortisol and $T_3$/$T_4$.

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cAMP pathway

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Involves hormone binding to a surface receptor that activates a G-protein, which then stimulates adenylate cyclase to convert ATP into cAMP. cAMP activates protein kinase A to phosphorylate target enzymes, and phosphodiesterase terminates the signal by degrading cAMP. This pathway mediates many peptide and glycoprotein hormone effects.

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PLC–IP3/DAG

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Is a second-messenger system where activated phospholipase C cleaves a membrane phospholipid into IP3 and DAG. IP3 triggers $Ca^{2+}$ release from the endoplasmic reticulum and DAG activates protein kinase C; $Ca^{2+}$ also binds calmodulin to activate additional kinases. This pathway mediates effects of some peptide hormones and neurotransmitters.

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Tyrosine kinase

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Receptors have intrinsic tyrosine kinase activity that autophosphorylates upon ligand-induced dimerization, initiating intracellular signaling cascades. This mechanism is used by insulin and many growth factors to regulate metabolism, growth, and gene expression.

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Insulin action

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Is produced by pancreatic beta cells and promotes glucose uptake into cells, glycogen synthesis, and fat storage. Insulin signaling causes translocation of GLUT4-containing vesicles to the plasma membrane, lowering blood glucose concentration. It also uses tyrosine kinase receptor-mediated pathways.

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Anterior pituitary

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Secretes six major trophic hormones including GH, TSH, ACTH, FSH, LH, and prolactin that regulate growth and endocrine target glands. Its secretion is controlled by hypothalamic releasing and inhibiting factors delivered via the hypophyseal portal system and by negative feedback from target gland hormones.

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Posterior pituitary

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Stores and releases hormones synthesized in the hypothalamus, principally antidiuretic hormone (ADH/vasopressin) and oxytocin. These hormones are transported down axons to the posterior lobe and released in response to neuroendocrine reflexes.

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Hypothalamo-pituitary portal

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Is a portal capillary system that carries hypothalamic releasing and inhibiting hormones directly to the anterior pituitary. This direct vascular link allows rapid, concentrated regulation of anterior pituitary secretion.

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Adrenal medulla

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Is neural tissue that secretes catecholamines, roughly 80% epinephrine and 20% norepinephrine, in response to sympathetic preganglionic stimulation. Its hormones prolong and amplify 'fight-or-flight' responses such as increased heart rate, respiration, and glycogenolysis.

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Adrenal cortex

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Is controlled by ACTH and secretes steroid hormones including cortisol, aldosterone, and some androgens. Cortisol promotes gluconeogenesis and inhibits glucose utilization, while aldosterone stimulates renal reabsorption of $Na^+$ and secretion of $K^+$. Chronic cortisol elevation from stress has multiple negative effects on physiology.

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Thyroid hormones

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The thyroid secretes mainly $T_4$ and some $T_3$, which set basal metabolic rate and are essential for growth and development. Most circulating $T_4$ is bound to thyroid-binding globulin (TBG) and must be converted to $T_3$ in target cells to exert genomic effects. Only the free fractions can enter cells to bind nuclear receptors.

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Goiter

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Is an enlarged thyroid that can result from iodine deficiency: low $T_4$/$T_3$ reduces negative feedback and elevates TSH, causing thyroid hypertrophy. Goiter can also occur in Graves' disease where autoantibodies stimulate the gland, producing hyperthyroidism.

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Parathyroid hormone

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Is secreted by parathyroid glands in response to low blood $Ca^{2+}$ and is the primary regulator of plasma calcium. PTH increases blood $Ca^{2+}$ by stimulating bone resorption, increasing renal $Ca^{2+}$ reabsorption, and promoting activation of vitamin D to enhance intestinal calcium absorption.

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Islets of Langerhans

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Are pancreatic endocrine cell clusters containing alpha and beta cells that regulate blood glucose. Alpha cells secrete glucagon to raise blood glucose via glycogenolysis and lipolysis, while beta cells secrete insulin to promote cellular glucose uptake and glycogen/fat synthesis, lowering blood glucose.

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Melatonin

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Is secreted by the pineal gland under control of the suprachiasmatic nucleus (SCN) and helps align circadian rhythms with the light/dark cycle. Its secretion is increased at night and inhibited by light, and it is implicated in sleep regulation and jet lag.

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Prostaglandins

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Are eicosanoids derived from arachidonic acid that have diverse, often tissue-specific effects such as modulating inflammation, vascular tone, smooth muscle activity, and gastric secretion. Cyclooxygenase enzymes COX-1 and COX-2 synthesize prostaglandins and are targets of NSAIDs; nonselective COX inhibitors cause gastric side effects while COX-2 selective drugs aim to reduce those effects.