Animal Endocrine Systems Biology 2: Form and Function

Hormones regulate... Together with nervous system, responsible for regulation of body organs Secreted by ductless glands into surrounding capillary network Long-term, narrow-to-broad impact mediated by contact of hormone with receptor molecules at target site Receptor molecules may be internal or external to cell Endocrine system is distinct to organ self-regulation, or autocrine/paracrine system

There are four classes of hormone Polypeptides - short chains < 100 amino acids in sequence, e.g., ADH Glycoproteins - long chains (100+) of amino acids connected to carbohydrate, e.g., FSH Amines - derivatives of tyrosine and tryptophan, e.g., epinephrine, thyroxine Steroids - lipid derived –sex steroids, e.g., testosterone –corticosteroids, e.g., aldosterone

In mammals, there is a close association between the endocrine and neural systems... Nervous system-controlled endocrine glands include: –Adrenal medulla –Posterior and anterior pituitary (via Hypothalamus) –Pineal gland Examples of non-nervous system controlled endocrine secretion include pancreas (insulin), and adrenal cortex (aldosterone)

Hormones that enter cells Includes all lipophilic (lipid-soluble) hormones (e.g., steroids, thyroxine) Bind to specific receptors in cytoplasm, which then moves to the nucleus, or binds directly to receptor proteins in nucleus Receptor molecule, once activated, binds to portions of DNA and stimulates transcription, ultimately effecting protein production and cell metabolism

Hormones that do not enter cells... Water soluble hormones that bind to the outside of cell membranes, requiring a secondary messenger inside cell to complete message Secondary messengers include –Cyclic AMP –IP 3 /Ca 2+

Cyclic AMP e.g., effects of epinephrine on  -adrenergic centers Binding of epinephrine to G-protein receptor causes G-protein sub-unit to disassociate G-protein sub-unit binds with membrane enzyme adenlyl cyclase Adenlyl cyclase, now activated, catalyzes formation of cAMP from ATP cAMP binds to and activates protein kinase-A, responsible for phosphorylation of certain proteins specific to tissue/cell –in liver, stimulates conversion of glycogen to glucose –In cardiac muscle, increases speed and force of heart beat

Inositol triphosphate/Ca 2+ e.g., effects of epinephrine on  -adrenergic centers Binding of epinephrine to G-protein receptor causes G-protein sub-unit to disassociate G-protein sub-unit binds with, and activates membrane enzyme phospholipase C Phospholipase C cleaves certain phospholipids to produce IP 3 IP 3 binds with receptors on endoplasmic reticulum, stimulates release of Ca 2+ Ca 2+ binds to calmodulin, which activates different types of protein kinases, causing phosphorylation of different cellular proteins

The Posterior Pituitary neurally derived: hormones are part of neuroendocrine reflex. Secretes Antidiuretic hormone (ADH) and Oxytocin (although both are made in the hypothalamus) –ADH stimulates water retention by the kidneys (alcohol inhibits ADH, causing dehydration) –Oxytocin stimulates uterine contractions and milk-ejection reflex

Anterior pituitary Epithelially-derived tissue, produces –GH, growth hormone (somatotropin) –ACTH, adrenocorticotropic hormone (corticotropin) –TSH, thyroid stimulating hormone (thyrotropin) –Gonadotropins LH (luteinizing hormone) and FSH (follicle stimulating hormone) –Prolactin (PRL) –MSH, Melanocyte-stimulating hormone

Control of the Anterior Pituitary by the Hypothalamus is still hormone-mediated Releasing and inhibitory hormones are secreted by the hypothalamus and carried via a c apillary network connected to a second caillary network - the hypothalamo- hypophyseal portal system for example, gonadotropin releasing hormone (GnRH) stimulates the release of FSH and LH In turn, hypothalamus is controlled by negative feedback inhibition

Other endocrine glands Adrenal glands –The adrenal medulla secrets epinephrine and norepinephrine, triggering alarm responses across the body at various targets, preparing the body for ‘fight or flight’ –The adrenal cortex secretes cortisol and other glucocorticoids, an aid in glucose homeostasis, as well as aldosterone, responsible for salt balance

The pancreas has both exocrine and endocrine functions