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

– IP3/Ca2+

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/Ca2+

• 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 IP3

• IP3 binds with receptors on endoplasmic reticulum, stimulates release of Ca2+

• Ca2+ 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