Endocrine Physiology

Direct Communication

cells are adjacent and directly connected with each other. substance pass from one cell to another due to gap junctions

Indirect Communication

cells are not directly connected

Paracrine ligands

chemical messengers are secreted by cells to extracellular fluid and affect neighboring cells of a different type

Autocrine Ligands

chemical messengers secreted to extracellular fluid that affect the same cells that produce them by binding to cell surface receptors

Neurotransmitters

secreted by neurons to synapse and affects function of adjacent neuron or muscle cell

Endocrine Ligands

secreted by glands to bloodstream and affect function of cell in another location

Receptors

Needed for the target cell to respond to such hormones. Lack of these will make it impossible for the target cell to respond to the hormone

Gamma-aminobutyric acid, Glutamate, Glycine, Aspartate

The 4 amino acids that act as messengers or neurotransmitters

Polypeptide and Protein Hormones

• most abundant type of hormones

• Building blocks are also amino acids

• Are water soluble and cannot enter the target cells and need to bind to cell surface receptors

• Synthesized in the Rough ER as large proteins called preprohormones

• Cleaved into smaller prehormones in the ER

• Transferred to the Golgi apparatus to be packed into secretory vesicles

• Vesicles stored within the cytoplasm

• Vesicles fuse with cell membrane and contents are released into the bloodstream

Synthesis of peptide hormones

Steroid Hormones

• synthesized from cholesterol in the Smooth ER

• Lipid soluble, requires binding proteins in the plasma

• Able to enter cells by diffusing across the plasma membrane and bind to intracellular receptors

Eicosanoids

• derived from arachidonic acid, cell membrane phospholipid

• involved in paracrine signaling

Hydrophilic messengers

• secreted by exocytosis from the vesicles into the blood as dissolved substances

Hydrophobic messengers

• secreted by diffusion, bound by carrier proteins

Midnight

When does growth hormone peak?

8am before midday

When does ACTH and Cortisol peak?

Night

When does Melatonin peak?

Adenohypophysis

• epithelial component

• 5 cell types that produce 6 hormones

Neurohypophysis

• neural component

• site of secretion for neurohormones from the hypothalamus

• Hypothalamus produces releasing hormones along the axon

• Releasing hormones are released into the bloodstream

• It travels to anterior pituitary where they stimulate pituitary cells to produce tropic hormones

• Tropic hormones are released into the circulation

Process of hypothalamus-pituitary complex

Leutenizing hormone

• acts on the gonads to produce androgen

Follicle-stimulating hormone

• acts on the gonads to produce estrogen and progesterone

Prolactin inhibitory hormone (Dopamine)

Predominantly under inhibitory control by PIH and only with a reduction in dopamine will prolactin be released and stimulate the breast.

Insulin Growth Factor I

• released by the liver when stimulated by growth hormone

• acts on the skeleton to induce chondrogenesis which leads to increase in height

• acts on muscles and adipose tissues to increase protein synthesis and cell proliferation for organ growth

Prolactin

• acts directly on a non-endocrine gland to induce physiologic change

• stimulates alveoli of breast to produce milk

Tonic Inhibitory control

• Releasing hormones stimulate anterior pituitary to produce hormones

• Prolactin will bre released when inhibitory effect of dopamine is produced

ADH and Vasopressin

• primary function is maintenance of osmolality and blood volume

• Presence increases urine osmolality. urine flow and serum osmolality decreases

• stimulated by low blood volume and pressure and high serum osmolality

Thyroid follicle cells

• many small bags containing amorphous fluid/colloid inside

• lined by cuboidal epithelium = thyroid follicle cells/thyrocytes

Parafollicular Cells/C cells

• cells that do not line the coloid

• secretes calcitonin

Blood vessels

• source of iodine for thyroid hormone synthesis

• Thyroid follicle cell produces a protein called thyroglobulin in the Rough ER

• Thyroglobulin is packaged in the golgi apparatus and released into vesicles

• Vesicles go through exocytosis and thyroglobulin is secreted into the lumen

• Lumen stores the protein and forms the colloid

• Iodide taken up by the thyroid is transported into the lumen via pendrin

• Iodide is oxidized and converted into iodine

• Thyroglobulin and iodine is incorporated, residues will be endocytosed

• Thyroxine (T4) and Triiodothyronine (T3) will be released via proteolysis

Process inside the Thyroid

Act for Salt Iodization Nationwide (ASIN Law)

• reduce iodine deficiency

• increase intelligence of population

• 15 IQ points

Blood ionized calcium

• has a narrow range of 1.12-1.32 mmol/L which makes it a narrower range

• decrease in levels also decrease PTH activity

• decrease in levels induce a sharp increase in PTH activity

Anticipatory relationship

• PTH glands anticipate a decrease in ionized calciumlevels

• Increase in activity to maintain ionized calcium in normal range

Maximal PTH hormone concentration

• does not occur below normal range

• occurs on the lower third of normal rage

Low serum calcium concentration

• main stimulus in PTH

• detected by calcium sensory receptors in parathyroid cells

Islets of Langerhans

• basic units of endocrine pancreas

• 1M in a healthy human pancreas

• 1 - 1.5 grams

A cells

• secrete glucagon

• 20% of cells

B cells

• secrete insulin

• 60% of cells

Insulin

• hormone of abundance

• secreted when nutrients are abundant

• promotes storage of glucose and fat

• Glucose goes down in concentration gradient via GLUT 2 receptor

• Glucose is converted by glucokinase into glucose-6-phosphate

• Glucose-6-phosphate undergoes bglycolysis oxidation

• There is an increase in ATP production which closes the ATPK channels

• This depolarizes the plasma membrane which opens the Ca++ channel and allows the entry of Ca

• Increse in cytosolic calcium is a stimulus for release of insulin

Beta cell response to increased level of glucose

• Insulit attaches to the receptor which is intimately associated with a tyrosine kinase

• Tyrosine kinase is stimulated, causing phosphorus moieties to be added to tyrosine residues

• Fusion of GLUT4 transporter is allowed, causing glucose to move via facilitated diffusion

What does the insulin do to the target cell?

Glycogenesis

• Glucose-P conversion to glycogen

• inhibited by insulin

Glycolysis

• Glucose-P into pyruvate

Glycogenolysis

• breakdown of glycogen to glucose

• inhibited by insulin

Gluconeogenesis

• uptake of amino acids by the liver converting pyruvate to glucose-p

• inhibited by insulin

Glucagon

• hyperglycemic hormone

• ensures adequate glucose levels when nutrients are scarce

• opposes the effects of insulin

• breaks down glycofen

Adrenal glands

• covered with thick connective tissue capsule which trabeculae extend to the parenchyma, carrying blood vessels and nerves

Outer Yellowish cortex

• steroid-secreting

• 90% of the gland by weight

• from the mesodermal mesenchyme

• controlled by anterior pituirary gland

• regulates metabolism and maintains balance

Dark inner medulla

• catecholamine-secreting

• forms the center of the gland

• neuroectoderm

• innervated by preganglionic sympathetic nerve

Chromaffin cells

• part of medulla that produces catecholamines

Zona Glomerulosa

• part of cortex

• outermost, produces mineralocoricoids

Zona Fasiculata

• part of cortex

• produces glucocorticoids

Zona Reticularis

• innermost layer of cortex

• produces androgens

Adrenal medulla

• produce stress hormone which stimulate sympathetic autonomic nervous system which secretes epinephrine and norepineohrine

Aldosterone and cortisol

main mineralocorticoid and glucocorticoids

Cholesterol

• precursor and backbone of steroid hormone

Low blood volume or pressure

• stimulant of RAAS

Adrenal medulla

• stimulated by the preganglionic fibers via splanchnic nerves

• sympathetic ganglion in postganglionic neurons that have lost their axons

• produces catecholamines

• not essential for life