16.11 Other endocrine organs

Pancreas;

Triangular gland located

partially behind stomach

• Has both exocrine and endocrine cells

Pancreas;

Acinar cells (exocrine) -

produce enzyme-rich juice for digestion

Pancreatic islets (islets of Langerhans)

contain endocrine cells

Pancreas;

Alpha (α) cells -

procuse glucagon (hyperglycemic hormone)

Pancreas

Beta (β) cells -

produce insulin (hypoglycemic hormone) 

Pancreas;

Glucagon -

• Extremely potent hyperglycemic agent

  • Triggered by decreased blood glucose levels, rising amino acid levels, or sympathetic nervous system

• Raises blood glucose levels by targeting liver to:

  • Break down glycogen into glucose

    • Glycogenolysis

  • Synthesize glucose from lactic acid and other noncarbohydrates

    • Gluconeogenesis

  • Release glucose into blood 

Pancreas;

• Insulin

• Secreted when blood glucose levels increase

• Insulin lowers blood glucose levels in three ways:

  • Enhances membrane transport of glucose into fat and muscle cells

  • Inhibits breakdown of glycogen to glucose

  • Inhibits conversion of amino acids or fats to glucose

• Not needed for glucose uptake in liver, kidney, or brain

• Plays a role in neuronal development, learning, and memory

• Insulin also triggers cells to:

  • Catalyze oxidation of glucose for ATP production: first priority

  • Polymerize glucose to form glycogen

  • Convert glucose to fat (particularly in adipose tissue

Pancreas;

• Factors that influence insulin release -

• Elevated blood glucose levels: primary stimulus

• Rising blood levels of amino acids and fatty acids

• Release of acetylcholine by parasympathetic nerve fibers

• Hormones glucagon, epinephrine, growth hormone, thyroxine, glucocorticoids

• Somatostatin and sympathetic nervous system inhibit insulin release

The Gonads and Placenta

• Gonads produce

same steroid sex hormones as those of adrenal cortex, just lesser amounts

The Gonads and Placenta

Ovaries produce

estrogens and progesterone

The Gonads and Placenta

Estrogen -

• Maturation of reproductive organs

• Appearance of secondary sexual characteristics 

• With progesterone, causes breast development and cyclic changes in uterine mucosa

Testes produce testosterone 

• Initiates maturation of male reproductive organs

• Causes appearance of male secondary sexual characteristics and sex drive

• Necessary for normal sperm production

• Maintains reproductive organs in functional state

Placenta -

secretes estrogens, progesterone, and human chorionic gonadotropin (hCG)

Adipose cells release:

• Leptin: appetite control; stimulates increased energy expenditure

• Resistin: insulin antagonist

• Adiponectin: enhances sensitivity to insulin

Gastrointestinal tract

• Enteroendocrine cells secrete these hormones:

• Gastrin stimulates release of HCl

• Ghrelin from stomach stimulates food intake

• Secretin stimulates liver and pancreas

• Cholecystokinin (CCK) activates pancreas, gallbladder, and hepatopancreatic sphincter

• Incretins enhance insulin release and inhibit glucagon

• Heart

  • Atrial natriuretic peptide (ANP) -

decreases blood Na⁺ concentration, therefore blood pressure and blood volume

Kidneys -

• Erythropoietin signals production of red blood cells

• Renin initiates the renin-angiotensin-aldosterone mechanism

Skeleton

Osteoblasts in bone secrete osteocalcin -

• Prods pancreas to secrete more insulin; restricts fat storage; improves glucose handling; reduces body fat

• Activated by insulin

• Low levels of osteocalcin are present in type 2 diabetes: perhaps increasing levels may be new treatment

Skin

Cholecalciferol,

precursor of vitamin D

Skin -

• Calcitriol: active form of vitamin D that helps absorb calcium from intestine

• Also modulates immunity, decreases inflammation, and may act as anticancer agent

Thymus -

• Large in infants and children; shrinks with age

• Thymulin, thymopoietins, and thymosins may be involved in normal development of T lymphocytes in immune response 

  • Classified as hormones but act as paracrines

Hormone-producing glands arise from

all three germ layers during embryonic development 

Effects of Environmental Pollutants

• Exposure to pesticides, industrial chemicals, arsenic, dioxin, and soil and water pollutants disrupts hormone function

• Sex hormones, thyroid hormone, glucocorticoids are all vulnerable to effects of pollutants

• Interference with glucocorticoids may help explain high cancer rates in certain areas