Actions of Adrenal Gland Hormones

A. Adrenal Cortex. Cortisol exerts major effects on glucose metabolism in several different ways. Cortisol combines with cytosolic glucocorticoid receptor proteins and, after interaction with nuclear acceptor sites, markedly increases hepatic gluconeogenesis (glucose production), partly by stimulation of transcription and translation of genes for enzymes, which convert amino acids to glucose, and also by increasing amino acid mobilization from muscle and other tissues. In addition, cortisol also causes an increase in glycogen synthesis by the liver and decreases the rate of peripheral glucose utilization.

With respect to protein and fat metabolism, cortisol stimulates catabolism (breakdown) of nonhepatic proteins and increases the rate of amino acid uptake into liver. In adipose tissue, cortisol stimulates lipolysis and the subsequent release of glycerol and free fatty acids, while enhancing the rate of fatty-acid oxidation.

In addition, cortisol is secreted in so many forms of stress, that an increase in cortisol secretion is often considered the result of what may be defined as a stressful stimulus. Cortisol may help an organism survive a stressful situation by improving the metabolic milieu by means of the energy-producing and biosynthetic pathways noted above. Cortisol may also help to minimize damage to the body due to excessive inflammation produced by illness or injury by stabilizing lysosomal membranes.

This prevents the release of proteolytic enzymes. Cortisol also reduces capillary permeability to avoid leakage of plasma and blood cells into an inflamed area. Immunological responses are decreased by high concentrations of cortisol.

DHAS and other adrenal androgens circulate in young adults at concentrations much higher than those of cortisol, but their functions have not been as clearly elucidated. Adrenal androgens may prevent osteoporosis (thinning of the bones), facilitate the birth process by causing cervical softening, mediate female libido, and serve as precursors for more potent sex steroids. In some species of animals, these steroids have been shown to protect against obesity, diabetes, and certain types of infections and tumors. DHAS is synthesized in rat brain and is present in the human brain, but its function in the nervous system of either species is not known.

Aldosterone and other mineralocorticoids maintain normal sodium and potassium concentrations and the volume of the extracellular fluid. After combining with an intracellular receptor in the target cell, aldosterone exerts actions on the nucleus with induction of protein synthesis and subsequent activation of a sodium pump, which transports sodium across cell membrane channels. In the kidney, sodium is absorbed, and hydrogen ion and potassium are secreted. Therefore, under the influence of aldosterone, sodium is conserved, and potassium and hydrogen ion are excreted into the urine. Aldosterone has similar effects in sweat glands, salivary glands, and the intestinal lumen.

B. Adrenal Medulla. Catecholamines exert their actions through two types of receptors, called alpha and beta, present on the surface of target cells. Norepinephrine acts on alpha receptors, with a variety of effects such as vasoconstriction and blood pressure elevation, iris dilatation, and bladder sphincter contraction. Epinephrine stimulates both alpha and beta receptors.

The latter interaction results in vasodilatation, acceleration of heart rate, bronchodilatation, glycogenolysis, and lipolysis. These actions are part of the physiological response to stressful stimuli and complement those of cortisol, secreted by the cortex.