Glucagon, secreted by the alpha cells of the pancreas is a polypeptide hormone with 29 amino acids residues.
The major function of glucagon in the body is to elevate the blood glucose level by glycogenolysis in the liver. As it raises the blood sugar level it is also called as Hyperglycemic hormone. A second important function of glucagon is the gluconeogenesis in the liver in which amino acids are used as substrates. It promotes lipolysis and the release of fatty acids in the adipose tissues. The increased fatty acid oxidation leads to ketogenesis. Glucagon also stimulates the myocardial contractility. Glucagon exerts a direct effect upon the kidneys and accelerates the renal plasma flow and glomerular filtration rate. A proper balance between insulin and glucagon production is necessary to maintain proper blood glucose level.
The adrenal gland or supra renal gland is composed of an outer cortex and an inner medulla. The adrenal cortex forms the major portion of the total mass of tissue of adrenal gland. In adults three concentric zones are discernible within the cortex. 1. A thin outer most layer, the Zona glomerulosa, 2. A thick middle region, Zona fasciculata and 3. A relatively thick inner layer, the Zona reticularis. In man, the cells of zona fasciculata and zona reticularis act as a single unit, the main function of which is to form glucocorticoids and to a lesser extent androgens and possibly oestrogens. The mineralocorticoid hormone, aldosterone is secreted by the cells of the zona glomerulosa. The enzymes necessary for its synthesis reside in the cells of the zona glomerulosa. All the adreno corticoid hormones are steroids.
The major glucocorticoids are cortisone and certain closely related steroids. These hormones stimulate the production of glucose from non-carbohydrate sources such as fats and amino acids. Glucocorticoids also decrease glucose utilization by tissues in general. All these effects lead to increased blood glucose level. Cortisone also acts as an anti-inflammatory agent.
The major effect is on the metabolism of sodium ions and indirectly potassium ions. The major mineralocorticoid hormone is Aldosterone. Its most important effect is to promote the resorption of sodium ions from the renal glomerular filtrate. Secondary effects of sodium retention are an increased chloride retention and a decreased potassium retention by the kidneys. The most important function of the adrenal cortex is its role in stress tolerance.
The medulla of adrenal gland differs both in morphology and physiology from the cortex portion. Both cortex and medulla are derived independently and from totally different tissues in the embryo. The adrenal medulla in the adult humans and other mammals is invested closely by the cortical tissue. The cells of the adrenal medulla are large ovoid and columnar in type and are grouped into clumps around the blood vessels. The hormones produced by adrenal medulla are 1. Adrenalin or epinephrine and 2. Nor adrenalinor nor epinephrine. The two hormones belong to a group called catecholamines. The various physiological and biochemical actions of adrenalin or epinephrine are the following:
1. Adrenalin stimulates constriction of blood vessels supplying the intestine, kidneys, viscera and skin and causes dilation of blood vessels supplying skeletal and heart muscle.
2. It increases the rate, amplitude and frequency of the heart beat.
3. It causes relaxation of the smooth muscles of the digestive tract and brings peristalsis to a halt
4. It causes relaxation of the bronchi, dilation of the pupil, closure of sphincters and increases sweating
5. It causes contraction of muscles associated with hair follicles and makes the hair 'stand on end' and causes goose flesh
6. It accelerates respiration and stimulates mental alertness
7. It stimulates the breakdown of glycogen to glucose, thereby increasing oxygen consumption and heat production.
8. Biochemically it releases the free fatty acids and increases blood glucose level.
9. Adrenalin prepares an individual during emergency or stress situations. Hence it is called the fight, flight and fright hormone.
Nor adrenalin has certain effects similar to that of adrenalin. For example, both the hormones dilate the coronary vessels. However, nor epinephrine cause vaso constriction in most organs other than heart. It increases both the systolic and diastolic blood pressures. It exerts a weakly inhibitory action upon the contractile activity of smooth muscle in the gastro intestinal tract. However, it does not relax the smooth musculature of the pulmonary bronchioles. Nor epinephrine exerts very little effect upon carbohydrate metabolism and oxygen consumption.