Endocrine glands and their hormones – overview

Anterior pituitary gland

The pituitary gland is a marble-sized gland at the base of the brain. The secretory action of the pituitary is controlled by either neural mechanisms or hormones secreted by the hypothalamus. Therefore, the pituitary gland can be thought of as the relay between central nervous system control centers and peripheral endocrine glands.

The pituitary gland is composed of three lobes: anterior, intermediate and posterior. The intermediate lobe is very small and is thought to play little or no role in humans, but both the posterior and anterior lobes have major endocrine functions. The anterior pituitary has a major role in fluid and electrolyte balance.

The anterior pituitary, also called the adenohypophysis, secretes six hormones in response to releasing factors and inhibiting factors(hormones) secreted by the hypothalamus. Communication between the hypothalamus and the anterior lobe of the pituitary occurs through a specialized circulatory system that transports the releasing and inhibiting factor from the hypothalamus to the anterior pituitary. The major functions of each of the anterior pituitary hormones, along with their releasing and inhibiting factors are discussed here. Exercise appears to be a strong stimulant to the hypothalamus because exercise increases the release rate of all anterior pituitary hormones.

Of the six anterior pituitary hormones, four are tropic hormones, meaning they affect the functioning of other endocrine glands. The exceptions are growth hormone and prolactin. Growth hormone is a potent anabolic agent(a substance that promotes constructive metabolism). It promotes muscle growth and hypertrophy by facilitating amino acid transport into the cells. In addition, growth hormone directly stimulates fat metabolism(lypolysis) by increasing the synthesis of enzymes involved in this process. Growth hormone concentrations are elevated during aerobic exercise, apparently in proportion to the exercise intensity, and typically remain elevated for some time after exercise.

Thyroid gland

The thyroid gland is located along the midline of the neck, immediately below the larynx. It secretes two important nonsteroid hormones, triiodothyronine(T₃) and thyroxine(T₄), which regulate metabolism in general, and an additional hormone, calcitonin, which assists in regulating calcium metabolism.

The two metabolic thyroid hormones share similar functions. Triiodothyronine and thyroxine increase the metabolic rate of almost all tissues and can increase the body’s basal metabolic rate by as much as 60% to 100%. These hormones also:

• Increase protin synthesis(also enzyme synthesis);
• Increase the size and number of mitochondria in most cells;
• Promote rapid cellular uptake of glucose;
• Enhance glycolysis and gluconeogenesis;
• Enhance lipid mobilization, increasing FFA availability for oxidation.

Release of thyrotropin(thyroid-stimulating hormone, or TSH) from the anterior pituitary increases during exercise. Thyroid-stimulating hormone controls the release of triiodothyronine and thyroxine, so the exercise-induced increase in TSH would be expected to stimulate the thyroid gland. Exercise does increase plasma thyroxine concentrations, but a delay occurs between the increase in TSH concentrations during exercise and the increase in TSH concentrations during exercise and the increase in plasma thyroxine concentration. Furthermore, during prolonged submaximal exercise, thyroxine concentrations remain relatively constant after a sharp initial increase as exercise begins, and triiodothyronine concentrations tend to decrease.

Adrenal glands

The adrenal glands are situated directly atop each kidney and are composed of the inner adrenal medulla and the outer adrenal cortex. The hormones secreted by these two parts are quite different, so we consider them separately. The adrenal medulla produces and releases two hormones, epinephrine and norepinephrine, which are collectively referred to as catecholamines. Because of its origin in the adrenal gland, a synonym for epinephrine is adrenaline. When the adrenal medulla is stimulated by the sympathetic nervous system, approximately 80% of its secretion is epinephrine and 20% is norepinephrine, although these percentages vary with different physiological conditions. The catecholamines have powerful effects similar to those of the sympathetic nervous system. Recall that these same catecholamines function as neurotransmitters in the sympathetic nervous system: however, the hormones’ effect last longer because these substances are removed from the blood relatively slowly compared to the quick reuptake and degradation of the neurotransmitters. These two hormones prepare a person for immediate action, often called the “fight-or-flight response”.

Although some of the specific actions of these two hormones differ, the two work together. Their combined effects include:

• Increased heart rate and force of concentration;
• Increased metabolic rate;
• Increased glycogenolysis(breakdown of glycogen to glucose) in the liver and muscle;
• Increased release of glucose and FFAs into the blood;
• Redistribution of blood to the skeletal muscles(through vasodilatation of vessels supplying skeletal muscles and vasoconstriction of vessels to the skin and viscera);
• Increased blood pressure;
• Increased respiration.

Release of epinephrine and norepinephrine is affected by a wide variety of factors, including changes in body position, psychological stress, and exercise. Plasma concentrations of these hormones increase as individuals gradually increase their exercise intensity. Plasma norepinephrine concentrations increase markedly at work rates above 50% of VO₂max, but epinephrine concentrations do not increase significantly until the exercise intensity exceeds 60% to 70% of VO₂max. During long-duration steady-state activity of moderate intensity, blood concentrations of both hormones increase. When the exercise bout ends, epinephrine returns to resting concentrations within only a few minutes of recovery, but norepinephrine can remain elevated for several hours.

The adrenal cortex secretes more than 30 different steroid hormones, referred to as corticosteroids. These generally are classified into three major types: mineralocorticoids, glucocorticoids, and gonadocorticoids(sex hormones).

The glucocorticoids are essential components in the ability to adapt to external changes and stress. They also help maintain fairly consistent plasma glucose concentrations even when we go for long periods without ingesting food. Cortisol, also known as hydrocortisone, is the major corticosteroid. It is responsible for about 95% of all glucocorticoid activity in the body. Cortisol:

• Stimulates gluconeogenesis to ensure an adequate fuel supply;
• Increases mobilization of FFAs, making them more available as an energy source;
• Decreases glucose utilization, sparing it for the brain;
• Stimulates protein catabolism to release amino acids for use in repair, enzyme synthesis, and energy production;
• Acts as an anti-inflammatory agent;
• Depresses immune reactions;
• Increases the vasoconstriction caused by epinephrine.

Pancreas

The pancreas is located behind and slightly below the stomach. Its two major hormones are insulin and glucagon. The balance of these two opposing hormones provides the major control of plasma glucose concentrations. When plasma glucose is elevated(hyperglycemia), as after a meal, the pancreas receives signals to release insulin into the blood.

Among its actions, insulin:

• Facilitates glucose transport into the cells, especially those in muscle;
• Promotes glycogenolysis;
• Inhibits gluconeogenesis.

Insulin’s main function is to reduce the amount of glucose circulating in the blood. But it is also involved in protein and fat metabolism, promoting cellular uptake of amino acids and enhancing synthesis of protein and fat.

The pancreas secretes glucagon when the plasma glucose concentration falls below normal concentrations(hypoglycemia). Its effects generally oppose those of insulin. Glucagon promotes increased breakdown of liver glycogen to glucose(glycogenolysis) and increased gluconeogenesis, both of which increase plasma glucose levels.

During exercise lasting 30 min or longer, the body attempts to maintain plasma glucose concentrations. However, insulin concentrations tend to decline. Research has shown that the ability of insulin to bind to its receptors on muscle cells increases during exercise,  due in large part to increased blood flow to muscle. This increases the body’s sensitivity to insulin and reduces the need to maintain high plasma insulin concentrations for transporting glucose into the muscle cells. Plasma glucagons, on the other hand, shows a gradual increase throughout exercise. Glucagon primarily maintains plasma glucose concentrations by stimulating liver glycogenolysis. This increases glucose availability to the cells, maintaining adequate plasma glucose concentrations to meet increased metabolic demands. The responses of these hormones are usually blunted in trained individuals, and those who are well trained are better able to maintain plasma glucose concentrations.

“Physiology of sport and exercise”, fourth edition; Jack H. Wilmore, David L. Costill, W. Larry Kenney