McKenna's Pharmacology for Nursing, 2e

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P A R T 6  Drugs acting on the endocrine system

■■ TABLE 34.1 Endocrine glands with associated hormones and clinical effects Gland Hormones produced Principal effects

Adrenal cortex

Cortisol

Increases glucose levels, suppresses inflammatory and immune reactions Decreases gastric movement, stimulates bile and pancreatic juice secretion Sodium retention, potassium excretion

Intestine

Aldosterone

Secretin, cholecystokinin Erythropoietin

Kidney (juxtaglomerular cells)

Increases red blood cell production

Renin

Stimulates increase in blood pressure and vascular volume Promotes secondary sex characteristics, prepares the female body for pregnancy Regulation of glucose, fat metabolism (islets of Langerhans)

Ovaries

Oestrogen,

progesterone

Pancreas

Insulin, glucagon, somatostatin Parathyroid hormone

Parathyroid glands

Increases serum calcium levels

Pineal gland

Melatonin

Affects secretion of hypothalamic hormones, particularly gonadotropin-releasing hormone Maintains fetal growth and development, prepares the body for birth

Placenta

Oestrogens,

progesterones

Stomach

Gastrin

Stimulates stomach acid production

Testes

Testosterone

Stimulates secondary sex characteristics in males Stimulates basal metabolic rate (how the body uses energy)

Thyroid

Thyroid hormone

Calcitonin

Decreases serum calcium levels

THE HYPOTHALAMUS The hypothalamus is the coordinating centre for the nervous and endocrine responses to internal and external stimuli. The hypothalamus constantly monitors the body’s homeostasis by analysing input from the periphery and the central nervous system (CNS) and coordinating responses through the auto­ nomic, endocrine and nervous systems. In effect, it is the “master gland” of the neuroendocrine system. This title was once given to the pituitary gland because of its many functions and well-protected location (see later discussion). The hypothalamus has various neurocentres—areas specifically sensitive to certain stimuli—that regulate a number of body functions, including body temperature, thirst, hunger, water retention, blood pressure, respir­ ation, reproduction and emotional reactions. Situated at the base of the forebrain, the hypothalamus receives input from virtually all other areas of the brain, includ­ ing the limbic system and the cerebral cortex. Because of its positioning, the hypothalamus is able to influence, and be influenced by, emotions and thoughts. The hypo­ thalamus is also located in an area of the brain that is poorly protected by the blood–brain barrier, so it is able to act as a sensor to various electrolytes, chemicals and hormones that are in circulation and do not affect other areas of the brain. The hypothalamus maintains internal homeostasis by sensing blood chemistries and by stimulating or sup­ pressing endocrine, autonomic and CNS activity. In

essence, it can turn the autonomic nervous system and its effects on or off. The hypothalamus also produces and secretes a number of releasing hormones or factors that stimulate the pituitary gland, which in turn stimu­ lates or inhibits various endocrine glands throughout the body (see Figure 34.1). These releasing hormones include growth hormone–releasing hormone (GHRH), thyrotropin-releasing hormone (TRH), gonadotropin- releasing hormone (GnRH), corticotropin-releasing hormone (CRH) and prolactin-releasing hormone (PRH). The hypothalamus also produces two inhibiting factors that act as regulators to shut off the produc­ tion of hormones when levels become too high: growth hormone release–inhibiting factor (somatostatin) and prolactin-inhibiting factor (PIF). Recent research has indicated that PIF may actually be dopamine, a neuro­ transmitter. People who are taking dopamine-blocking drugs often develop galactorrhoea (inappropriate milk production) and breast enlargement, theoretically because PIF is also blocked and prolactin levels continue to rise, stimulating breast tissue and milk production. Research is ongoing about the chemical structure of several of the releasing factors. The hypothalamus is connected to the pituitary gland by two networks: A vascular network carries the hypothalamic releasing factors directly into the anterior pituitary, and a neurological network delivers two other hypothalamic hormones—antidiuretic hormone (ADH) and oxytocin—to the posterior pituitary to be stored. These hormones are released as needed by the body when stimulated by the hypothalamus.