McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 3 8 Agents to control blood glucose levels

release, blood glucose levels fall and insulin release drops off. Sometimes, an insufficient amount of insulin is released. This may occur because the pancreas cannot produce enough insulin, the insulin receptor sites have lost their sensitivity to insulin and they require more insulin to lower glucose effectively or the person does not have enough receptor sites to support his or her body size, as in obesity. Glucagon Glucagon is released from the alpha cells in the islets of Langerhans located in the pancreas in response to low blood glucose levels. Glucagon causes an immediate mobilisation of glycogen stored in the liver and raises blood glucose levels. Other factors affecting glucose control Other factors in the body have been found to have an impact on glucose, fat and protein metabolism. These factors play a role in the overall energy balance in the body. Adipocytes, or fat cells, were once thought to just store fat for energy. However, they have been found to have a major impact on glucose and fat metabolism throughout the body through the secretion of adiponec- tin . This hormone acts to increase insulin sensitivity, decrease the release of glucose from the liver and protect the blood vessels from inflammatory changes. When adi- ponectin levels are high, it exerts a protective effect on the body. When adiponectin levels are low, as in cases of intra-abdominal fat accumulation, glucose levels rise and blood vessel injury increases. Endocannabinoid receptors have been identified in the adipose tissue, muscles, liver, the satiety centre and the GI tract. These receptors seem to be part of a sig- nalling system within the body to keep the body in a state of energy gain, to prepare for stressful situations. When stimulated, these receptors promote food intake, decrease adiponectin release, increase fat breakdown, decrease insulin sensitivity, increase fat storage and alter gastric emptying to promote greater nutrient absorption. People who are obese have been shown to have increased stimulation of these receptors. The sympathetic nervous system, through noradren- aline and adrenaline effects, directly causes a decrease in insulin release, an increase in the release of stored glucose and an increase in fat breakdown. A person under stress will have increased glucose levels and increased free fatty acids (FFAs) levels, which will provide the energy needed for the immediate “fight or flight” associated with a stress reaction. Prolonged stress can alter the control of metabolism that regulates the body’s energy balance. Corticosteroids, which are released diurnally but also during a stress reaction, decrease insulin sensitivity,

increase glucose release and decrease protein building. All of these actions conserve energy and provide imme- diate glucose for any stressful situation. Growth hormone causes decreased insulin sensitiv- ity, increase of FFAs and increase in protein building. Fluctuating levels of growth hormone can upset the metabolic homeostasis. Box 38.2 summarises effects of various factors on blood glucose levels. Loss of blood glucose control When an insufficient amount of insulin is released or insulin receptors are no longer responding, several metabolic changes occur, beginning with hyperglycae- mia, or increased blood sugar. Hyperglycaemia results in glycosuria : Sugar is excreted into the urine because the concentration of glucose in the blood is too high for complete reabsorption. Because this sugar-rich urine is an ideal environment for bacteria, cystitis is a common finding. The person experiences fatigue because the body’s cells cannot use the glucose that is there; they need insulin to facilitate transport of the glucose into the cells. Polyphagia (increased hunger) occurs because the hypothalamic centres cannot take in glucose; thus the cells sense that they are requiring glucose. Polydipsia

■■ BOX 38.2  Glucose control mechanisms

Insulin

Decreases blood glucose; glycogen storage; adipose tissue deposit; synthesis of proteins to form amino acids

Glucagon

Increases blood glucose

Somatostatin

Decreases insulin release; decreases glucagon release; slows GI emptying

Growth hormone Decreases insulin sensitivity;

increases protein building; increases free fatty acid formation Increases insulin release; decreases glucagon release; stimulates satiety centre; slows GI emptying

Incretins

Adiponectin Increases insulin sensitivity; decreases glucose output from liver; protects vessels from inflammatory reactions Catecholamines Decreases insulin release; increases glucose output from liver and muscles; increases breakdown of fat to free fatty acids Corticosteroids Increases glucose output; decreases insulin sensitivity Endocannabinoid system Increases food intake by blocking satiety signals; decreases adiponectin release; decreases insulin sensitivity; increases fat synthesis; alters gastric motility