Alpha cells secrete glucagon in response to a fall in blood glucose concentrations. Glucagon stimulates the liver to
Decreasing blood Ca2+
Reabsorption of Ca2+
Increased blood Ca2+
Decreased urinary excretion of Ca2+
■ Figure 11.29 The actions of parathyroid hormone and the control of its secretion. An increased level of parathyroid hormone causes the bones to release calcium and the kidneys to conserve calcium that would otherwise be lost through the urine. A rise in blood Ca+ can then exert negative feedback inhibition on parathyroid hormone secretion.
hydrolyze glycogen to glucose (glycogenolysis), which causes the blood glucose level to rise. This effect represents the completion of a negative feedback loop. Glucagon also stimulates the hydrolysis of stored fat (lipolysis) and the consequent release of free fatty acids into the blood. This effect helps to provide energy substrates for the body during fasting, when blood glucose levels decrease. Glucagon, together with other hormones, also stimulates the conversion of fatty acids to ketone bodies, which can be secreted by the liver into the blood and used by other organs as an energy source. Glucagon is thus a hormone that helps to maintain homeostasis during times of fasting, when the body's energy reserves must be utilized.
Beta cells secrete insulin in response to a rise in blood glucose concentrations (fig 11.31). Insulin promotes the entry of glucose into tissue cells, and the conversion of this glucose into energy storage molecules of glycogen and fat. Insulin also aids the entry of amino acids into cells and the production of cellular protein. Thus, insulin promotes the deposition of energy storage molecules (primarily glycogen and fat) following meals, when the blood glucose concentration rises. This action is antagonistic to that of glucagon, and the secretion of glucagon is normally decreased when insulin secretion increases. During times of fasting, conversely, the secretion of insulin is decreased while the secretion of glucagon is increased. The secretion and actions of insulin and glucagon, and the association of these hormones with diabetes mellitus, is more fully explained in chapter 19.
Body of pancreas
■ Figure 11.30 The pancreas and associated pancreatic islets (islets of Langerhans). Alpha cells secrete glucagon and beta cells secrete insulin. The pancreas is also exocrine, producing pancreatic juice for transport via the pancreatic duct to the duodenum of the small intestine.
t Blood glucose
P cells in pancreatic islets f Insulin secretion f Cellular uptake-of blood glucose
i Blood glucose
Diabetes mellitus is characterized by fasting hyperglycemia and the presence of glucose in the urine. q There are two forms of this disease. Type I, or insulin-dependent diabetes mellitus, is caused by destruction of the beta cells and the resulting lack of insulin secretion. Type 2, or non-insulin-dependent diabetes mellitus (the more common form), is caused by decreased tissue sensitivity to the effects of insulin, so that larger than normal amounts of insulin are required to produce a normal effect. Both types of diabetes mellitus are also associated with abnormally high levels of glucagon secretion. The causes and symptoms of diabetes mellitus are described in more detail in chapter 19.
Liver and skeletal muscle
■ Figure 11.31 Homeostasis of blood glucose. A rise in blood glucose concentration stimulates insulin secretion. Insulin promotes a fall in blood glucose by stimulating the cellular uptake of glucose and the conversion of glucose to glycogen and fat.
Clinical Investigation Clues
Remember that Rosemary has hyperglycemia and hypertension, and that her oral glucose tolerance test (which tests a person's insulin secretion and action) was normal. Was Rosemary's hyperglycemia and hypertension the result of diabetes mellitus?
If not, what did produce these symptoms?
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