Other Factors That Stimulate Insulin Secretion
Amino Acids. In addition to the stimulation of insulinsecretion by excess blood glucose, some of the amino acids have a similar effect. The most potent of these are arginine and lysine. This effect differs from glucose stim-ulation of insulin secretion in the following way: Amino acids administered in the absence of a rise in blood glucose cause only a small increase in insulin secretion. However, when administered at the same time that the blood glucose concentration is elevated, the glucose-induced secretion of insulin may be as much as doubled in the presence of the excess amino acids. Thus, the amino acids strongly potentiate the glucose stimulus for insulin secretion.
The stimulation of insulin secretion by amino acids is important, because the insulin in turn promotes transport of amino acids into the tissue cells as well as intracellular formation of protein. That is, insulin is important for proper utilization of excess amino acids in the same way that it is important for the utilization of carbohydrates.
Gastrointestinal Hormones. A mixture of several impor-tant gastrointestinal hormones—gastrin, secretin, cholecystokinin, and gastric inhibitory peptide (whichseems to be the most potent)—causes a moderate increase in insulin secretion. These hormones are released in the gastrointestinal tract after a person eats a meal. They then cause an “anticipatory” increase in blood insulin in preparation for the glucose and amino acids to be absorbed from the meal. These gastroin-testinal hormones generally act the same way as amino acids to increase the sensitivity of insulin response to increased blood glucose, almost doubling the rate of insulin secretion as the blood glucose level rises.
Other Hormones and the Autonomic Nervous System. Other hor-mones that either directly increase insulin secretion or potentiate the glucose stimulus for insulin secretion include glucagon, growth hormone, cortisol, and, to a lesser extent, progesterone and estrogen. The impor-tance of the stimulatory effects of these hormones is that prolonged secretion of any one of them in large quantities can occasionally lead to exhaustion of the beta cells of the islets of Langerhans and thereby increase the risk for developing diabetes mellitus. Indeed, diabetes often occurs in people who are main-tained on high pharmacological doses of some of these hormones. Diabetes is particularly common in giants or acromegalic people with growth hormone-secreting tumors, or in people whose adrenal glands secrete excess glucocorticoids.
Under some conditions, stimulation of the parasym-pathetic nerves to the pancreas can increase insulin secretion. However, it is doubtful that this effect is of physiologic significance for regulating insulin secretion.
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