Hormonal feedback loop components & Glucagon (glycogenolysis and gluconeogenesis)

The endocrine system maintains many body conditions within normal limits with feedback loops. Each endocrine feedback loop maintains homeostasis using the following components: • Stimulus - a change in a body condition. • Production cell - an endocrine cell that produces a hormone after being affected by stimulus. • Hormone - the signaling chemical. • Target cell - a cell receptive to the hormone. • Action - what the cell does when affected by the hormone. • Response - the overall change in controlled body condition as a result of the feedback loop. • Exercise, or not having eaten recently, causes a decline in blood glucose concentration. • Low blood glucose stimulates alpha cells in the pancreatic islets to secrete glucagon. • Glucagon targets liver cells. • Glucagon causes liver cells to undergo: • Glycogenolysis, a process which breaks down glycogen into glucose. • Gluconeogenesis, a process which produces glucose from non-carbohydrate sources like amino adds and glycerol. • The glucose is released into the blood and blood glucose concentration is restored to normal levels. GHRH/hGH (glycogenolysis) • A decline in blood glucose concentration stimulates the release of growth hormone releasing (GHRH) hormone from the hypothalamus. • Growth hormone releasing hormone targets the somatotroph cells in the anterior pituitary. • GHRH causes somatotroph cells of anterior pituitary cells to release human growth hormone (hGH). • Human growth hormone targets liver cells. • Liver cells undergo glycogenolysis - breaking down glycogen into glucose. • The glucose is released into the blood and blood glucose concentration is restored to normal levels. GHRH/hGH (lipolysis, summary) • A decline in blood glucose concentration stimulates the release of growth hormone releasing (GHRH) hormone from the hypothalamus. • Human growth hormone targets adipose cells. • Increased hGH causes adipose cells to undergo lipolysis, a process which mobilizes stored lipids for energy use. • Glucose is spared thereby blood glucose levels increase to normal. • hGH causes livers cells to undergo glycogenolysis. • hGH causes adipose cells to undergo lipolysis. The resulting response of hGH secretion to both cell types is the increase in blood glucose levels.

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