BSC 215 Lecture Notes - Lecture 36: Pancreatic Islets, Alpha Cell, Blood Sugar
11 Jun 2018
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Endocrine Organs and Tissues
A summary of the various endocrine organs, their hormones, and their
functions is given in Table 1 in "The Hypothalamus and Pituitary Glands."
Also listed are some organs whose major function is not the secretion of
hormones, but which, nonetheless, contain some specialized cells that
produce hormones. These organs include the heart, the gastrointestinal
tract, the placenta, the kidneys, and the skin.
In addition, all cells (except red blood cells) secrete a class of hormones
called eicosanoids. These hormones are paracrines, or local hormones, that primarily affect
neighboring cells. Two groups of eicosanoids, the prostaglandins (PGs) and the leukotrienes
(LTs), have a wide range of varying effects that depend on the nature of the target cell.
Eicosanoid activity, for example, may impact blood pressure, blood clotting, immune and
inflammatory responses, reproductive processes, and the contraction of smooth muscles.
Antagonistic Hormones
Maintaining homeostasis often requires conditions to be limited to a narrow range. When
conditions exceed the upper limit of homeostasis, a specific action, usually the production of a
hormone, is triggered. When conditions return to normal, hormone production is discontinued.
If conditions exceed the lower limit of homeostasis, a different action, usually the production of
a second hormone, is triggered. Hormones that act to return body conditions to within
acceptable limits from opposite extremes are called antagonistic hormones.
The regulation of blood glucose concentration (through negative feedback)
illustrates how the endocrine system maintains homeostasis by the action of
antagonistic hormones. Bundles of cells in the pancreas called pancreatic
islets contain two kinds of cells, alpha cells and beta cells. These cells control
blood glucose concentration by producing the antagonistic hormones insulin
and glucagon:
• Beta cells secrete insulin. When the concentration of blood glucose
rises (after eating, for example), beta cells secrete insulin into the
blood. Insulin stimulates the liver and most other body cells to absorb
glucose. Liver and muscle cells convert the glucose to glycogen (for
short‐term storage), and adipose cells convert the glucose to fat. In
response, glucose concentration decreases in the blood, and insulin
secretion discontinues (through negative feedback from declining
levels of glucose).
• Alpha cells secrete glucagon. When the concentration of blood glucose
drops (during exercise, for example), alpha cells secrete glucagon into
the blood. Glucagon stimulates the liver to release glucose. The
glucose in the liver originates from the breakdown of glycogen and the
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