Human Physiology, 14th edition (2016)

Regulation of Metabolism 677

19.3 ENERGY REGULATION BY THE

PANCREATIC ISLETS

Insulin secretion is stimulated by a rise in the plasma glu-

cose concentration, and insulin lowers the blood glucose

while it promotes the synthesis of glycogen and fat. Gluca-

gon secretion is stimulated by a fall in the plasma glucose

concentration, and glucagon promotes increased blood

glucose due to glycogenolysis in the liver.

Figure 19.7 Hormonal interactions

in metabolic regulation. Different hormones

may work together synergistically, or they

may have antagonistic effects on metabolism.

(⊕ 5  stimulatory effects;  5  inhibitory effects.)

Liver cell

Insulin

Insulin

Insulin

Glucose

Glucose

Free

fatty

acids

Free

fatty

acids

Glycerol

Triglyceride

Insulin

Glucagon

Growth hormone

Corticosteroids

Epinephrine

Adipose cell

Glucose

Glycogen

CO 2 + H 2 O

Growth

hormone

Glucagon

Epinephrine

+

+

+

+

+

• 
  

  –

  
  

LEARNING OUTCOMES

After studying this section, you should be able to:

8. Explain how the secretions of insulin and glucagon
   are regulated and how they change during the
   absorptive and postabsorptive states.


9. Explain how the liver produces glucose and why the
   liver but not skeletal muscles can secrete glucose
   into the blood.

Scattered within a “sea” of pancreatic exocrine tissue (the acini)
are about a million islands of hormone-secreting cells (chapter 18;
see fig. 18.27). These pancreatic islets (islets of Langerhans)
contain three distinct cell types that secrete different hormones.

The most numerous are the beta cells, which secrete the hor-

mone insulin. About 60% of each islet consists of beta cells. The

alpha cells form about 25% of each islet and secrete the hormone

glucagon. The least numerous cell type, the delta cells, produce

somatostatin, the composition of which is identical to the soma-

tostatin produced by the hypothalamus and the intestine.

Insulin is the major hormone that maintains homeostasis

of the plasma glucose concentration, with glucagon playing an

important supporting role. This homeostasis is required because

the brain uses plasma glucose as its primary energy source, and

indeed the brain uses about 60% of the blood glucose when the

person is at rest. The plasma glucose concentration is maintained

relatively constant even during exercise, when the skeletal mus-

cle glucose metabolism can increase tenfold. This is possible

because the pancreatic islet hormones regulate the ability of the

liver to produce glucose (from stored glycogen and noncarbohy-

drate molecules) and secrete it into the blood.

Regulation of Insulin

and Glucagon Secretion

The secretion of insulin and glucagon is largely regulated by

the plasma concentrations of glucose and, to a lesser degree, of

amino acids. These concentrations rise during the absorption

of a meal (the absorptive state ) and fall during times of fasting