Hormones are compounds secreted by specific tissues called endocrine glands (Figure 23.6). Endocrine glands secrete hormones directly into the bloodstream, so the response to a hormonal signal comes as a direct and rapid result of its secretion. Insulin is a 5.8-kilodalton protein hormone synthesized in the so-called B cells, which are endocrine cells in the pancreas.

Insulin promotes

1. Uptake of fuel substrates into some cells;

2. Storage of fuels (lipids and glycogen); and

3. Biosynthesis of macromolecules (nucleic acids and protein).

Because insulin promotes biosynthesis, insulin can be considered a growth hormone. Specific effects of insulin include the following:

1. Increased uptake of glucose in muscle and adipose tissue;

2. Activation of glycolysis in liver;

3. Increased synthesis of fatty acids and triacylglycerols in liver and adipose tissue;

4. Inhibition of gluconeogenesis in liver;

5. Increased glycogen synthesis in liver and muscle;

6. Increased uptake of amino acids into muscle with consequent activation of muscle protein synthesis; and

7. Inhibition of protein degradation.

Insulin stimulates glucose uptake into muscle and adipose cells, at least partly by translocating the glucose transporter (a membrane protein that carries out facilitated diffusion of glucose) from the cytosol (where it resides in the absence of insulin) to the cell surface, in response to insulin.