Файл: BRS_Physiology_6E_2014_1.pdf

GIP

ACh

ACh = acetylcholine; GIP = glucose-dependent insulinotropic peptide.

c.  The insulin–receptor complexes enter the target cells.

d.  Insulin down-regulates its own receptors in target tissues.

■Therefore, the number of insulin receptors is increased in starvation and decreased in obesity (e.g., type 2 diabetes mellitus).

3.  Actions of insulin

■ Insulin acts on the liver, adipose tissue, and muscle.

a.  Insulin decreases blood glucose concentration by the following mechanisms:

(1)  It increases uptake of glucose into target cells by directing the insertion of glucose transporters into cell membranes. As glucose enters the cells, the blood glucose

concentration decreases.

(2)  It promotes formation of glycogen from glucose in muscle and liver, and simultane-

ously inhibits glycogenolysis.

(3)  It decreases gluconeogenesis. Insulin increases the production of fructose 2,6-bisphosphate, increasing phosphofructokinase activity. In effect, substrate is directed away from glucose formation.

b.  Insulin decreases blood fatty acid and ketoacid concentrations.

■In adipose tissue, insulin stimulates fat deposition and inhibits lipolysis.

■Insulin inhibits ketoacid formation in the liver because decreased fatty acid degradation provides less acetyl CoA substrate for ketoacid formation.

c.  Insulin decreases blood amino acid concentration.

■Insulin stimulates amino acid uptake into cells, increases protein synthesis, and inhibits protein degradation. Thus, insulin is anabolic.

d.  Insulin decreases blood K+ concentration.

■ Insulin increases K+ uptake into cells, thereby decreasing blood [K+].

4.  Insulin pathophysiology—diabetes mellitus

■Case study: A woman is brought to the emergency room. She is hypotensive and breathing rapidly; her breath has the odor of ketones. Analysis of her blood shows severe hyperglycemia, hyperkalemia, and blood gas values that are consistent with metabolic acidosis.

■Explanation:

a.  Hyperglycemia

■is consistent with insulin deficiency.

■In the absence of insulin, glucose uptake into cells is decreased, as is storage of glucose as glycogen.

■If tests were performed, the woman’s blood would have shown increased levels of both amino acids (because of increased protein catabolism) and fatty acids (because of increased lipolysis).

Ingested

Ca2+

1,25-Dihydroxycholecalciferol

+

Urinary Ca2+ excretion

Figure 7.13 Hormonal regulation of Ca2+ metabolism. ECF = extracellular fluid; PTH = parathyroid hormone.

1.  Positive Ca2+ balance

■is seen in growing children.

■Intestinal Ca2+ absorption exceeds urinary excretion, and the excess is deposited in the growing bones.

2.  Negative Ca2+ balance

■is seen in women during pregnancy or lactation.

■Intestinal Ca2+ absorption is less than Ca2+ excretion, and the deficit comes from the maternal bones.

B.Parathyroid hormone (PTH)

■is the major hormone for the regulation of serum [Ca2+].

■is synthesized and secreted by the chief cells of the parathyroid glands.

1.  Secretion of PTH

■is controlled by the serum [Ca2+] binding to Ca2+-sensing receptors in the parathyroid cell membrane. Decreased serum [Ca2+] increases PTH secretion, whereas increased serum Ca2+ decreases PTH secretion.

■Decreased serum Ca2+ causes decreased binding to the Ca2+-sensing receptor, which stimulates PTH secretion.

■Mild decreases in serum [Mg2+] stimulate PTH secretion.

■Severe decreases in serum [Mg2+] inhibit PTH secretion and produce symptoms of hypoparathyroidism (e.g., hypocalcemia).

■The second messenger for PTH secretion by the parathyroid gland is cAMP.

2.  Actions of PTH

■are coordinated to produce an increase in serum [Ca2+] and a decrease in serum

[phosphate].