354
Chapter 11
Parathyroid hormone exerts multiple actions that increase
extracellular calcium concentration, thus compensating for
the decreased concentration that originally stimulated secre-
tion of this hormone (
Figure 11–32
):
1.
It directly increases the resorption of bone by
osteoclasts, which causes calcium (and phosphate) to
move from bone into extracellular fl
uid.
2.
It directly stimulates the formation of another
hormone, called 1,25-dihydroxyvitamin D, which
then increases intestinal absorption of calcium (and
phosphate). Thus, the effect of parathyroid hormone on
the intestinal tract is indirect.
3.
It directly increases calcium reabsorption in the
kidneys, thus decreasing urinary calcium excretion.
4. It directly
reduces
the reabsorption of phosphate in
the kidneys, thus raising its urinary excretion. This
keeps plasma phosphate from increasing at a time when
parathyroid hormone causes an increased release of
both calcium and phosphate from bone, and increased
1,25-dihydroxyvitamin D is increasing both calcium
and phosphate absorption in the intestine.
1,25-Dihydroxyvitamin D
The term
vitamin D
denotes a group of closely related com-
pounds.
Vitamin D
3
(cholecalciferol)
is formed by the action
of ultraviolet radiation (from sunlight, usually) on a cholesterol
derivative (7-dehydrocholesterol) in skin.
Vitamin D
2
(ergo-
calciferol)
is derived from plants. Both can be found in vitamin
pills and enriched foods and are collectively called vitamin D.
Because of clothing and decreased outdoor living,
people are often dependent upon dietary vitamin D. For this
reason, it was originally classifi
ed as a vitamin. Regardless of
source, vitamin D is metabolized by the addition of hydroxyl
groups, fi rst in the liver by the enzyme 25-hydroxylase and
then in certain kidney cells by 1-hydroxylase (
Figure 11–33
).
The end result of these changes is
1,25-dihydroxyvitamin D
Begin
Release of calcium
into plasma
Bone
Resorption
Intestine
Calcium absorption into blood
Urinary excretion
of calcium
Plasma
1,25–(OH)
2
D
Kidneys
Calcium
reabsorption
1,25–(OH)
2
D
formation
Plasma parathyroid hormone
Parathyroid glands
Parathyroid hormone secretion
Plasma calcium
Restoration of plasma calcium levels toward normal
Figure 11–32
Mechanisms that allow parathyroid hormone to reverse a reduction
in plasma calcium concentration toward normal. See Figure
11–33 for a more complete description of 1,25-(OH)
2
D (1,25-
dihydroxyvitamin D). Parathyroid hormone and 1,25-(OH)
2
D are
also involved in the control of phosphate levels.
Begin
Plasma 1,25–(OH)
2
D
Kidneys
25–OH D
1,25–(OH)
2
D
Liver
Vitamin D
25–OH D
Plasma vitamin D
Skin
7-dehydrocholesterol
Vitamin D
3
Dietary vitamin D
2
or D
3
Sunlight
Parathyroid
hormone
(stimulates activity
of 1-hydroxylase)
GI tract
Absorption of calcium
(and phosphate) into blood
1–hydroxylase
25–hydroxylase
Figure 11–33
Metabolism of vitamin D to the active form, 1,25-(OH)
2
D.
Figure 11–33
physiological
inquiry
Sarcoidosis is a disease that affects a variety of organs (usually
the lungs). It is characterized by the development of nodules of
infl
amed tissue known as granulomas. These granulomas can
express signifi cant 1-hydroxylase activity that is not controlled by
parathyroid hormone. What will happen to plasma calcium and
parathyroid hormone levels under these circumstances?
Answer can be found at end of chapter.
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