Page 339 - Color Atlas of Biochemistry
P. 339
330 Tissues and organs
Renal hormones the α 2 -globulin group (see p. 276), which
like almost all plasma proteins is synthesized
in the liver. The decapeptide cleaved off by
A. Renal hormones
renin is called angiotensin I. Further cleavage
In addition to their involvement in excretion by peptidyl dipeptidase A (angiotensin-con-
and metabolism, the kidneys also have endo- verting enzyme, ACE), a membrane enzyme
crine functions. They produce the hormones located on the vascular endothelium in the
erythropoietin and calcitriol and play a deci- lungs and other tissues, gives rise to the
sive part in producing the hormone angioten- octapeptide angiotensin II [3], which acts as
sin II by releasing the enzyme renin. Renal a hormone and neurotransmitter. The lifespan
prostaglandins (see p. 390) have a local effect of angiotensin II in the plasma is only a few
+
on Na resorption. minutes, as it is rapidly broken down by other
Calcitriol (vitamin D hormone, 1α,25-dihy- peptidases (angiotensinases [4]), which occur
droxycholecalciferol) is a hormone closely re- in many different tissues.
lated to the steroids that is involved in Ca 2+ The plasma level of angiotensin II is mainly
homeostasis (see p. 342). In the kidney, it is determined by therateatwhich renin is re-
formed from calcidiol by hydroxylation at C-1. leased by the kidneys. Renin is synthesized by
The activity of calcidiol-1-monooxygenase [1] juxtaglomerular cells, which release it when
is enhanced by the hormone parathyrin sodium levels decline or there is a fall in blood
(PTH). pressure.
Erythropoietin is a peptide hormone that is Effects of angiotensin II. Angiotensin II has
formed predominantly by the kidneys, but effects on the kidneys, brain stem, pituitary
also by the liver. Together with other factors gland, adrenal cortex, blood vessel walls, and
known as “colony-stimulating factors” (CSF; heart via membrane-located receptors. It in-
see p. 392), it regulates the differentiation of creases blood pressure by triggering vasocon-
stem cells in thebonemarrow. striction (narrowing of the blood vessels). In
Erythropoietin release is stimulated by hy- the kidneys, it promotes the retention of Na +
poxia (low pO 2 ). Within hours, the hormone and water and reduces potassium secretion.
ensures that erythrocyte precursor cells in the In the brain stem and at nerve endings in the
bone marrow are converted to erythrocytes, sympathetic nervous system, the effects of
so that their numbers in the blood increase. angiotensin II lead to increased tonicity (neu-
Renal damage leads to reduced erythropoie- rotransmitter effect). In addition, it triggers
tin release, whichinturnresults in anemia. the sensation of thirst. In the pituitary gland,
Forms of anemia with renal causes can now angiotensin II stimulates vasopressin release
be successfully treated using erythropoietin (antidiuretic hormone) and corticotropin
produced by genetic engineering techniques. (ACTH) release. In the adrenal cortex, it in-
The hormone is also administered to dialysis creases the biosynthesis and release of aldo-
patients. Among athletes and sports profes- sterone, which promotes sodium and water
sionals, there have been repeated cases of retention in the kidneys. All of the effects of
erythropoietin being misused for doping pur- angiotensin II lead directly or indirectly to
poses. increased blood pressure, as well as increased
sodium and water retention. This important
hormonal system for blood pressure regula-
B. Renin–angiotensin system
tion can be pharmacologically influenced by
The peptide hormone angiotensin II is not inhibitors at various points:
synthesized in a hormonal gland, but in the • Using angiotensinogen analogs that inhibit
blood. The kidneys take part in this process by renin.
releasing the enzyme renin. • Using angiotensin I analogs that competi-
Renin [2] is an aspartate proteinase (see tively inhibit the enzyme ACE [3].
p. 176). Itisformedby the kidneysasapre- • Using hormone antagonists that block the
cursor (prorenin), which is proteolytically binding of angiotensin II to its receptors.
activated into renin and released into the
blood. In the blood plasma, renin acts on
angiotensinogen, a plasma glycoprotein in
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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