CLINICAL CASES
419
Angiotensinogen: a2-Globulin; a 14-amino acid peptide that circulates in
the plasma. It is cleaved by the protease renin to give the inactive
decapeptide angiotensin I. Angiotensin converting enzyme hydrolyzes
two amino acids from the C-terminus of angiotensin I to give the active
angiotensin II, which gives rise to angiotensin III by the action of an
aminopeptidase.
Diabetes insipidus: The chronic excretion of large quantities of very pale
urine of low specific gravity resulting in dehydration and extreme thirst.
Neurophysin: A protein that is secreted with oxytocin and vasopressin
from the posterior pituitary gland. Neurophysin binds to these two hor-
mones and stabilizes them.
Renin: A protease that is synthesized by the juxtaglomerular cells of the
kidney and secreted into the bloodstream in response to conditions of
hypovolemia and hyponatremia. It hydrolyzes circulating angiotensino-
gen to angiotensin I.
Vasopressin: Antidiuretic hormone; a nine-amino acid peptide that is syn-
thesized by the hypothalamus and controls resorption of water by distal
tubules of the kidney. It stimulates the insertion of water channels (aqua-
porins) into the apical membranes of kidney tubules.
D ISC U SSIO N
Vasopressin (antidiuretic hormone) is a nonapeptide that controls resorp-
tion of water by distal tubules of the kidney to regulate the osmotic pressure
of blood. It functions to conserve body water by reducing the output of urine,
and thus it is known as an antidiuretic. Vasopressin is synthesized in the
supraoptic nucleus of the hypothalamus where it is bound to a neurophysin
protein carrier, packaged in granules, and delivered by intracellular transport to
nerve terminals in the posterior pituitary. Vasopressin bound to neurophysin is
released from the granules in response to increased extracellular osmolarity
sensed by hypothalamic osmoreceptors, signaling by atrial stretch receptors or
after a rise in angiotensin II levels. Its secretion is increased by dehydration
or stress and decreased after alcohol consumption.
Vasopressin promotes increased resorption of water in the renal distal
tubule by stimulating insertion of water channels or aquaporins into the
apical membranes of kidney tubules. Water is resorbed across the renal
epithelium into the blood leading to a decrease in plasma osmolarity and an
increase in the osmolarity of urine. In DI, this process is impaired, leading to
excessive urine production. In the absence of vasopressin, the kidney cannot
resorb water and it flows out as urine. This condition can arise from a defi-
ciency in vasopressin secretion from the posterior pituitary as a result of hypo-
thalamic tumors, injury (as in the case of this patient) or infection. Alternately,
previous page 433 Case Files   Biochemistry read online next page 435 Case Files   Biochemistry read online Home Toggle text on/off