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1 Institut für Anatomie, Charité, Humboldt Universität, 10115 Berlin; 2 Forschungsinstitut für Molekulare Pharmakologie, Campus Berlin-Buch, 13125 Berlin, Germany; and 3 Division of Nephrology, Hypertension, and Clinical Pharmacology, Oregon Health Sciences University, Portland, Oregon 97201
Hypothyroidism is associated
with significant abnormalities in the renal handling of salt and water.
To address the involvement of tubular transport proteins in these
abnormalities, rats were rendered pharmacologically hypothyroid and the
abundance of major tubular transport proteins was assessed by
immunoblot and immunohistochemistry. Hypothyroidism resulted in a
marked reduction in kidney size and creatinine clearance along with
decreased or unchanged total kidney abundance of the transport
proteins. Whereas the proximal tubular type 3 Na/H exchanger (NHE3) and
type 2 Na-phosphate cotransporter (NaPi2) stood out by their
disproportionately reduced abundance, the bumetanide-sensitive type 2 Na-K-2Cl cotransporter (NKCC2) and aquaporin-2 (AQP2) were unaltered in
their total kidney abundance despite a markedly lower kidney mass. The
latter proteins in fact showed enhanced immunostaining. Decreased NHE3
and NaPi2 expression was most likely due to a combination of
triiodo-L-thyronine (T3) deficiency along with
a reduced glomerular filtration rate. The increased abundance
of NKCC2 and AQP2 may have been caused by an increased action of
vasopressin since urinary excretion of this hormone was elevated. On
the other hand, the thiazide-sensitive Na-Cl cotransporter; the
-,
-, and
-subunits of the amiloride-sensitive epithelial Na
channel; and the
1-subunit of Na-K-ATPase showed a
moderate decrease in total kidney abundance that was largely proportional to the smaller kidney mass. Although the observed expression of transporters was associated with a balanced renal sodium
handling, altered transporter abundance may become functionally relevant if the hypothyroid kidney is challenged by an additional destabilization of the milieu interieur that has previously been shown
to result in an inadequate natriuresis and clinical symptoms.
sodium transport; vasopressin; distal tubule
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