Protein-enriched diet increases water absorption via the aquaporin-2 water channels in healthy humans.
BACKGROUND
According to animal experiments, a protein-enriched diet increased renal absorption of sodium and water. We wanted to test the hypothesis that a protein-enriched diet would increase the expression of the aquaporin-2 water channels and the epithelial sodium channels in the distal part of the nephron using biomarkers for the activity of the two channels.


METHODS
We performed a randomized, placebo controlled crossover study in 13 healthy humans to examine the effect of a protein-enriched diet on renal handling of water and sodium during baseline condition and during hypertonic saline infusion. We measured the effect of the protein-enriched diet on urinary excretions of aquaporin-2 (u-AQP2), the beta-fraction of the epithelial sodium channels (u-ENaC(beta)), free water clearance (C(H2O)), fractional excretion of sodium and vasoactive hormones.


RESULTS
During baseline conditions, u-AQP2 increased, and C(H2O) decreased during the protein-enriched diet, whereas u-ENaC(beta) was unchanged, although the urinary sodium excretion increased. During hypertonic saline infusion, the response in the effect variables did not deviate between protein-enriched and normal diet. Plasma concentrations of angiotensin II and aldosterone increased as well as pulse rate. Vasopressin in plasma was unchanged, and prostaglandin E(2) fell during the protein-enriched diet.


CONCLUSIONS
The protein-enriched diet increased water absorption via an increased transport via the aquaporin-2 water channels. The increased u-AQP2 might be due to a reduced prostaglandin level. The increase in renal sodium excretion seems to be mediated in another part of the nephron than the epithelial sodium channels.
Department of Medicine, Department of Medical Research, Regional Hospital Holstebro, Lagaardvej 12, Holstebro, Denmark. thomlaur@rm.dk
Thomas Guldager
TG
Lauridsen
Henrik
H
Vase
Jørn
J
Starklint
Jesper N
JN
Bech
Erling B
EB
Pedersen
D000042:Absorption / Q000187:drug effects; D000293:Adolescent; D000328:Adult; D000368:Aged; D000450:Aldosterone / Q000097:blood; D000804:Angiotensin II / Q000097:blood; D051399:Aquaporin 2 / Q000378:metabolism*; D009320:Atrial Natriuretic Factor / Q000097:blood; D001794:Blood Pressure / Q000187:drug effects; D018592:Cross-Over Studies; D000242:Cyclic AMP / Q000652:urine; D004044:Dietary Proteins / Q000494:pharmacology*; D015232:Dinoprostone / Q000652:urine; D053503:Epithelial Sodium Channels / Q000378:metabolism; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D008297:Male; D008875:Middle Aged; D020097:Natriuretic Peptide, Brain / Q000097:blood; D009399:Nephrons / Q000187:drug effects* / Q000378:metabolism*; D012083:Renin / Q000097:blood; D012964:Sodium / Q000652:urine; D014667:Vasopressins / Q000097:blood; D014867:Water / Q000378:metabolism*; D055815:Young Adult
C496023:AQP2 protein, human; D051399:Aquaporin 2; D004044:Dietary Proteins; D053503:Epithelial Sodium Channels; D014867:Water; D014667:Vasopressins; D000804:Angiotensin II; D020097:Natriuretic Peptide, Brain; D000450:Aldosterone; D009320:Atrial Natriuretic Factor; D012964:Sodium; D000242:Cyclic AMP; D012083:Renin; D015232:Dinoprostone