AJP - Regulatory, Integrative and Comparative Physiology, Vol 269, Issue 1 15-R22, Copyright © 1995 by American Physiological Society
Chronic sodium loading augments natriuretic response to acute volume expansion in the preweaned rat
D. G. Muchant, B. A. Thornhill, D. C. Belmonte, R. A. Felder, A. Baertschi and R. L. Chevalier
Department of Pediatrics, University of Virginia, Charlottesville 22908, USA.
Positive sodium balance is necessary for normal somatic growth of the
neonate, and the neonatal renal response to volume expansion (VE) is
attenuated compared with the adult. To test the hypothesis that dietary
sodium modulates the developmental response to VE, preweaned rats were
artificially reared with either a normal (25 meq/l)- or high-sodium (145
meq/l) diet for 7-8 days and were compared with adult rats receiving normal
or high sodium. Serum sodium concentration remained normal in adults on
high sodium, whereas neonates became hypernatremic. Glomerular filtration
rate (GFR), urinary flow (V), and urinary sodium (UNaV) were measured
before and after acute saline VE (1% body wt). While remaining constant in
preweaned rats, GFR increased > 50% in adult rats after VE (P <
0.05). High sodium intake augmented V and UNaV after VE but was not
sustained in neonates as in adults. Plasma atrial natriuretic peptide (ANP)
and guanosine 3',5'-cyclic monophosphate excretion (UcGMPV) were measured,
and baseline UcGMPV was lower in preweaned rats receiving normal sodium but
increased to levels similar to adult levels after VE. Postexpansion plasma
ANP was higher in preweaned rats than in adult rats and was not affected by
dietary sodium regardless of age. We conclude that the attenuated
postexpansion natriuresis in the neonate is due in part to an adaptive
response to limited sodium intake. However, neonatal compensation to
increased sodium intake is incomplete and independent of plasma ANP.
