Chronic sodium loading augments natriuretic response to acute volume expansion in the preweaned rat

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.