Abnormal neonatal sodium handling in skin precedes hypertension in the SAME rat

We discovered high Na + and water content in the skin of newborn Sprague–Dawley rats, which reduced ~ 2.5-fold by 7 days of age, indicating rapid changes in extracellular volume (ECV). Equivalent changes in ECV post birth were also observed in C57Bl/6 J mice, with a fourfold reduction over 7 days, to approximately adult levels. This established the generality of increased ECV at birth. We investigated early sodium and water handling in neonates from a second rat strain, Fischer, and an Hsd11b2-knockout rat modelling the syndrome of apparent mineralocorticoid excess (SAME). Despite Hsd11b2 −/− animals exhibiting lower skin Na + and water levels than controls at birth, they retained ~ 30% higher Na + content in their pelts at the expense of K + thereafter. Hsd11b2 −/− neonates exhibited incipient hypokalaemia from 15 days of age and became increasingly polydipsic and polyuric from weaning. As with adults, they excreted a high proportion of ingested Na + through the kidney, (56.15 ± 8.21% versus control 34.15 ± 8.23%; n = 4; P