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Internal sodium balance in DOCA-salt rats: a body composition study.

Titze J, Bauer K, Schafflhuber M, Dietsch P, Lang R, Schwind KH, Luft FC, Eckardt KU, Hilgers KF

Forschungslabor der Medizinischen Klinik IV, Kreuzburger Strasse 2, 90471 Nürnberg, Germany. jens.titze@rzmail.uni-erlangen.de

The idea that Na(+) retention inevitably leads to water retention is compelling; however, were Na(+) accumulation in part osmotically inactive, regulatory alternatives would be available. We speculated that in DOCA-salt rats Na(+) accumulation is excessive relative to water. Forty female Sprague-Dawley rats were divided into four subgroups. Groups 1 and 2 (controls) received tap water or 1% saline (salt) for 5 wk. Groups 3 and 4 received subcutaneous DOCA pellets and tap water or salt. Na(+), K(+), and water were measured in skin, bone, muscle, and total body by desiccation and consecutive dry ashing. DOCA-salt led to total body Na(+) excess (0.255 +/- 0.022 vs. 0.170 +/- 0.010 mmol/g dry wt; P < 0.001), whereas water retention was only moderate (0.685 +/- 0.119 vs. 0.648 +/- 0.130 ml/g wet wt; P < 0.001). Muscle Na(+) retention (0.220 +/- 0.029 vs. 0.145 +/- 0.021 mmol/g dry wt; P < 0.01) in DOCA-salt was compensated by muscle K(+) loss, indicating osmotically neutral Na(+)/K(+) exchange. Skin Na(+) retention (0.267 +/- 0.049 vs. 0.152 +/- 0.014 mmol/g dry wt; P < 0.001) in DOCA-salt rats was not balanced by K(+) loss, indicating osmotically inactive skin Na(+) storage. We conclude that DOCA-salt leads to tissue Na(+) excess relative to water. The relative Na(+) excess is achieved by two distinct mechanisms, namely, osmotically inactive Na(+) storage and osmotically neutral Na(+) retention balanced by K(+) loss. This "internal Na(+) escape" allows the maintenance of volume homeostasis despite increased total body Na(+).

Published 14 September 2005 in Am J Physiol Renal Physiol, 289(4): F793-802.
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