Osmotically inactive skin Na+storage in rats

Compared with age-matched men, women are resistant to the hypertensive effects of dietary NaCl; however, after menopause, the incidence of salt-sensitive hypertension is similar in women and men. We recently suggested that osmotically inactive Na+storage contributes to the development of salt-sensitive hypertension. The connective tissues, including those immediately below the skin that may serve as a reservoir for osmotically inactive Na+storage, are affected by menopause. We tested the hypothesis that ovariectomy (OVX) might reduce osmotically inactive Na+storage capacity in the body, particularly in the skin. Male, female-fertile, and female OVX Sprague-Dawley (SD) rats were fed a high (8%)- or low (0.1%)-NaCl diet. The groups received the diet for 4 or 8 wk. At the end of the experiment, subgroups received 0.9% saline infusion and urinary Na+and K+excretion was measured. Wet and dry weight (DW), water content in the body and skin, total body Na+(rTBNa+) and skin Na+(rSKNa+) content were measured relative to DW by desiccation and dry ashing. There were no gender differences in osmotically inactive Na+storage in SD rats. All SD rats accumulated Na+if fed 8% NaCl, but rTBNa+was lower in OVX rats than in fertile rats on a low (P< 0.001)- and a high (P< 0.05)-salt diet. OVX decreased rSKNa+(P< 0.01) in the rats. A high-salt diet led to Na+accumulation (ΔSKNa+) in the skin in all SD rats. Osmotically inactive skin Na+accumulation was ∼66% of ΔSKNa+in female and 82% in male-fertile rats, but there was no osmotically inactive Na+accumulation in OVX rats fed 8% NaCl. We conclude that skin is an osmotically inactive Na+reservoir that accumulates Na+when dietary NaCl is excessive. OVX leads to an acquired reduction of osmotically inactive Na+storage in SD rats that predisposes the rats to volume excess despite a reduced Na+content relative to body weight.