Osmotically inactive skin Na⁺storage in rats

Abstract

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 (DeltaSKNa+) in the skin in all SD rats. Osmotically inactive skin Na+ accumulation was approximately 66% of DeltaSKNa+ 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.

Feasibility and Impact of the Measurement of Extracellular Fluid Volume Simultaneous with GFR by 125I-Iothalamate

“…In rat models, they found that an increase in sodium intake went along with a less than anticipated rise in ECFV, which could be attributed to nonosmotic storage of sodium in tissues as the skin (32). Whether such a nonosmotic storage exists in human has not been established yet.…”

Extrarenal Na + Balance, Volume, and Blood Pressure Homeostasis in Intact and Ovariectomized Deoxycorticosterone-Acetate Salt Rats

“…This finding is in line with previous experiments, where we demonstrated that OVX led to skin Na ϩ storage incapacity in rats without mineralocorticoid excess. 10 However, the loss in Na ϩ storage capacity was not enough to cause a salt-sensitive blood pressure increase in untreated OVX Sprague-Dawley rats. We, thus, combined OVX and DOCA salt in this present experiment.…”

“…In Sprague-Dawley rats, we found that ovariectomy (OVX) reduced the osmotically inactive Na ϩ storage capacity with a high-salt diet. 10 However, whether or not osmotically inactive Na ϩ storage attenuates blood pressure is unclear. We now investigated the osmotically inactive Na ϩ storage capacity in a prototype model of salt-sensitive hypertension with or without OVX.…”

Exercise-Associated Hyponatremia

“…These studies indicated that up to one fourth of the total body sodium may exist in bone and cartilage stores that are not osmotically active (i.e., in an insoluble crystal compound) but potentially recruitable into an osmotically active form (81)(82)(83). In rats, this nonosmotically active sodium may reside bound to skin proteoglycans (87,88). This dynamic pool of exchangeable sodium also can lead to the osmotic inactivation of sodium if sodium moves into this compartment.…”