Tissue sodium content in patients with systemic lupus erythematosus: association with disease activity and markers of inflammation

OBJECTIVES

Sodium (Na⁺) is stored in the skin and muscle and plays an important role in immune regulation. In animal models, increased tissue Na⁺ is associated with activation of the immune system, and high salt intake exacerbates autoimmune disease and worsens hypertension. However, there is no information about tissue Na⁺ and human autoimmune disease. We hypothesized that muscle and skin Na⁺ content is (a) higher in patients with systemic lupus erythematosus (SLE) than in control subjects, and (b) associated with blood pressure, disease activity, and inflammation markers (interleukin (IL)-6, IL-10 and IL-17 A) in SLE.

METHODS

Lower-leg skin and muscle Na⁺ content was measured in 23 patients with SLE and in 28 control subjects using ²³Na⁺ magnetic resonance imaging. Demographic and clinical information was collected from interviews and chart review, and blood pressure was measured. Disease activity was assessed using the SLE Disease Activity Index (SLEDAI). Plasma inflammation markers were measured by multiplex immunoassay.

RESULTS

Muscle Na⁺ content was higher in patients with SLE (18.8 (16.7-18.3) mmol/L) than in control subjects (15.8 (14.7-18.3) mmol/L; p < 0.001). Skin Na⁺ content was also higher in SLE patients than in controls, but this difference was not statistically significant. Among patients with SLE, muscle Na⁺ was associated with SLEDAI and higher concentrations of IL-10 after adjusting for age, race, and sex. Skin Na⁺ was significantly associated with systolic blood pressure, but this was attenuated after covariate adjustment.

CONCLUSION

Patients with SLE had higher muscle Na⁺ content than control subjects. In patients with SLE, higher muscle Na⁺ content was associated with higher disease activity and IL-10 concentrations.