Insulin Resistance and the Relationship Between Urinary Na+/K+ and Ambulatory Blood Pressure in a Community of African Ancestry

BACKGROUND Although groups of African descent are particularly sensitive to blood pressure (BP) effects of salt intake, the role of obesity and insulin resistance in mediating this effect is uncertain. We determined whether obesity or insulin resistance is independently associated with urinary Na+/K+ BP relationships in a community sample of African ancestry. METHODS We measured 24-hour urinary Na+/K+, homeostasis model assessment of insulin resistance (HOMA-IR), and nurse-derived conventional and 24-hour ambulatory BP in 331 participants from a South African community sample of black African descent not receiving treatment for hypertension. RESULTS With adjustments for diabetes mellitus and the individual terms, an interaction between waist circumference and urinary Na+/K+ was associated with day diastolic BP (P < 0.05) and an interaction between log HOMA-IR and urinary Na+/K+ was associated with 24-hour and day systolic (P < 0.05) and 24-hour, day, and night diastolic (P < 0.002; P < 0.001) BP. The multivariable-adjusted relationship between urinary Na+/K+ and night diastolic BP increased across tertiles of HOMA-IR (tertile 1: beta-coefficient = -0.79 +/- 0.47; tertile 2: beta-coefficient = 0.65 +/- 0.35; tertile 3: beta-coefficient = 1.03 +/- 0.46; P < 0.05 tertiles 3 and 2 vs. 1). The partial correlation coefficients for relationships between urinary Na+/K+ and 24-hour (partial r = 0.19; P < 0.02), day (partial r = 0.17; P < 0.05), and night (partial r = 0.18; P < 0.02) diastolic BP in participants with log HOMA-IR greater than or equal to the median were greater than those for relationships between urinary Na+/K+ and 24-hour (partial r = -0.08; P = 0.29), day (partial r = -0.10; P < 0.22), and night (partial r = -0.06; P = 0.40) diastolic BP in participants with log HOMA-IR less than the median (comparisons of r values: P < 0.05). CONCLUSIONS Insulin resistance may modify the relationship between salt intake, indexed by urinary Na+/K+, and ambulatory BP in groups of African descent.