Serum Insulin, Insulin Sensitivity, and Erythrocyte Sodium Metabolism in Normotensive and Essential Hypertensive Subjects with and Without Overweight

Abstract

Increased insulin circulating levels and perturbations of intracellular sodium metabolism have been reported in essential hypertensive patients, leading to postulate their involvement in the pathophysiology of the disease. In-vitro studies have shown that insulin modulates the activity of some transmembrane sodium transporters. The aim of this investigation was to assess in subjects with essential hypertension and/or overweight, the levels of fasting serum insulin, the activity of sodium transporters and their possible relationships. In 18 lean normotensive, 12 overweight normotensive, 18 untreated lean essential hypertensive, and 16 untreated overweight essential hypertensive subjects, we measured the fasting levels of blood glucose and serum insulin, and calculated the glucose/insulin ratio as an index of sensitivity to insulin. In addition, in the red blood cells of these subjects, we evaluated the maximal rate of ouabain-sensitive Na/K pump, furosemide-sensitive outward Na/K cotransport, Na_i/Li_o countertransport, and the constant rate of passive permeabilty to Na. When compared to lean normotensive, overweight normotensive, lean hypertensive, and overweight hypertensive subjects exhibited significantly higher fasting insulin levels, with lower glucose/insulin ratio. No significant difference was found in the activity of Na/K pump, Na/K cotransport, and passive permeability to Na. The Na_i/Li_o exchange was significantly increased in both hypertensive groups. Mean blood pressure correlated positively and independently with body mass index and fasting insulinemia, and inversely with the glucose/insulin ratio. No relationships were found between blood pressure, fasting insulin levels or glucose/insulin ratio and the activity of sodium transport systems. We conclude that hyperinsulinemia and insulin resistance are associated with essential hypertension independently of overweight. These data lend support to the hypothesis that insulin is involved, concurrently with other factors, in the pathogenesis of essential hypertension in both lean and obese subjects.

Key Words:

• insulin
• hypertension
• overweight
• sodium transport