Enhanced activity of sodium-lithium countertransport in patients with cardiac syndrome X: a potential link between cardiac and metabolic syndrome X.

OBJECTIVES This study was aimed at assessing both stimulated insulinemia and the sodium-lithium countertransport in a selected group of patients with cardiac syndrome X. BACKGROUND Hyperinsulinemia, which is frequently present in patients with cardiac syndrome X, is often associated with an enhanced activity of the sodium-lithium countertransport, an in vitro marker of sodium-hydrogen exchange. METHODS Fifteen patients with syndrome X and 14 matched controls were studied. After pharmacological washout, sodium-lithium countertransport was assessed from lithium-loaded red blood cells. Postload insulin levels were evaluated by a double-antibody radioimmunoassay. RESULTS Maximal velocity of sodium-lithium countertransport was higher in patients with syndrome X compared to controls (635+/-200 vs. 324+/-49 micromol/liter/h, p = 0.001). Fourteen of the 15 patients with syndrome X (93%) presented sodium-lithium countertransport values higher than the mean +2 SD of the control group. At 120 min, 12 patients with syndrome X (80%) had plasma levels of insulin >420 pmol/liter, which corresponds to the mean value +2 SD of controls (p = 0.006). CONCLUSIONS Both enhanced activity of the sodium-lithium countertransport and stimulated hyperinsulinemia are present in the vast majority of patients with cardiac syndrome X. As enhanced activity of the sodium-lithium countertransport has the potential to cause both glucose intolerance and smooth muscle hyperreactivity, it might represent a common cause of the metabolic and vascular alterations frequently found in syndrome X.