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 a nd 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,vith an enhanced activity of the so dium-lithium countertransport, an in vitro marker of sodium-hydrogen exchan ge. Methods. Fifteen patients with syndrome X and 14 matched controls were stud ied, After pharmacological washout, sodium-lithium countertransport was ass essed from lithium-loaded red bleed cells. Postload insulin levels were eva luated 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 m u mol/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 val ue +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 patient s with cardiac syndrome X. As enhanced activity of the sodium-lithium count ertransport 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. (J Am Coil Cardiol 1998;32:2031-4) (C) 1998 by the American College of Cardiology.