Hyponatraemia aggravates cardiac susceptibility to ischaemia/reperfusion injury

Hyponatraemia is defined as a serum sodium concentration of <135 mEql/L and is the most common electrolyte disturbance in patients with chronic heart failure. We hypothesize that hyponatraemia may induce Ca²⁺ overload and enhance reactive oxygen species (ROS) production, which will exacerbate myocardial injury more than normonatraemia. We investigated the effect of hyponatraemia on the ability of the heart to recover from ischaemia/reperfusion episodes. Cardiomyocytes were obtained from 1- to 3-day-old Sprague Dawley rats. After isolation, cardiomyocytes were placed in Dulbecco's modified Eagle's medium (DMEM) containing low sodium concentration (110, 120, or 130 mEq/L) or normal sodium concentration (140 mEq/L) for 72 hours. Exposure of cardiomyocytes to each of the low-sodium medium significantly increased both ROS and intracellular Ca²⁺ levels compared with the exposure to the normal-sodium medium. In vivo, 8-week-old male Sprague Dawley rats were divided into four groups: control group (Con), furosemide group (Fur), low-sodium diet group (Lsd) and both furosemide and low-sodium diet group (Fur + Lsd). The hearts subjected to global ischaemia exhibited considerable decrease in left ventricular developed pressure during reperfusion, and the size of infarcts induced by ischaemia/reperfusion significantly increased in the Fur, Lsd and Fur + Lsd compared with that in the Con. Hyponatraemia aggravates cardiac susceptibility to ischaemia/reperfusion injury by Ca²⁺ overload and increasing in ROS levels.