Increased Na⁺/H⁺-exchange activity is the cause of increased [Na⁺]_i and underlies disturbed calcium handling in the rabbit pressure and volume overload heart failure model

Objective: Cytosolic sodium ([Na⁺]_i) is increased in heart failure (HF). We hypothesize that up-regulation of Na⁺/H⁺-exchanger (NHE) in heart failure is causal to the increase of [Na⁺]_i and underlies disturbance of cytosolic calcium ([Ca²⁺]_i) handling. Methods: Heart failure was induced in rabbits by combined volume and pressure overload. Age-matched animals served as control. [Na⁺]_i, cytosolic calcium [Ca²⁺]_i and cytosolic pH (pH_i) were measured in isolated left ventricular midmural myocytes with SBFI, indo-1 and SNARF. SR calcium content was measured as the response of [Ca²⁺]_i to rapid cooling (RC). Calcium after-transients were elicited by cessation of rapid stimulation (3 Hz) in the presence of 100 nmol/l noradrenalin. NHE and Na⁺/K⁺-ATPase activity were inhibited with 10 μmol/l cariporide and 100 μmol/l ouabain, respectively. Results: At all stimulation rates (0–3 Hz) [Na⁺]_i and diastolic [Ca²⁺]_i were significantly higher in HF than in control. With increasing frequency [Na⁺]_i and diastolic [Ca²⁺]_i progressively increased in HF and control, and the calcium transient amplitude (measured as total calcium released from SR) decreased in HF and increased in control. In HF (at 2 Hz), SR calcium content was reduced by 40% and the calcium gradient across the SR membrane by 60%. Fractional systolic SR calcium release was 90% in HF and 60% in control. In HF the rate of pH_i recovery following acid loading was much faster at all pH_i and NHE dependent sodium influx was almost twice as high as in control. In HF cariporide (10 μmol/l, 5 min) reduced [Na⁺]_i and end diastolic [Ca²⁺]_i to almost control values, and reversed the relation between calcium transient amplitude and stimulation rate from negative to positive. It increased SR calcium content and SR membrane gradient and decreased fractional systolic SR depletion to 60%. Cariporide greatly reduced the susceptibility to develop calcium after-transients. In control animals, cariporide had only minor effects on all these parameters. Increase of [Na⁺]_i with ouabain in control myocytes induced abnormal calcium handling as found in HF. Conclusions: In HF up-regulation of NHE activity is causal to increased [Na⁺]_i and secondarily to disturbed diastolic, systolic and SR calcium handling. Specific inhibition of NHE partly normalized [Na⁺]_i, end diastolic [Ca²⁺]_i, and SR calcium handling and reduced the incidence of calcium after-transients. Chronic treatment with specific NHE inhibitors may provide a useful future therapeutic option in treatment of developing hypertrophy and heart failure.