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 micromol/l cariporide and 100 micromol/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 micromol/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.