Impaired Ca2+ handling in perfused hypertrophic hearts from Dahl salt-sensitive rats

To clarify the correlation between intracellular Ca2+ dynamics and level of Ca2+-regulatory proteins, changes in Ca2+ handling and these proteins were investigated in a whole-heart experimental model of pressure-overload hypertrophy. We used 17-18-week-old male Dahl salt-sensitive rats (DS) and Dahl salt-resistant rats (DR) fed a high-salt diet. We monitored the fura-2 fluorescence ratio, an index of cytoplasmic Ca2+ concentration ([Ca2+]i), using a Ca2+ analyzer in a retrograde perfused heart. Left ventricular pressure (LVP) and an electrocardiogram were simultaneously recorded. Ca2+ handling was assessed by exposing the hearts to 2 min of low-Na+ (70 mmol/l) perfusion to produce an increase in [Ca2+]i (n = 6), which was sensitive to Ni2+, a blocker of the Na+/Ca2+ exchanger (NCX). In another series, the hearts were stimulated at 2.5 to 5 Hz to determine the Ca2+-force-frequency relationship (n = 6). DS rats showed marked cardiac hypertrophy without any signs of failure. The time-to-peak Ca2+ transient was prolonged in DS compared with that in DR during normal beating. During low-Na+ exposure, the time-to-peak diastolic [Ca2+]i (TTP) and the decay-time from peak [Ca2+]i (DT) were prolonged in DS compared with DR (TTP, 43.3 +/- 4.0 vs. 32.5 +/- 2.5 s, p < 0.05; DT, 70.0 +/- 8.8 vs. 29.2 +/- 2.7 s, p < 0.005). Following pretreatment with 10 mmol/l caffeine to inhibit sarcoplasmic reticulum (SR) function, TTP and DT were still prolonged in DS compared with DR (TTP, 64.2 +/- 9.7 vs. 37.0 +/- 5.8 s, p < 0.05; DT, 55.8 +/- 12.6 vs. 26.0 +/- 5.7 s, p < 0.05). The force (LVP)-frequency relationship was initially positive in DR but was negative at all times in DS (%LVP/2.5 Hz: DS, 90.3 +/- 2.0%; DR, 112.2 +/- 4.5%; p < 0.05). Elevation of diastolic [Ca2+]i (percent increase of baseline) was greater in DS than in DR with increased stimulation (5 Hz: DS, 80.7 +/- 6.7%; DR, 52.1 +/- 5.9%; p < 0.05). In Western blot analysis, the protein level of NCX was equivalent, whereas that of SR Ca2+ ATPase (SERCA2) was significantly decreased in DS compared with DR. These results suggest that slowing of cellular Ca2+ mobilization and removal is related to impaired Ca2+ handling in late-phase cardiac hypertrophy. Both the activity of the NCX and that of the SR may be affected. The SR dysfunction may be associated with change in protein level of SERCA2.