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. (Hypertens Res 2003; 26: 643-653)