Effects of inhibition of sarcoplasmic reticulum calcium uptake on contraction in myocytes isolated from failing human ventricle

Objectives: There has been debate regarding the level of sarcoplasmic reticulum (SR) Ca2+ ATPase protein in heart failure. We have used the SR Ca2+ ATPase inhibitor thapsigargin to investigate the functional contribution of this uptake system to contraction and relaxation in myocytes from failing and non-failing human ventricle. Methods: Myocytes were isolated from 28 failing and 18 non-failing ventricles and stimulated at 0.2 Hz, 32 degrees C in Krebs-Henseleit solution. Contraction amplitude and speed were compared before and after treatment with 1 mu mol/l thapsigargin for 20 min to deplete SR Ca2+ stores. Results: initial beat duration was longer in myocytes from failing hearts. Addition of thapsigargin significantly prolonged the kat in myocytes from both groups, but the increase was greater in non-failing hearts (beat duration increased by 0.79+/-0.12 s in myocytes from non-failing hearts compared with 0.37+/-0.12 a in those from failing, P <0.02). The contraction amplitude increased at high stimulation frequencies in myocytes from non-failing hearts (from 2.6% shortening at 0.1 Hz to 4.6% at 1 Hz, P <0.001, n=9), but not in those from failing hearts (1.8% at 0.1 Hz compared with 1.7% at 1 Hz, n=5). Thapsigargin abolished the positive staircase in the non-failing, but had no effect in the failing group. Contraction amplitude following a rest interval was significantly depressed relative to steady-state levels in myocytes from the non-failing hearts (44.8+/-10.3% at 3 min), but not in failing (102+/-18%, P <0.01 compared to non-failing at 3 min). Following thapsigargin treatment there were no longer significant differences between failing and non-hiring myocytes in the time course of the beat, frequency response or post-rest behaviour. Conclusion: The differences between myocytes from failing and non-failing hearts were reduced by inhibition of SR function. These results are consistent with the hypothesis that the: initial differences had been due to decreased SR Ca2+ uptake.