Die Na+/H+-Austauscher-abhängige pH-Regulation in Vorhof- und Ventrikelmyozyten

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The Na+/H+-exchanger (NHE) is significantly involved in pHi regulation. Its activation is of particular importance from the pathophysiological perspective. In order to investigate the question of whether there are differences in NHE-1-dependent, global and sub-cellular pHi regulation between atrial and ventricular myocytes and whether there are differences in NHE expression between human atrial and venricular tissue and in different cardiac diseases, sub-cellular H+ homeostasis and the activity of NHE-1 was assessed in different sub-cellular regions of atrial and ventricular myocytes from the heart of the rabbit, and the expression of NHE-1 in healthy and insufficient human myocardium was measured and compared using the Western Blot method. The activity of NHE-1 was determined with reference to NHE-dependent pHi changes in different sub-cellular regions in atrial and ventricular myocytes of rabbits. For this purpose, the NH4Cl acid pulse method in conjunction with a pH-sensitive fluorescent dye, 5-carboxy-SNARF®-1, was used. The use of a confocal microscope allowed the measurement of sub-cellular changes in fluorescence to be made. The specificity of the method was demonstrated in control experiments with the NHE-1-specific inhibitor HOE642. The changes in fluorescence were calibrated and converted into changes in pHi. The rate of H+ efflux at pH 6.9 (JpH6,9) was chosen as a measure of the activity of NHE-1. It was found that the global NHE-1-dependent-JpH6,9 in ventricular myocytes was significantly larger than in atrial myocytes and the JpH6,9 in the centre of the ventricular myocytes was significantly larger than in the atrial myocytes, respectively. The sub-cellular NHE-1-dependent JpH6,9 was of a similar size in both the ventricular myocytes and the atrial myocytes in all sub-cellular regions. In quantitative measurements of NHE-1 protein, likewise no difference was found between human non-insufficient ventricular and atrial myocardium using the Western Blot method. It is therefore concluded that ventricular myocytes show significantly greater NHE-1-mediated H+ elimination (5 mM/min versus 3.2 mM/min in atrial myocytes). Measurable radial pH gradients could neither be observed in ventricular nor in atrial myocytes. Comparing NHE-1 expression between insufficient and non-insufficient ventricular myocardium, an increased expression of NHE-1 in insufficient human ventricular myocardium was revealed.