Effect of the Na+-channel modulator BDF 9148 on Ca2+-sensitivity and force of contraction of hypertrophic myocardium from transgene rats harboring the mouse Renin gene (TG(mREN2)27)

The present study aimed to investigate the inotropic effect of the Na+-channel modulator BDF 9148 in hypertrophic myocardium compared to control tissue. Thus, TG(mREN2)27 rats (TGR), a model with hypertension induced cardiac hypertrophy, was compared with age matched Sprague-Dawley rats (SPDR). The effect of BDF 9148 (0.01-10 mu M) on force of contraction (1 Hz, 37 degrees C), the force-frequency relationship (0.5-7 Hz) and the frequency-dependent diastolic tension (0.5-7 Hz) was studied on leftventricular papillary muscles from SPDR and TGR. Chemically skinned muscle fibers of the same hearts were used to examine the influence of BDF 9148 on the Ca2+-sensitivity of the contractile proteins. For control the Ca2+-sensitizer EMD 57033 was examined. In addition the Na+/K+-ATPase activity was measured in both, SPDR and TGR. BDF 9148 showed a concentration dependent positive inotropic effect in SPDR and TGR cardiac preparations. Comparing SPDR and TGR, a higher effectiveness of BDF 9148 on TGR was found, while the potency was unchanged. With increasing stimulation rates a significant higher decrease in force of contraction in TGR compared to SPDR was observed. In addition, a significant higher increase in diastolic tension was found in TGR. After exposure to 1 mu M BDF 9148 the decrease in force of contraction was significantly reduced in both SPDR and TGR, while only in TGR the increase in diastolic tension was reduced. BDF 9148 had no effect on the Ca2+-sensitivity or maximal developed tension of skinned fiber preparations from SPDR or TGR. In contrast, the Ca2+-sensitizer EMD 57033 increased the Ca2+-sensitivity. The activity of the Na+/K+-ATPase was significantly reduced in TGR compared to controls. Conclusions: The Na+-channel modulator BDF 9148 was more effective in hypertrophic compared to control myocardium in increasing force of contraction, enhancing frequency-dependent force generation and reducing diastolic tension. These effects were not mediated via interaction with the contractile apparatus. The enhanced effectiveness of Na+-channel modulation in hypertrophic myocardium could result from alterations of the Na+ homeostasis, i.e. a reduced Na+/K+-ATPase activity.