Background: Although it is known that endothelin (ET-1) is elevated in heart failure (HF), it remains unclear if chronic ET(A/B) receptor antagonism affects the progression of HF, particularly by affecting coronary vasoactivity and left ventricular (LV) diastolic function.
Methods: We examined the effects of an ET(A/B) receptor antagonist, L-753,037 (oral bid for 6 weeks, n=7), and vehicle (n=8) in conscious dogs with previously implanted aortic, coronary sinus and left atrial catheters, LV pressure gauge, aortic flow probe, LV dimension crystals and pacers.
Results: Baseline hemodynamics were similar in the two groups. During the development of rapid pacing-induced HF, treatment with the ET(A/B) antagonist significantly reduced total peripheral resistance and increased cardiac output compared to vehicle. After 2 weeks of pacing, LV diastolic function (tau) was improved (P<0.05) in the ET(A/B) antagonist group (+6+/-2 ms) compared to the vehicle group (+12+/-2 ms). In addition, ET(A/B) antagonist treatment attenuated the increase in mean left atrial pressure and LV end-diastolic pressure that occurred during heart failure in vehicle-treated animals. However, LV systolic function (LV dP/dt, fractional shortening and Vcfc) neither at rest nor in response to dobutamine was altered by ET(A/B) antagonist treatment. Also, ET(A/B) antagonist treatment did not affect the progressive increases in LV dimension. After 6 weeks of pacing, maximal Ca(2+) transport in isolated cardiac sarcoplasmic reticulum (SR) was reduced (P<0.02) in the vehicle-treated compared to the ET(A/B) antagonist-treated dogs (1.34+/-0.09 vs. 1.60+/-0.06 micromol/mg/min, respectively). The improvement in SR function in the ET(A/B) antagonist-treated dogs was associated with a significant attenuation of the reduction in protein expression of SERCA2a and calsequestrin observed in the vehicle-treated dogs. Coronary arteries isolated from the dogs treated with the ET(A/B) antagonist exhibited enhanced (P<0.01) coronary endothelium-dependent relaxation compared to the vehicle group, while coronary responses to an NO donor were identical in the two groups. Plasma NO levels in the coronary sinus during the late stage of HF were higher (P<0.05) in the ET(A/B) antagonist group (40+/-2 microM) compared to the vehicle group (18+/-2 microM).
Conclusions: We conclude that in conscious dogs during the development of HF induced by rapid pacing, chronic inhibition of ET(A/B) receptors does not affect resting myocardial contractile function nor reserve, but reduces vascular resistance and improves LV diastolic function. After 6 weeks of pacing, the reduction in intracellular Ca(2+) regulation by the SR is also attenuated, and endothelium-dependent coronary relaxation is improved, which appears to be related to the preservation of coronary NO levels.