Beside wall tension and contractility, heart rate is a major determinant of myocardial oxygen consumption. Therefore, a decrease in heart rate could prevent ischemia or reduce its consequences. We examined the effect of a new bradycardic agent of the benzazepinone-type (DK-AH 269) on eight isolated, saline-perfused rabbit hearts; bradycardia resulted from a specific blockade of i(f)-channels in sinus node cells. After control measurements (C), the substance was added in three increasing concentrations (D-1: 10(-8) M, D-2: 10(-7) M, D-3: 10(-6) M). We observed a dose-dependent reduction in heart rate (C: 206 +/- 25, D-1: 195 +/- 30, D-2: 77 +/- 41, D-3: 154 +/- 48/min). In the highest dosage, the duration of diastole was increased by 100%. To characterize systolic function, we measured stroke volume (SV), peak left ventricular pressure (LVP(max)) and its first derivative (dP/dt(max)). Aortic flow was slightly decreased whereas SV increased to 108 % of control after initial reduction at the two lower dosages. LVP(max) remained unchanged, and dP/dt(max) was dose-dependently reduced to 91, 81, and 70 % of control (C: 1885 +/- 376, D-1: 1712 +/- 525, D-2: 1526 +/- 504, D-3: 1327 +/- 337 mm Hg/s); dP/dt(min) as a measure of early relaxation was also reduced. The coronary flow per beat did not change compared with control in the presence of the two lower doses of DK-AH 269, but was significantly increased with the highest dose (C: 0.29 +/- 0.06, D-1: 0.28 +/- 0.07, D-2: 0.29 +/- 0.09, D-3: 0.34 +/- O.11 ml). The myocardial oxygen demand was dose-dependently decreased (C: 10.4 +/- 2.5, D-1: 9.6 +/- 2.5, D-2: 8.8 +/- 2.6, D-3: 7.9 +/- 2.4 ml/min/100 g). The relation between subendocardial and subepicardial flow, assessed with colored microspheres, exhibited no changes in the presence of the highest dose of DK-AH 269 (C: 1.28 +/- 0.09, D-3: 1.27 +/- 0.08). DK-AH 269 reduced heart rate in isolated rabbit hearts and increased the duration of diastole. Whereas systolic function was primarily left unchanged, coronary flow per beat and oxygen consumption were decreased. According to our results, this new bradycardic agent could be useful in treating coronary heart disease.