Anesthesia, monitoring and baseline measurements 
Animals were acclimatized for at least 7 days. The night before experiments, they were fasted. In all animals, premedication was provided with an intramuscular injection of midazolam (0.3 mg/kg), ketamine (20 mg/kg) and atropine (0.04 mg/kg). Anesthesia was induced using inhalational isoflurane (end tidal concentration: 1-3%) and maintained on the same concentration through an oral endotracheal tube and mechanical ventilation was established with a tidal volume of 6-8 ml/kg. An arterial line was inserted through the right common carotid artery and a central venous line was introduced into the left jugular vein. A pulmonary artery catheter was inserted via the right jugular vein and directed into position beyond the pulmonary artery bifurcation. Standard monitoring consisted of: EKG, pulse oximetry, end tidal CO2 and invasive arterial pressure. After a median sternotomy and pericardiotomy, the heart and great vessels were exposed. An umbilical tape was placed around the inferior vena cava and a pressure-volume conductance catheter (Millar Instruments Inc, Houston, TX, USA) was inserted into the left ventricular apex. Systemic anticoagulation was achieved with an intravenous injection of heparin (30 000 IU).