EFFECTS OF INTRINSIC SYMPATHOMIMETIC ACTIVITY ON CARDIAC-FUNCTION DURING VARIATIONS IN PRELOAD

The effect of intrinsic sympathomimetic activity (ISA) on ventricular function during wide variations in preload was investigated in 10 heal thy men. Hemodynamic and ventricular performance responses were measur ed during minute 90 of 5 degrees head-down tilt (TILT) and -40 Torr lo wer body negative pressure (LBNP) after bolus injections of either sal ine, pindolol (0.01 mg/kg) or propranolol (0.1 mg/kg). Differences in response between drugs were attributed to ISA effects. Heart rate, cha mber volumes, ejection fraction, mean velocity of circumferential fibe r shortening (V-cf) and diastolic and systolic wall stress were determ ined by M-mode echocardiographic and electrocardiographic measurements . Propranolol decreased V-cf and heart rate (p<0.05) relative to both saline and pindolol during all preload conditions, whereas the respons es during pindolol treatment were not different from those during sali ne. Systolic and diastolic wall stress were increased (p<0.05) by prop ranolol treatment regardless of preload. Both systolic and diastolic w all stress increased >12% during TILT with propranolol while no change s were observed with saline (-1%) or pindolol (+2%). These data indica te that the noncardiodepressive effects of beta-blockade with ISA pers ist regardless of variations in preload. In addition, increases in wal l stress by beta-blockade (propranolol), particularly during increases in preload (TILT), are prevented by blockade with ISA (pindolol). Thu s, under certain conditions, pindolol does not elicit the wall-stress- related increases in myocardial oxygen demand that occur with proprano lol treatment.