THE RATE OF FORCE GENERATION BY THE MYOCARDIUM IS NOT INFLUENCED BY AFTERLOAD

The influence of afterload on the rate of force generation by the myoc ardium was investigated using two types of preparations: the in situ d og heart (dP/dt) and isolated papillary muscle of rats (dT/dt). Thirte en anesthetized, mechanically ventilated and thoracotomized dogs were submitted to pharmacological autonomic blockade (3.0 mg/kg oxprenolol plus 0.5 mg/kg atropine). A reservoir connected to the left atrium per mitted the control of left ventricular end-diastole pressure (LVEDP). A mechanical constriction of the descending thoracic aorta allowed to increase the systolic pressure in two steps of 20 mmHg (conditions H-1 and H-2) above control values (condition C). After arterial pressure elevations (systolic pressure C: 119 +/- 8.1; H-1: 142 +/- 7.9; H-2: 1 66 +/- 7.7 mmHg; P<0.01), there were no significant differences in hea rt rate (C: 125 +/- 13.9; H-1: 125 +/- 13.5; H-2: 123 +/- 14.1 bpm; P> 0.05) or LVEDP (C: 6.2 +/- 2.48; H-1: 6.3 +/- 2.43; H-2: 6.1 +/- 2.51 mmHg; P>0.05). The values of dP/dt did not change after each elevation of arterial pressure (C: 3,068 +/- 1,057; H-1: 3,112 +/- 996; H-2: 3, 086 +/- 980 mmHg/s; P>0.05). In isolated rat papillary muscle, an afte rload corresponding to 50% and 75% of the maximal developed tension di d not alter the values of the maximum rate of tension development (100 %: 78 +/- 13; 75%: 80 +/- 13; 50%: 79 +/- 11 g mm(-2) s(-1), P>0.05). The results show that the rise in afterload per se does not cause chan ges in dP/dt or dT/dt.