Dependence of peak dP/dt and mean ejection rate on load and effect of inotropic agents on the relationship between peak dP/dt and left ventricular developed pressure--assessed in the isolated working rat heart and cardiac muscles.

To examine the effects of preload (mean left atrial pressure) and afterload on two so-called "contractility indices" and the effects of inotropic agents (isoproterenol and calcium) on the relationship between left ventricular developed pressure and peak dP/dt when afterload was increased, we used a modified working rat heart preparation (perfused with Krebs-Henseleit solution bubbled with a 95% O2 -5% CO2 gas mixture at 37 degrees C). The atrium was stimulated at a rate of 240/min. An increase in preload from 5 to 15 mmHg caused an increase in peak dP/dt from 1,711 +/- 293 mmHg/s to 1,971 +/- 387 mmHg/s (p less than 0.001, n = 15), and an increase in the mean systolic ejection rate from 1.28 +/- 0.31 ml/s to 2.74 +/- 0.80 ml/s (p less than 0.001). An increase in afterload (left ventricular developed pressure) from 66 to 97 mmHg produced by elevating aortic pressure caused an increase in peak dP/dt from 1,829 +/- 222 mmHg/s to 2,449 +/- 254 mmHg/s (p less than 0.001, n = 7), and a decrease in the mean systolic ejection rate from 2.12 +/- 0.36 ml/s to 1.95 +/- 0.33 ml/s (p less than 0.001). Studies using isolated rat papillary muscles ruled out the contribution of an increase in myocardial perfusion to the increase in peak dP/dt, since the maximum rate of rise in tension (dT/dt) increased with an increase in afterload during afterloaded isotonic contraction. Peak dP/dt showed a linear relationship to left ventricular developed pressure when the latter was increased by elevating the aortic reservoir and then clamping the aortic outflow tube (n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)