Mechanical determinants of maximum isotonic lengthening rate in rat left ventricular myocardium.

The effects of changing loading conditions and inotropic state on maximum isotonic lengthening rate (+dL/dt, muscle lengths/sec) were examined in isolated rat myocardium. Physiologically sequenced contractions were studied in 18 left ventricular papillary muscle preparations (stimulation rate, 12/min). To study the effects of changing loading conditions, only one loading variable (preload, total load, or late load) was changed during each contraction, while the others were held constant. To study the effects of isoproterenol (10(-6) M) and temperature (28 vs. 33 degrees C) on maximum isotonic lengthening rate, preload and late load were held constant and +dL/dt was examined at a common total load. When preload was increased from 0.7 +/- 0.1 to 1.4 +/- 0.1 g/mm2, muscle length increased from 0.98 +/- 0.003 to 1.01 +/- 0.002 muscle lengths, the extent of shortening increased from 0.05 +/- 0.003 to 0.08 +/- 0.003 muscle lengths, but minimum length (0.93 +/- 0.01 muscle lengths) and +dL/dt (1.1 +/- 0.1 muscle...