The effect of controlled changes in volume on the active state of the rabbit isolated left ventricle

The contractile properties of isovolumically contracting isolated rabbit left ventricles are studied under the influence of controlled rapid volume changes during systole and diastole. The time integral of the pressure curve (TTI), representing the active state, is used to quantify the energy consumption of the ventricle. Steady state conditions resulting from an introduced volume change show a TTI/EDV relation which represents the Starling curve. However, immediately after a quick volume increase (decrease) introduced in diastole, the TTI/EDV ratio has a higher (lower) value than indicated by the Starling relation. This shows a volume dependent activation (deactivation), related to changes in the inotropic state of the heart muscle cells within the ventricular wall.