DIRECT RECORDING OF EDP-EDV RELATIONSHIP IN ISOLATED RAT LEFT-VENTRICLE - EFFECT OF DIASTOLIC CROSSBRIDGE FORMATION

Objective: The aim was to investigate whether the end diastolic pressu re - end diastolic volume (EDP-EDV) relationship of the left ventricle can be influenced by calcium dependent elements, especially at low va lues of end diastolic pressure. Methods: Isolated rat hearts were perf used in a modified Langendorff perfusion system. The EDP-EDV relations hip of the left ventricle was investigated. Pressure was recorded with a microtip pressure catheter and volume with a microconductance cathe ter. Crossbridge cycling was affected by adding calcium antagonists (v erapamil, diltiazem, nifedipine at 2.10(-7) M) or by adding the Mg-ATP ase blocker BDM (2,3-butanedione-2-monoxime, 10(-3) M) to the perfusat e. Results: The above drugs had a negative inotropic effect in systole . At EDP = 0 after stimulation the active isovolumetric pressure was z ero. In diastole, BDM shifted the EDP-EDV relationship to slightly sma ller EDVs. A decrease of about 5% in the EDV was found at lower EDP va lues. Ca2+ antagonists increased the EDV up to 40-80% at low EDP value s. At higher EDP values only a small increase of EDV (about 10%) was f ound after verapamil perfusion. The results obtained are interpreted i n terms of a three step crossbridge model. Conclusions: At low EDP, di astolic volume is dependent upon weakly bound crossbridges as a functi on of the [Ca2+] in the cardiac cell.