Preload does not influence nonmechanical O2 consumption in isolated rabbit heart

Area of Science:

• Cardiology
• Physiology
• Biochemistry

Background:

• Myocardial nonmechanical activity, crucial for excitation-contraction coupling, exhibits length dependence in isolated muscle studies.
• Previous whole-heart studies suggested minimal preload effects on nonmechanical energy consumption.
• Unloaded oxygen consumption (VO2) may inaccurately estimate nonmechanical VO2 due to residual cross-bridge cycling.

Purpose of the Study:

• To accurately quantify the influence of left ventricular (LV) diastolic volume on nonmechanical VO2 in the whole heart.
• To investigate the energetic cost of length-dependent activation in the intact heart.

Main Methods:

• Utilized 2,3-butanedione monoxime (BDM) to quantify nonmechanical VO2 in excised rabbit ventricles.
• Measured VO2 and force-time integral under high (VH) and low (VL) LV volumes.
• Estimated nonmechanical VO2 as the VO2-axis intercept of the VO2-force-time integral relationship during BDM infusion.

Main Results:

• Nonmechanical VO2 did not significantly differ between high and low LV volumes (P = 0.702).
• Multiple linear regression analysis confirmed no significant effect of LV diastolic volume on nonmechanical VO2 (P = 0.361).
• Physiological ranges of LV end-diastolic pressure were maintained.

Conclusions:

• Left ventricular diastolic volume does not significantly affect nonmechanical energy consumption in the physiological range in rabbit hearts.
• Length-dependent activation does not appear to have an energetic cost in the whole rabbit heart.
• The findings suggest increased Ca2+ affinity for contractile proteins is the primary mechanism for length-dependent activation.