Mechanical Properties of Coronary Arterioles under Pulsation

Summary

Recently it has become clear that the deep myocardial vessels are pulsating due to cardiac contraction. Although it is known that the coronary arterioles show myogenic responses and play a role in the regulation of blood flow, the influence of pulsation on the mechanical propertes of the arterioles is not clear. We evaluated the mechanical properties of the coronary arterioles under pulsation with basic physiological frequency and different basic tone. Isolated porcine coronary arterioles (I.D. = 100–150 μm) were cannulated with two micropipettes. The luminal cross-sectional area (CSA) was measured using a fluorescence technique under cyclically (1 Hz) changing transmural pressure. After vascular tone was induced (active) by acetylcholine or abolished (passive) by bradykinin, the CSA was measured while the pressure amplitude was changed at a fixed mean level (60 mmHg). At all amplitudes tested, the isolated coronary arterioles in the active condition showed smaller compliance compared to those in the passive condition. The compliance was larger with greater amplitude of the pulsatile transmural pressure than with smaller amplitude of the pulsatile transmural pressure for both the active and passive coronary arterioles. Therefore, the physiological active arterioles pulsating with larger transmural pressure may have benefits in accommodating and discharging blood, and thus may contribute to the compliant blood flow. Raising the amplitude caused the normalized mean CSA in a steady state to increase under active conditions, and to decrease under passive conditions. The vasodilating effects of the pulsation may compensate for the extra compressing effects with increasing cardiac contraction.