The purpose of this study was to determine whether sonicated albumin bubbles that were larger than the red
blood cells impair the responses of resistance arterioles to vasoactive drugs during myocardial contrast
echocardiography using direct in vitro approaches. We injected two sizes of bubbles (diameter: 5 μm and 50 μm) at random into the microcirculation of the cremaster muscle of Wistar-Kyoto rats (n=12). We then
dissected the arteriole (mean diameter: 97 μm) and pressurized at 40 mmHg in isolated organ chambers. The
diameters were measured in a no-flow, constant pressure state using video microvessel imaging apparatus.
Endothelium-dependent relaxations to acetylcholine were significantly impaired in the arterioles from rats
treated with 50 μm-bubbles, while endothelium-independent responses to KCl were not influenced by the size
of bubbles. Thus, bubbles larger than the red blood cells but smaller than the vessel diameter could injure the
endothelium, but not the vascular smooth muscle, of resistance arterioles. A light microscopic examination
revealed the damage of endothelial cells in the resistance vessels treated with 50 μm-bubbles. We should pay
much more attention to the size of bubbles, especially when we perform the myocardial contrast echocardiography on diseased hearts which are expected to have coronary arteries with already damaged endothelium.