Microvascular origin of laser-Doppler flux signal from the surface of normal and injured liver of the rat

Although laser-Doppler flowmetry (LDF) is a widely used technique, little is known about its microvascular origin from the surface of the liver. In the present study, we attempted to identify the microvascular bed interrogated by the laser light, by correlating perfusion and red blood cell velocity data obtained using intravital fluorescence microscopy (IVFM) with the LDF signal recorded under normal conditions and following liver injury. Sprague-Dawley rats which underwent hepatic artery ligation (HAL) or nonarterialized liver transplantation (NOLT) served as in vivo models of reduced sinusoidal red blood cell velocity (RBC(vel)) and altered homogeneity of perfusion, respectively. Three groups of animals were studied: HAL (n = 8), NOLT (n = 8), and sham-operated control (n = 8). Homogeneous perfusion of the sinusoids and liver lobules and reduced sinusoidal RBC(vel) were initially confirmed in the HAL group. In addition, significantly reduced sinusoidal and lobular perfusion was observed in the NOLT liver. HAL caused a 28% reduction in the LDF signal (P < 0.05 vs sham), whereas the LDF signal from the NOLT liver (101 +/- 11 PU) was lower than that from the native recipient liver with HAL (128 +/- 11 PU). Under control conditions, and following HAL and NOLT, a linear correlation could be established only between sinusoidal RBC(vel) and LDF signal (y = 0.78x + 50.98, r2 = 0.71, P < 0.001). Furthermore, using IVFM images and NIH Image analysis, we estimated a high contribution of the sinusoids to the total vascular space at the liver periphery. Our data indicate that under normal conditions and those of altered microcirculation, blood flow in the sinusoids, compared to other microvessels, has a significant influence on the LDF signal from the surface of the liver.