Laser-Doppler measurements of concentration and velocity of moving blood cells in rat cerebral circulation

In brain cortex all capillaries are perfused with plasma at anyone time while the flow of blood cells is heterogenous. Increased blood flow is associated with increased number of moving erythrocytes in the microcirculation, while capillary recruitment in its classical anatomical sense appears not to exist in the brain. Modulation of the concentration of flowing erythrocytes may influence the oxygen supply to the tissue. Therefore, we examined the possibility that laser-Doppler flowmetry (LDF) could be used to quantify changes in the microvascular concentration of moving blood cells (CMBC) and blood cell velocity (< v >) by comparing LDF measurements with electromagnetic flow measurements in vitro, and confocal laser-scanning microscopy in vivo in the brain of anaesthetized male Wistar rats. in vitro measurements showed that CMBC was affected by changes in haematocrit, while < v > correlated almost linearly with blood cell velocity measured electromagnetically within a relevant physiological range. In vivo studies during hypercapnia (Paco(2) from 39 +/- 4 to 66 +/- 5 mmHg) with confocal laser scanning microscopy disclosed a 39 +/- 10% increase of cortical capillary erythrocytes, while CMBC measured with LDF increased by 37 +/- 5%. Erythrocyte flow velocity in brain cortex capillaries increased by 65 +/- 17% with confocal microscopy as compared to 72 +/- 8% with LDF. Local electrical stimulation of cerebellar cortex, and application of adenosine or sodium-nitroprusside, increased CMBC and < v > simultaneously, while during hypercapnia the < v > increase preceded the CMBC increase by 30 s. The CMBC rise rapidly reached a steady stale in response to all types of stimulation, while < v > continued to increase during the major part, or the entire stimulation period. In conclusion, our data support the hypothesis that LDF may be useful for haemodynamic studies of brain microcirculation.