Decreased heterogeneity of capillary plasma flow in the rat whisker-barrel cortex during functional hyperemia

The pattern of capillary plasma perfusion was investigated in the rat brain during functional activation. Functional hyperemia was induced in the left whisker-barrel cortex by deflection of the right mystacial vibrissae for 2 min at frequencies of 1-7 Hz. Rats were decapitated under anesthesia 3-4 s after i.v. bolus injection of Evans blue dye. The steep increase of the arterial dye concentration ensures that divergent capillary plasma transit times result in unequal intracapillary dye concentrations. Plasma perfusion heterogeneity was determined from the coefficient of variation (CV) of Evans blue concentrations measured in numerous single capillaries of the whisker-barrel cortex. Functional hyperemia was quantified from measurements of CBF using the [14C]-iodoantipyrine technique in a second experimental group. CBF in the left whisker-barrel cortex increased with the stimulation frequency and was maximal at 5 Hz compared to the right side. Conversely, plasma perfusion heterogeneity decreased with stimulation frequency in a reciprocal way, being minimal at 5 Hz. Results indicate a decrease in the microcirculatory flow heterogeneity during functional hyperemia in the brain.