Intravital Microreflectometry of Individual Pial Vessels and Capillary Region of Rat

A microscopic reflectance spectrophotometer was constructed to obtain the spectra of single pial vessels and of a region containing only capillaries (capillary region). The difference in the oxygen saturation (So 2 ) of hemoglobin between the regional arteriole and venule [R(A – V)] and that between the regional arteriole or capillaries [R(A – C)] were calculated. The reduction of cytochrome aa 3 was also estimated in the capillary region. This method was applied to the brain surface of spontaneously breathing rats subjected to hypoxic and anemic hypoxia. On decreasing the inhaled O 2 from 100 to 15%, elevation of R(A – V) and R(A – C) with slight arteriolar dilatation (though statistically not significant) was observed. Below 10% O 2 (especially at 4 and 3% O 2 ), the R(A – V) and R(A – C) decreased in spite of significant arteriolar dilatation with progressive reduction of cytochrome aa 3 , indicating suppression of oxygen transport to mitochondria. In the case of hemodilution down to 37% hematocrit (Ht), elevation of R(A – V) and R(A – C) occurred with a slight tendency toward arteriolar dilatation. Below 32% Ht, the R(A – V) decreased but the R(A – C) remained steady, while reduction of cytochrome aa 3 progressed. Altogether, the So 2 in the capillary region decreased and the reduction of cytochrome aa 3 progressed with the decline of arteriolar O 2 supply in both hypoxic and anemic hypoxia.