Experimental effects

We have long known about the existence of arterial hyperoxic vasoconstriction, particularly in the areas of the brain44,50, the retina63, the kidneys64 or the skeletal muscles65,66. However hyperoxic vasoconstriction is not uniform, neither in all organs nor along the arterial axes. Accordingly there is a large number of contradictory studies with varying models and experimental protocols.

Hyperoxic vasoconstriction varies: - with the location on the artery axis : large diameter arteries (over 80 ^m) undergo much less vasoconstriction than finer arteries and terminal arterioles67,68. Bertuglia69 using video-microscopes and Strahler's topographical artery classification70 showed that the maximum decrease in diameter affected mainly arterioles of the1st and 2nd order, whereas arterioles of the 3rd and 4th order were little affected. However, it must not be concluded that sensitivity of vascular walls to oxygen varies with diameter, since Sullivan & Johnson71 found no evidence of a variation in constrictive response to oxygen for the various arterial diameters for identical PO2 in the environment. Duling72,73 had made the hypothesis that arteriole vasoconstriction depended on periarteriolar PO2. Granger74 showed that vasoconstriction affected vessels of increasing diameters in accordance with the degree of increase in PO2.

The variation of intensity and location of hyperoxic vasoconstriction was confirmed by Sonny75 who showed that the greatest amount of vasoconstriction occurred between 1 ata 100% O2 and 2 ata 100% O2, but beyond that no significant further changes occurred between 2 ata 100% O2 and 3 ata 100% O2 - which agrees with the results of Dooley & Mehm study22.

- with blood flow : it was observed a long time ago that hyperoxic vasoconstriction affects organs differently. Vasoconstrictive response is stronger in skeletal muscles76 than in mesenteric vessels77. In an arterial segment model it has been shown that an increase in blood flow causes greater vasoconstriction in hyperoxia78. Thus, the vasoconstrictive response to oxygen is flow-dependent.

- with the pre-existing arterial contraction : Hoogerwerf79 et al. have provided evidence of an increase in vasoconstrictive responsiveness in vessels that have previously undergone noradrenergic constriction.

Vasomotion is also affected by hyperoxia: it decreases its frequency and amplitude. Many capillaries seem to be totally closed and only open briefly. Perfusion time decreases and tends towards zero68,80.

All these phenomena are reversible and microcirculation reverts to a normal condition 15 to 20 seconds after return to normal oxygenation conditions.

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