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Erythrocyte-associated transients in PO₂ revealed in capillaries of rat mesentery

Department of Physiology, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, Virginia

Submitted 16 July 2004 ; accepted in final form 27 January 2005

Mathematical models have predicted the existence of PO₂ gradients between erythrocytes in capillaries in the usual case where plasma contributes substantial resistance to oxygen diffusion. According to theoretical predictions, these gradients could be detected as rapid PO₂ fluctuations (erythrocyte-associated transients, EATs) along the capillary. However, verification of a model and correct choice of its parameters can be made only on the basis of direct experimental measurements. We used phosphorescence quenching microscopy to measure PO₂ in 52 capillaries of rat mesentery to obtain plasma PO₂ values 100 times/s at a given point along a capillary. A 532-nm laser generated 10-µs pulses of light, concentrated by a x100 objective, onto a spot 0.9 µm in diameter. The presence of erythrocytes in the excitation region was detected on the basis of phosphorescence amplitude (PA), proportional to the amount of plasma encountered by the laser beam, and on the basis of the intensity of transmitted laser light (LT), detected by a photodiode placed under the capillary. The data revealed correlated waveforms in PA, LT, and PO₂ in capillaries. The magnitude of the PO₂ gradients between erythrocytes and plasma was correlated with average capillary PO₂. EATs in PO₂ were more readily detected in capillaries with relatively low oxygenation. The correlation coefficients between PA and PO₂ for the half of the capillaries (n = 26) below the median PO₂ (mean PO₂ = 17 mmHg; R = –0.72) was higher than that for the other half (mean PO₂ = 39 mmHg; R = –0.38). These results support the theoretical predictions of EATs and plasma PO₂ gradients in capillaries.

microcirculation; red blood cells; oxygen transport; oxygen tension gradients; phosphorescence quenching microscopy

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