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1 Department of Physiology, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, Virginia, USA
* To whom correspondence should be addressed. E-mail: pittman{at}hsc.vcu.edu.
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 the correct choice of its parameters can be made only on the basis of direct experimental measurements. We employed phosphorescence quenching microscopy to measure PO2 in 52 capillaries of rat mesentery to obtain plasma PO2 values 100 times per second at a given point along a capillary. A 532 nm laser generated 10 µs pulses of light, concentrated by a 100X 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 the average capillary PO2. Erythrocyte-associated transients 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.
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