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J Appl Physiol 102: 2083, 2007; doi:10.1152/japplphysiol.00188.2007
8750-7587/07 $8.00
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LETTER TO THE EDITOR

Reply to Tsai, Cabrales, Johnson, and Intaglietta

Reply: Tsai and coworkers (1) have the right to reinterpret our published data (3), but they should take responsibility for the new interpretations and acknowledge that they are inconsistent with the limits on the data expressed in the original paper. On p. 1649, we state: "According to Eq. 1, intensities (amplitudes) of phosphorescent signals decrease with increasing PO2 values ... histograms are very reliable at lower PO2 values where there is little broadening ... above ~ 80 Torr ... they should be used only for qualitatively comparisons."

Histogram measurement is based on numerical inversions of convoluted phosphorescence signals containing responses from all of the differently oxygenated regions (2). Phosphorescence is quenched by oxygen (Eq. 1 in the paper), and regions with higher PO2 values contribute much less to the overall signal than do regions with lower PO2 values, even when amounts of the Oxyphor in these regions are the same. Weaker signals cause greater uncertainty in determination of underlying distributions, and this is inherent to any kind of distribution or image analysis. Therefore, histograms at higher PO2, (shorter lifetimes, weaker phosphorescence) are "blurred" (broadened) compared with histograms at lower PO2 values (longer lifetimes, stronger phosphorescence). It is the blurring effect that makes regions of excessively high PO2 appear in the histograms, and this should not be misinterpreted. On p. 1654, right column, we note that "With further technical development, it should become possible to quantify the volume mismatch between the blood and the interstitial space in the high-oxygen region," making it clear that quantification of the presented data was not possible.

Oxyphor G3 was specifically developed as an oxygen sensor not affected by biomolecules, such as albumin, and is therefore suited for use in the interstitial space. Our interpretations are conservative and appropriate. Experimental demonstration that in the lower PO2 region the intravascular and interstitial spaces are not significantly different suggests that the walls of the capillaries are not an important barrier to oxygen diffusion. Tsai and coworkers reported large PO2 differences (>20 Torr) between the interstitial space and the blood of arterioles, not capillaries. They have, however, proposed a model for tissue oxygen delivery in which arterioles have a primary role. This model (and the underlying data) is inconsistent with a large body of data on oxygen delivery to tissue, but this cannot be presented here due to space limitations. In addition, Tsai and coworkers have not provided convincing estimates of oxygen consumed by the measurements or accurate calibrations for Oxyphor R0 in the interstitial space. If the oxygen consumed by their measurements were underestimated, as appears likely, this could account for much of the reported oxygen pressure difference across the vessel wall.

We look forward to resolution/reconciliation of the differences. A firm knowledge of the oxygen pressure difference from the blood to the cell is of critical importance to understanding both oxygen delivery to tissue and the oxygen dependence of tissue metabolism.

REFERENCES

  1. Tsai AG, Cabrales P, Johnson PC, Intaglietta M. New phosphorescence quenching oxygen measurements technique yields unusual tissue and plasma PO2 distributions. J Appl Physiol. 102: 2081–2082, 2007.[Free Full Text]
  2. Vinogradov SA, Fernandez-Seara MA, Dugan BW, Wilson, DF. Frequency domain instrument for measuring phosphorescence lifetime distributions in heterogeneous samples. Rev Sci Instrum 72: 3396–3406, 2001.[CrossRef][ISI]
  3. Wilson DF, Lee WM, Makonnen S, Finikova O, Aprelva S, Vinogradov SA. Oxygen pressures in the interstitial space and their relationship to those in the blood plasma in resting skeletal muscle. J Appl Physiol 101: 1648–1656, 2006.[Abstract/Free Full Text]

David F. Wilson1
William M. F. Lee2
Sosina Makonnen2
Olga Finikova1
Sofia Apreleva1
Sergei A. Vinogradov1
 Departments of 1Biochemistry and Biophysics and 2Medicine, University of Pennsylvania, Philadelphia, Pennsylvania





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