Response to Microdialysis of Prostaglandins, Thromboxane, and Other Eicosanoids: Recall Past Knowledge
We thank Tsikas et al1 for the interest in our findings. An underlying concern of Tsikas et al1 is our use of an immunoassay for the determination of prostaglandins,2 a method that most studies on prostaglandins in the literature have used. Their point is that our prostaglandin values in muscle microdialysate are too high. Our measured levels are very similar to those observed by others in interstitial fluid from skeletal muscle,3 as well as from peritendinous tissue.4 The prostaglandin concentrations in these studies have been measured by radioimmunoassay, different than the immunoassay used by our group, yet resulting in similar values. The observation that our concentrations are higher than those observed by others in plasma is not surprising; we often observe large differences in concentrations of compounds between plasma and interstitial fluid. It may be true that, for methodologic/technical reasons, urine is a better fluid to measure prostaglandins in; however, the origin of prostaglandins determined in urine cannot be known, thus, as we are specifically interested in what occurs in skeletal muscle, it is very important for us to measure directly in this tissue.
Tsikas et al1 provide data from 1 obese hypertensive subject showing that the prostaglandin levels in the muscle microdialysate are several-fold lower than reported by us and others. From the description of the measurements, it appears that the reported values do not include calculations of relative loss, which indicates the recovery of a compound across the microdialysis membrane. Thus, the actual interstitial concentration may be many-fold higher than the dialysate concentrations reported in their letter. We have found that several kinds of commercial microdialysis probes, despite adequate cutoff, have very poor in vivo recoveries, as assessed by calculation of relative loss by inclusion of a radioactive tracer in the perfusate. For this reason we prefer to use our laboratory-made probes with much better recovery. In addition, the data of Tsikas et al1 originate from only 1 probe in 1 subject, and from long-term experience with microdialysis we know that there can be large individual variations and that positioning of the probe can be an important issue, in particular, in obese subjects. Also, we have found that prostaglandin levels can be reduced by sample handling, for example, freeze/thaw cycles.
Finally, we want to emphasize the important point, also brought up by Tsikas et al,1 that our finding that exercise training shifts the thromboxane/prostaglandin ratio toward dilation in the skeletal muscle interstitium of individuals with essential hypertension agrees well with the results of a previous study in which prostaglandins were measured after exercise in urine by gas chromatography-mass spectrometry.5 Thus, even if there are some differences in concentrations with the 2 methods used, the different methods and sites of measurements complement each other well. Tsikas et al1 point out that the thromboxane and prostacyclin synthase protein amounts do not match the prostaglandin alterations. However, clearly enzymatic reactions depend on many factors, such as the degree of activation and substrate availability, thus the discrepancy could easily be explained by such alterations.
Department of Exercise and Sport Sciences
University of Copenhagen
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- © 2012 American Heart Association, Inc.
- Tsikas D,
- Zoerner AA,
- Haufe S,
- Engeli S,
- Stichtenoth DO,
- Jordan J
- Hansen AH,
- Nyberg M,
- Bangsbo J,
- Saltin B,
- Hellsten Y