Cutaneous Respirometry as Novel Technique to Monitor Mitochondrial Function: A Feasibility Study in Healthy Volunteers

Abstract

Background: The protoporphyrin IX-triplet state lifetime technique (PpIX-TSLT) is proposed as a potential clinical non-invasive tool to monitor mitochondrial function. This technique has been evaluated in several animal studies. Mitochondrial respirometry allows measurement in vivo of mitochondrial oxygen tension (mitoPO2) and mitochondrial oxygen consumption (mitoVO2) in skin. This study describes the first use of a clinical prototype in skin of humans.

Methods: The clinical prototype was tested in 30 healthy volunteers. A self-adhesive patch containing 2 mg 5-aminolevulinic acid (ALA) was applied on the skin of the anterior chest wall (sternal) for induction of mitochondrial protoporphyrin IX and was protected from light for 5 h. MitoPO2 was measured by means of oxygen-dependent delayed fluorescence of protoporphyrin IX. MitoVO2 was determined by dynamic mitoPO2 measurements on the primed skin, while locally blocking oxygen supply by applying local pressure with the measurement probe. MitoPO2 was recorded before and during a 60-s period of compression of the microcirculation, at an interval of 1 Hz. Oxygen consumption (i.e. the local oxygen disappearance rate) was calculated from the decay of the mitoPO2 slope.

Results: Oxygen-dependent delayed fluorescence measurements were successfully performed in the skin of 27 volunteers. The average value (± SD) of mitoPO2 was 44 ± 17 mmHg and mean mitoVO2 values were 5.8 ± 2.3 and 6.1 ± 1.6 mmHg s-1 at a skin temperature of 34°C and 40°C, respectively. No major discomfort during measurement and no long-term dermatological abnormalities were reported in a survey performed 1 month after measurements.

Conclusion: These results show that the clinical prototype allows measurement of mitochondrial oxygenation and oxygen consumption in humans. The development of this clinically applicable device offers opportunities for further evaluation of the technique in humans and the start of first clinical studies.

Fig 1. Principle of respirometry by local cessation of the oxygen supply.

Mitochondrial oxygen consumption (mitoVO₂) is calculated from the linear part of the oxygen disappearance curve following local compression of the microcirculation by the measurement probe.

Fig 2

A. Example of a baseline mitoPO₂ measurement. The mean mitochondrial oxygen tension (mitoPO₂) (red line) is calculated over a period of 90 second. B. The mitoPO₂ is presented in a box-and-whisker graph, C. Frequency distribution of all ratio differences of the mitoPO_2.

Fig 3

A. Typical example of the analysis of the mitochondrial oxygen consumption (mitoVO₂). The first panel shows an example of the oxygen disappearance rate after local occlusion of the microcirculation. In the second panel we demonstrate the analysis of the mitochondrial oxygen tension (mitoPO₂), the green line represents the linear fit (ΔPO₂(t)/Δt) of the oxygen disappearance rate. B. The mitoVO₂ are presented in a box-and-whiskers graph. The boxes extend from the 25^th percentile to the 75^th percentile, with a line at the median, the whiskers extend above and below the box to show the highest and lowest values. Presented data are measured with two different probe temperatures (34°C and 40°C, respectively). C. Frequency distribution of all ratio differences of the mitoVO₂ at 34°C and 40°C.

Fig 4

Correlation plot of the initial mitochondrial oxygen tension (mitoPO₂) and the mitochondrial oxygen consumption (mitoVO₂) at 34°C (A) and 40°C (B).