In situ NADH laser fluorimetry during muscle contraction in humans

Summary

The aim of the present study was to use nicotinamide adenine dinucleotide phosphate, reduced (NADH) fluorimetry, to investigate in situ NADH changes during muscle contraction in humans on an isokinetic dynamometer. Thirteen healthy male subjects each performed one maximal voluntary contraction (MVC) with the knee extensor muscle. The NADH muscle fluorescence was monitored by a double beam laser fluorimeter which uses an optical fibre, percutaneously inserted through a needle into the vastus lateral muscle, to guide the light. The NADH fluorescence was continuously measured at a wavelength of 337 nm. To estimate the haemodynamic artefact, blood backscattering was simultaneously determined at a wavelength of 586 nm. The fluorescence signal was recorded before, during and after contractions at 50% of MVC. The fibre was kept out of contact with the muscle during contractions at 100% of MVC and was only put into contact with it at the end of the contraction. At the onset of contractions at 50% of MVC, NADH fluorescence increased rapidly for 3 s and remained stable thereafter until exhaustion. After a muscle measurement had been made, the optical fibre was put successively into solutions of increasing NADH concentration to ascertain the relationship between the muscle fluorescence signal and the muscle NADH level. This procedure yielded estimated mean values for muscle NADH of 0.172 mmol · kg⁻¹, SEM 0.028 and of 0.184 mmol·kg⁻¹ SEM 0.027 after contractions at 50% and 100% of MVC, respectively, from a resting value of 0.087 mmol·kg⁻¹, SEM 0.015. These results indicated that in situ laser fluorimetry could be used to evaluate NADH changes in humans during muscle contraction. The early increase in NADH at the onset of muscle contraction suggested that metabolic factors other than local hypoxia were involved in the NADH increase during contraction. The comparison between contractions at 50% and 100% of MVC suggested that the NADH concentration was increased maximally within the first seconds of a submaximal muscle contraction.