Optimal levels of membrane fluidity are essential for numerous cell functions including cell growth, solute transport and signal transduction. Our aim was to investigate the membrane fluidity of human erythrocytes before and immediately after several cycloergometric protocols. Eight healthy males (23.25±0.7 years and maximal oxygen uptake, VO₂max: 49.73±2.60 mL/Kg/min) were enrolled in this study. All participants were informed on the potential risks and signed an informed consent statement. Subjects performed 3 cycloergometric tests: 1) For the calculation of VO₂max during a continuous progressive exercise test; 2) a strenuous test until exhaustion, 3) an ergometry at an intensity corresponding to 70% of VO₂max for 30 minutes. Blood samples were collected from the antecubital vein just before first ergometry (A) and immediately after the 3 ergometric tests (B, C and D respectively). Membranes from erythrocytes were isolated using hypotonic hemolysis and a centrifugation method. Membrane fluidity was monitored by fluorescence spectroscopy in a Perkin-Elmer LS-55 Luminiscence Spectrometer using TMA-DPH as a probe.

Results of membrane fluidity were expressed as the inverse of polarization. Membrane fluidity levels (A: 3.46±0.07; B: 3.29±0.08; C: 3.22±0.07; D: 3.18±0.08) decreased immediately after exercise (p<0.05). Physical exercise is characterized by an increase in oxygen consumption by the whole body. This increase is related to a rise in the production of free radicals. Since it is well-known that free radicals induce rigidity in biological membranes, overproduction of oxidative stress may be responsible, at least partially, for the rigidity in the membrane of the erythrocytes due to acute exercise.