Sildenafil works by inhibiting phosphodiestrase-5, an enzyme which degrades cyclic guanosine monophosphate (cGMP) a cell messenger that causes the blood vessels to relax, Friedlander explained. By inhibiting the enzyme, the drug allows greater vasodilation and greater blood flow. Although the drug works in different target sites, this study focused on the lungs.
The researchers hypothesized that the drug would allow the study's participants to improve their performance at altitude because it would reduce the constriction of vessels in the lungs that sometimes occurs at altitude. In turn, that would allow greater blood flow through the heart, better transfer of oxygen from the lungs to the blood and improved oxygen delivery to working muscles.
The participants, all trained cyclists, performed a total of 10 cycling trials, with and without sildenafil at sea level and at simulated altitude of 3,874 meters. Neither the participants nor the researchers knew whether the trial included a placebo or one of the two sildenafil doses, 50 mg or 100 mg.
The high altitude simulation was achieved by changing the mix of air. The cyclists began breathing the high altitude mix starting one hour before the exercise session and continuing through the session. The simulation did not include the lower air pressure that would occur at altitude, Friedlander said.
Researchers analyzed the changes in each individual's performance under various exercise conditions and also compared the group's performance under different drug conditions.
Responders versus non-responders
Four of the 10 participants responded
Source:American Physiological Society