Glycine propionyl-L-carnitine increases plasma nitrate/nitrite in resistance trained men

We have recently demonstrated that oral intake of glycine propionyl-L-carnitine (GPLC) increases plasma nitrate/nitrite (NOx), a surrogate measure of nitric oxide production. However, these findings were observed at rest, and in previously sedentary subjects. In the present study, we sought to determine the impact of oral GPLC on plasma NOx at rest and in response to a period of reactive hyperemia in resistance trained men. Using a double blind, crossover design, 15 healthy men (24 ± 4 years) were assigned to GPLC (3 g/d PLC + 1044 mg glycine) and a placebo in random order, for a four-week period, with a two-week washout between condition assignment. Blood samples were taken from subjects at rest and at 0, 3, and 10 minutes following an ischemia-reperfusion protocol (six minutes of upper arm cuff occlusion at 200 mmHg followed by rapid reperfusion with cuff removal). Blood samples were taken from a forearm vein from the same arm used for the protocol and analyzed for total nitrate/nitrite. Data are presented as mean ± SEM. A condition main effect (p = 0.0008) was noted for NOx, with higher values in subjects when using GPLC (45.6 ± 2.8 μmol·L(-1)) compared to placebo (34.9 ± 1.2 μmol·L(-1)). No time main effect was noted (p = 0.7099), although values increased approximately 12% from rest (37.7 ± 2.7 μmol·L(-1)) to a peak at 10 minutes post protocol (42.3 ± 3.3 μmol·L(-1)). The interaction effect was not significant (p = 0.8809), although paired time contrasts revealed higher values for GPLC compared to placebo at 3 (48.2 ± 6.7 vs. 34.9 ± 2.4 μmol·L(-1); p = 0.033) and 10 (48.8 ± 5.9 vs. 35.7 ± 2.1 μmol·L(-1); p = 0.036) minutes post protocol, with non-statistically significant differences noted at rest (41.8 ± 4.5 vs. 33.6 ± 2.5 μmol·L(-1); p = 0.189) and at 0 minutes (43.6 ± 5.1 vs. 35.4 ± 2.7 μmol·L(-1); p = 0.187) post protocol. An analysis by subject (collapsed across time) indicated that 11 of the 15 subjects experienced an increase in NOx with GPLC treatment. These findings indicate that short-term oral GPLC supplementation can increase NOx in resistance trained men. However, as with many dietary supplements, there exist both "responders" and "non-responders" to treatment. Future work may focus on the mechanisms for the discrepancy in response to GPLC supplementation for purposes of NOx elevation.