A Comparison of 2H2O and Phenylalanine Flooding Dose to Investigate Muscle Protein Synthesis with Acute Exercise in Rats

The primary objective of this investigation was to determine whether (2)H(2)O and phenylalanine (Phe) flooding dose methods yield comparable fractional rates of protein synthesis (FSR) in skeletal muscle following a single bout of high-intensity resistance exercise (RE). Sprague-Dawley rats were assigned by body mass to either 4-h control (CON 4 h; n = 6), 4-h resistance exercise (RE 4 h; n = 6), 24-h control (CON 24 h; n = 6), or 24-h resistance exercise (RE 24 h; n = 6). The RE groups were operantly conditioned to engage in a single bout of high-intensity, "squat-like" RE. All rats were given an intraperitoneal injection of 99.9% (2)H(2)O and provided 4.0% (2)H(2)O drinking water for either 24 (n = 12) or 4 h (n = 12) prior to receiving a flooding dose of l-[2,3,4,5,6-(3)H]Phe 16 h post-RE. Neither method detected an effect of RE on FSR in the mixed gastrocnemius, plantaris, or soleus muscle. Aside from the qualitative similarities between methods, the 4-h (2)H(2)O FSR measurements, when expressed in percent per hour, were quantitatively greater than the 24-h (2)H(2)O and Phe flooding in all muscles (P < 0.001), and the 24-h (2)H(2)O was greater than the Phe flooding dose in the mixed gastrocnemius and plantaris (P < 0.05). In contrast, the actual percentage of newly synthesized protein was significantly higher in the 24- vs. 4-h (2)H(2)O and Phe flooding dose groups (P < 0.001). These results suggest that the methodologies provide "qualitatively" similar results when a perturbation such as RE is studied. However, due to potential quantitative differences between methods, the experimental question should determine what approach should be used.