Differential stimulation of myofibrillar and sarcoplasmic protein synthesis with protein ingestion at rest and after resistance exercise

Abstract

We aimed to determine whether there is a differential stimulation of the contractile myofibrillar and the cellular sarcoplasmic proteins after ingestion of protein and how this is affected by resistance exercise. Fasted (FAST) muscle protein synthesis was measured in seven healthy young men with a primed constant infusion of L-[ring-(13)C(6)]phenylalanine. Participants then performed an intense bout of unilateral resistance exercise followed by the consumption of 25 g of whey protein to maximally stimulate protein synthesis. In the rested (FED) leg myofibrillar (MYO) protein synthesis was elevated (P < 0.01) above FAST at 3 h (approximately 163%) but not at 1 and 5 h (P > 0.05). In contrast, MYO protein synthesis in the exercised (FED-EX) leg was stimulated above FAST at 1, 3 and 5 h (approximately 100, 216, and 229%, respectively; P < 0.01) with the increase at 5 h being greater than FED (P < 0.01). Thus, the synthesis of muscle contractile proteins is stimulated by both feeding and resistance exercise early (1 h) but has a greater duration and amplitude after resistance exercise. Sarcoplasmic (SARC) protein synthesis was similarly elevated (P < 0.01) above FAST by approximately 104% at 3 h in both FED and FED-EX suggesting SARC protein synthesis is stimulated by feeding but that this response is not augmented by resistance exercise. In conclusion, myofibrillar and sarcoplasmic protein synthesis are similarly, but transiently, stimulated with protein feeding. In contrast, resistance exercise rapidly stimulates and sustains the synthesis of only the myofibrillar protein fraction after protein ingestion. These data highlight the importance of measuring the synthetic response of specific muscle protein fractions when examining the effects of exercise and nutrition.

Figure 1. Schematic diagram of the experimental protocol

Single muscle biopsies (1×) were taken from alternate thighs to characterize fasted protein synthesis after which a unilateral bout of resistance exercise (EX) was performed. After a drink containing 25 g of whey protein (Protein) was ingested at 0 h, bilateral muscle biopsies (2×) were taken from the rested (FED) and exercised (FED-EX) legs.

Figure 2. Blood essential amino acid (EAA; top) and leucine (bottom) concentration after ingestion of 25 g of protein (Drink)

Values are means ±s.e.m. (n= 7). EAA are sum of His, Ile, Leu, Lys, Met, Phe, Thr, Val (note: Cys not measured). Means with different letters are significantly different from each other, P < 0.05.

Figure 3. Myofibrillar (top) and sarcoplasmic (bottom) protein synthesis (FSR) in the fasted state (Fast) and after protein ingestion at rest (FED) and after resistance exercise (FED-EX)

Values are means ±s.e.m. (n= 7). *Significantly different from Fast, P < 0.01. †Significantly different from FED at same time point, P < 0.01. Times with different letters are significantly different from each other within each condition, P < 0.01.