Cross-training in birds: cold and exercise training produce similar changes in maximal metabolic output, muscle masses and myostatin expression in house sparrows (Passer domesticus)

Abstract

Maximal metabolic outputs for exercise and thermogenesis in birds presumably influence fitness through effects on flight and shivering performance. Because both summit (Msum, maximum thermoregulatory metabolic rate) and maximum (MMR, maximum exercise metabolic rate) metabolic rates are functions of skeletal muscle activity, correlations between these measurements and their mechanistic underpinnings might occur. To examine whether such correlations occur, we measured the effects of experimental cold and exercise training protocols for 3 weeks on body (Mb) and muscle (Mpec) masses, basal metabolic rate (BMR), Msum, MMR, pectoralis mRNA and protein expression for myostatin, and mRNA expression of TLL-1 and TLL-2 (metalloproteinase activators of myostatin) in house sparrows (Passer domesticus). Both training protocols increased Msum, MMR, Mb and Mpec, but BMR increased with cold training and decreased with exercise training. No significant differences occurred for pectoralis myostatin mRNA expression, but cold and exercise increased the expression of TLL-1 and TLL-2. Pectoralis myostatin protein levels were generally reduced for both training groups. These data clearly demonstrate cross-training effects of cold and exercise in birds, and are consistent with a role for myostatin in increasing pectoralis muscle mass and driving organismal increases in metabolic capacities.

Keywords: BMR; Birds; Cold training; Exercise training; MMR; Msum; Myostatin; Pectoralis; Phenotypic flexibility.

Fig. 1.

BMR, M_sum and MMR in cold-trained house sparrows. (A–C) Least-squares linear regressions of log₁₀-transformed metabolic rates against log₁₀ body mass (M_b) for cold-trained and control house sparrows. Slopes did not differ significantly for all regressions. (D–F) Least-squares means (±s.e.) for each metabolic rate for cold-trained and control sparrows. *P<0.05.

Fig. 2.

BMR, M_sum and MMR in exercise-trained house sparrows. (A–C) Least-squares linear regressions of log₁₀-transformed metabolic rates against log₁₀ body mass (M_b) for exercise-trained and control house sparrows. Slopes did not differ significantly for all regressions. (D–F) Least-squares means (±s.e.) for each metabolic rate for exercise-trained and control sparrows. *P<0.05.

Fig. 3.

Mass of pectoralis and heart muscle in cold-trained and exercise-trained house sparrows.  Linear least-squares regressions of log₁₀-transformed pectoralis (M_pec; right side only) and heart (M_heart) masses on log₁₀ body mass (M_b) for cold-trained (A,B) and exercise-trained (E,F) house sparrows. Body mass values for these regressions were M_b minus the mass of the organ in question. Slopes did not differ significantly for all regressions. Bar graphs show least squares means (±s.e.) in trained and control birds for M_pec (C,G) and M_heart (D,H). *P<0.05.

Fig. 4.

Protein and mRNA expression of myostatin, TLL-1 and TLL-2 in cold-trained and exercise-trained house sparrows. Relative mRNA expression from qRT-PCR for Myostatin and its metalloproteinase activators TLL1 and TLL2 in pectoralis for cold- (A) and exercise-trained (B) sparrows relative to control birds. Relative protein levels from western blot for both unprocessed latent and active forms of myostatin (MSTN) for cold- (C) and exercise-trained (D) sparrows relative to control birds. *P<0.05.