Aminopeptidase-N modulation assists lean mass anabolism during refuelling in the white-throated sparrow

Abstract

Songbirds meet the extreme metabolic demands of migration by burning both stored fat and protein. However, catabolizing these endogenous tissues for energy leads to organ atrophy, and reductions in gastrointestinal tissue can be as great as 50% of the pre-flight mass. Remarkably, during stopover refuelling birds quickly regain digestive mass and performance. Aminopeptidase-N (APN) is a brush-border enzyme responsible for late-stage protein digestion and may critically assist tissue reconstruction during the stopover, thus compensating for reduced gut size. We hypothesized that birds recovering from a fast would differentially upregulate APN activity relative to disaccharidases to rapidly process and assimilate dietary protein into lean mass. We fasted 23 wild-caught migratory white-throated sparrows (Zonotrichia albicollis) for 48 h to mimic mass reductions experienced during migratory flight and measured intestinal APN activity before the fast, immediately after the fast, and during recovery at 24 h and 48 h post-fast. Total fat mass, lean mass and basal metabolic rate were measured daily. We show that fasted birds maintain APN activity through the fast, despite a 30% reduction in intestine mass, but during refuelling, APN activity increases nearly twofold over pre-fasted individuals. This suggests that dynamically regulating APN may be necessary for rapid protein reconstruction during the stopover.

Keywords: body condition; migratory refuelling; stopover ecology.

Figure 1.

Four experimental groups underwent distinct fasting and recovery treatments concurrently. Light-fill indicates the fasting period for three groups (recovery 48 h n = 5, recovery 24 h n = 5 and fasted n = 5) which began at 07.00 on day ‘0'. Dark fill indicates the time period which birds had ad libitum access to food and water during a refuelling period; groups recovery 48 h and 24 h were given access to food at 07.00 on day 2. The control group (n = 4) had ad libitum access during the entire duration of the experiment. Dashed lines indicate the daily photo period split with lights on at 07.00 and dark at 19.00 and groups entered respirometry at dark on their scheduled days. Tissue collection began within 30 min of birds exiting respirometry the following morning (red dashed line). (Online version in colour.)

Figure 2.

Experimental mass changes. Individuals were measured for total, fat and lean mass prior to and after an induced fast. The three groups which underwent the fast all had significantly greater reductions in total mass by the end of their respective experimental periods. However, only the fasted group (which was sampled without the opportunity to refuel after the fast) had significantly greater lean mass reductions compared to the control group. Recovery 24 h and 48 h groups continued catabolizing fat through recovery, but did not have significantly less lean mass reduction compared to the control, indicating that protein was replenished in as little as 24 h after the fast.

Figure 3.

Intestinal mass and length changes. The fasted group had significantly smaller length and mass compared to the control, a result of 48 h of fasting. After 24 h of recovery, both the recovery 24 and recovery 48 h groups regained intestinal length matching that of the control (a). After 48 h of recovery, the recovery 48 h group had on average 10% more small intestine mass compared to the control group (b).

Figure 4.

Activity rates of APN in the distal and proximal sections of the small intestine at time of dissection. There were no differences in activity between the ‘control' and ‘fasted' groups at the time of sampling. However, after the fasted birds were allowed to refuel, individuals in the ‘recovery 24' and ‘recovery 48' groups increased their APN activity. The ‘recovery 48 h’ group had significantly higher APN activity rates than the ‘fasted' group and near-significant increase over the ‘control' group.

Figure 5.

Whole- animal basal metabolic rates of experimental groups. Fasting had no effect on metabolic rate despite significant lean mass reductions resulting from experimental manipulation.