Meeting Abstract
In preparation for long-distance migration, birds dramatically modify major aspects of their anatomy, physiology and behavior. Prior to migration, Gambel’s White-crowned Sparrows (WCS) become hyperphagic, accumulate lipid stores, gain weight, and increase fiber size of the pectoralis (flight) but not gastrocnemius (leg) muscles. Mechanisms regulating muscle hypertrophy are unknown, but androgens, that are anabolic, might be involved; however, plasma testosterone (T) and 5-dihydrotestosterone (5α-DHT) levels are basal at this time. We hypothesized that local androgen signaling within muscle stimulates flight muscle hypertrophy prior to migration. As a test, male WCS were sampled for androgen signaling molecules across three stages of migratory preparedness: winter (February), prenuptial molt (March), and spring departure (April). Despite the fact that T levels were low and generally invariant in blood and muscle, T was detectable in all tissues at each stage of migratory preparedness. Interestingly, in both muscles, mRNA expression levels of androgen receptor (AR) and 5α-reductase Type 1 (converts T to 5α-DHT) increased dramatically at departure compared to previous stages. Thus increased synthesis of 5α-DHT as well as an increased sensitivity to active androgen may enable muscle tissue to utilize existing androgenic substrate to activate AR-dependent anabolic properties of muscle in preparation for migration. The magnitude of the changes in both AR and 5α-reductase Type 1 were greater in the pectoralis as compared to the gastrocnemius. These data suggest that hypertrophy of flight muscles are especially susceptible to this enhanced androgenic signaling to promote increased endurance and power for long distance flight.
