Effects of muscle type, castration, age, and compensatory growth rate on androgen receptor mRNA expression in bovine skeletal muscle

The effect of testosterone on sexual dimorphism is evident by differential growth of forelimb and neck muscles in bulls and steers. Divergent hormone sensitivites may account for the differential growth rates of individual mu scles. Therefore, the objective of this study was to compare androgen recep tor (AR) expression in three different muscles of bulls and steers at vario us ages and growth rates. Thirty Montbeliard bulls and 30 steers were assig ned to four slaughter age groups. Four or five animals of each sex were sla ughtered at 4 and 8 mo of age. Animals in the remaining two slaughter group s (12 and 16 mo) were divided into groups of either restricted (R) or ad li bitum (AL) access to feed. Five animals of each sex and diet were slaughter ed at the end of the restricted intake period at 12 mo of age. To simulate compensatory growth, the remaining animals (R and AL) were allowed ad libit um access to feed until slaughter at 16 mo of age. Total RNA was extracted from samples of semitendinosus (ST), triceps brachii (TB), and splenius (SP ) muscles. Androgen receptor mRNA was quantified in 200-ng total RNA prepar ations using an internally standardized reverse transcription (RT) PCR assa y. Data were analyzed using 18S ribosomal RNA concentrations as a covariabl e. Steers had higher AR mRNA levels per RNA unit than bulls (P < .01). Andr ogen receptor mRNA levels differed between muscles (P < .05), with lowest e xpression in the SP. The pattern of AR expression differed (P < .05) for ea ch muscle with increasing age. Between 4 and 12 mo of age, AR mRNA levels i ncreased (P < .05) in SP but remained unchanged in the ST and TB. Feeding r egimen had no effect on muscle AR expression, but steers exhibiting compens atory growth had higher AR mRNA levels than AL steers (P < .01) or bulls (P < .01). Our results show that AR expression is muscle-specific and may be modulated by circulating testicular hormones. These data suggest that the r egulation of AR expression may be linked to allometric muscle growth patter ns in cattle and compensatory gain in steers.