TEMPORAL RESPONSE OF RABBITS TO BETA-ADRENERGIC AGONIST FEEDING - TISSUE WEIGHT, CALPAINS AND CALPASTATIN ACTIVITIES, AND NUCLEIC-ACID AND PROTEIN CONCENTRATIONS

Forty-eight crossbred rabbits were used in three replications of a 2 x 4 factorial arrangement to investigate the short-term responses of ti ssue accretion, calpains and calpastatin activity, and nucleic acid an d protein concentrations to beta-adrenergic agonist (BAA) feeding. Rab bits were fed a 17% CP diet with or without 7 ppm of L(644,969) and sl aughtered after 1, 4, 8, or 16 d of treatment. Empty body dressing per centage and biceps femoris weight (as a percentage of empty body weigh t [EBW]) were significantly higher in the treated rabbits than in the controls after 16 d of treatment. Heart and Liver weights (as a percen tage of EBW) were higher (P < .05) after 1 d and liver weight (as a pe rcentage of EBW) was lower (P < .05) after 16 d in treated vs controls . Except for an elevation of skeletal muscle m-calpain after 16 d, BAA -supplementation did not affect the calpain-calpastatin system. Muscle RNA concentrations and RNA:DNA ratios were higher (P < .05) in treate d rabbits after 1 d and remained higher thereafter. Protein:RNA ratios were lower (P < .01) in treated than in control rabbits after 4 d and remained lower throughout the trial. Muscle DNA content was lower aft er 4 d and higher after 16 d; RNA content was higher after 4, 8, and 1 6 d; and protein content was higher after 16 d in treated vs control r abbits. Liver nucleic acid and protein concentrations were not affecte d,by BAA treatment. Heart RNA:DNA ratios mere higher; (P < .01) after I d and protein:RNA ratios were lower (P < .05) after 4 d in the treat ed rabbits than in the controls. Collectively, these data imply that B AA-induced muscle growth in rabbits occurs through hyperplasia and see ms to be related to elevated protein synthetic capacity.