Alterations in postmortem degradation of myofibrillar proteins in muscle of lambs fed a beta-adrenergic agonist.

Dietary administration of 4 ppm of the beta-agonist L-644,969 (Merck Sharpe and Dohme Research Laboratories) to finishing lambs induced a decrease (10 to 14%, P less than .05) in extractable calpain I activity in the longissimus muscle (LD) at death (d 0). At 4 d postmortem (d 4), extractable calpain I levels in the LD of both control and treated lambs were reduced (P less than .001) from those present at d 0, but the extractable activity in the LD was reduced to a greater extent in control than in treated lambs. Calpain II activity was increased 42% (P less than .005) in LD of treated lambs; however, no significant differences were observed between d 0 and d 4 calpain II activity within treated or control LD samples (P greater than .1). Calpastatin activity was higher in the LD of treated lambs (74% on d 0, P less than .001 and 430% on d 4, P less than .001) than in the LD of control lambs. Measurable cathepsin B activity was decreased (29% on d 0, P less than .05) and measurable cathepsin H activity was increased (10% on d 0, P less than .05 and 10% on d 4, P less than .05) in the LD of treated lambs compared with controls. On d 2, 4 and 6 postmortem, degradation in myofibrils isolated from the LD was lower for treated than for control lambs. Warner-Bratzler shear values for loin chops from treated lambs were higher on both d 3 (111%) and 6 (108%) postmortem than for chops from control lambs (P less than .001). L-644,969-induced decreases in muscle proteolytic capacity may limit postmortem myofibril degradation and contribute to the reduced tenderness observed. This decreased proteolytic capacity may contribute to increased muscularity of L-644,969-treated lambs.