Materials and Methods
Animals
Forty-seven purebred Angus bulls were born in the fall of 1996 at the EORDC. Bulls were divergently selected in high and low lines for blood serum IGF-I concentration. Calves were weaned at an average age of approximately 140 days, and were fed a corn and soybean meal diet, which was formulated to meet NRC requirements for growing beef cattle. After a 140-day postweaning performance test, bulls were slaughtered at a commercial packing facility in Columbus, OH at 13 to 15 months of age. Longissimus muscles, which were separated between the 12th and 13th ribs, were removed from the carcasses at 2 days postmortem.
Sample Preparation
Steaks were cut approximately 5 cm thick from the longissimus muscle. After removing all external fat and peripheral connective tissue, the samples were prepared for measurement of activity of CAC, WBS, pH, and myofibril fragmentation index (MFI).
Warner-Bratzler Shear Force
The method of WBS was described by Koohmaraie (1994) and was used to measure meat tenderness. Shear force was reported in kilograms and was scored from 1 to 10 (lower score indicates more tender meat).
Myofibril Fragmentation Index
Myofibril isolation was performed using a modification of the method described by Uytterhaegen et al. (1994); 5 grams of muscle was used. The MFI was calculated after measuring absorbence at 590 nm using 200 x absorbency.
pH
pH was measured after homogenizing the meat samples with homogenizing buffer (50 mM Tris and 10 mM EDTA, pH 7.0).
Calpastatin Activity
The assay of calpastatin activity was described by Koohmaraie et al. (1994); 5 grams of muscle tissue were extracted for this analysis. The Angstrom 278 was determined, and calpastatin activity was reported as units of activity per gram of tissue.
Carcass Traits
The following carcass measurements were obtained: Backfat thickness (FAT) between the 12th and 13th rib; kidney, pelvic, and heart fat percentage (KPH); marbling score (MAR) (1= devoid, 2 = practically devoid, 3 = traces, 4 = slight, 5 = small, 6 = modest, 7 = moderate, 8 = slightly abundant, 9 = moderately abundant, and 10= abundant); loineye area (LEA); hot carcass weight (HCW); and quality grade (QUL) (6 = standard-, 7 = standard, 8 = standard+, 9 = select-, 10 = select, 11= select+, 12 = choice-, 13 = choice, and 14 = choice+).
PCR Conditions
The PCR conditions were 94OC / 1 min, 63OC / 1 min, 72OC / 2 min for CAST28, and 94 OC / 1 min, 60 OC / 1 min, 72 OC / 1.5 min for CAST67 using a themocycler (MJ Research, Inc, PTC-200, Watertown, MA).
Statistical Analysis
Least squares means and standard errors were determined for all measurements with a model that included fixed effects of calpastatin genotypes, age of dam, line, and age of bull as a covariate. Analysis of variance was conducted using statistical analysis system (SAS) general linear models (GLM) procedures, and least squares means were compared using Fisher's least significant difference test (SAS, 1985) with a comparison error rate of 0.05.