In vitro ouabain-sensitive respiration and protein synthesis in ruminal epithelial papillae of Hereford steers fed either alfalfa or bromegrass hay once daily

Abstract

Seven Hereford steers (457 +/- 8.5 kg BW) with ruminal cannulas were fed either long or short chopped bromegrass (B) or alfalfa (A) hay at 90% of a 3-h ad libitum intake once daily between 0900 and 1200 in a 4 x 4 complete and 3 x 4 incomplete Latin square design. After evacuation of ruminal contents, papillae from the ventral sac of the rumen were excised at 0900, 1200, and 2100 and placed in oxygenated media. Oxygen consumption was determined polarographically, after which either 10(-4) M ouabain or 10(-4) M cycloheximide was added for determination of ouabain-sensitive (OSO2) or cycloheximide-sensitive (CSO2) O2 consumption, respectively. Additionally, protein synthesis was measured by uptake of [3H]phenylalanine. Twenty-four-hour patterns of ruminal fluid pH, osmolality, ammonia N, and VFA were also determined. Steers fed A exhibited a rapid rise in total O2, OSO2, and CSO2 immediately after consumption of a meal; the respiration patterns of ruminal epithelial papillae from animals fed B lagged behind those observed for animals fed A. Patterns of O2 consumption for both diets paralleled those observed for ruminal concentration of products of fermentation; those fed A had a larger magnitude of change in both O2 consumption and fermentation products. Ruminal epithelial O2 consumption seems to be determined by substrate availability and products of fermentation, and Na+,K+ATPase and protein synthetic activity each account for one-fifth of ruminal papillae O2 consumption. Fractional rates of protein synthesis were unaffected by type of forage consumed.