Behavioral stress and skeletal muscle glycogen metabolism in young bulls

Friesian bulls were penned with an established group of bulls for 6 h. This mixing procedure normally resulted in an intense behavioral interaction between the established herd and each of the newly introduced experimental animals. Muscle needle biopsy samples and blood samples were taken before stress and at intervals during the recovery period. Stressed bulls showed increases over controls in body temperature (P less than .001), heart rate (P less than .001), serum nonesterified fatty acids (P less than .05) and plasma creatine kinase (P less than .01). Muscle glycogen content fell during stress to 41% of the value for the control animals (P less than .001). On the first day of the recovery period, the glycogen content increased slightly, to 45% of the mean control value, and on d 2 it increased substantially, to 70%. A significant difference existed between experimental and control groups until d 7 of the recovery period. Blood glucose did not differ significantly between the control and experimental groups immediately after stress or during the recovery period. Muscle glucose-6-phosphate was lower in the experimental animals immediately after stress (P less than .01) and also on d 1 of recovery (P less than .05). Plasma immunoreactive insulin remained close to 40 microunits/ml in both groups, except on the day after stress, when it was below (P less than .05) the prestress value. The results show that substantial muscle glycogen breakdown occurred during the period of behavioral stress and that recovery to resting values was a comparatively slow process. It was concluded that the inherently lower blood glucose concentration and insulin activity in cattle than in nonruminant species may reduce glucose availability in cattle and thereby delay muscle glycogen recovery.