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Effect of Propranolol on Muscle Glycogen Metabolism during Social Regrouping of Young Bulls

The Agricultural Institute^,2, Castleknock, Co Dublin, Ireland

Abstract

The objective of the investigation was to determine the role of epinephrine in muscle glycogen breakdown during mixed penning of cattle. Experiments were performed on young Friesian bulls in which the β-adrenergic receptor sites in muscle were blocked with propranolol. In an initial trial, effective blockade was demonstrated by the absence of the glycogenolytic response in muscle to injected epinephrine. In a second trial, propranolol-treated and untreated bulls were stressed by mixing with strangers for 5 h. At intervals during stress and during a 3-d recovery period, needle biopsy samples were taken from M. longissimus for glycogen determination. During mixing stress in untreated bulls, highly significant increases above resting levels were observed in body temperature, heart rate, serum nonesterified fatty acid concentration and plasma creatine kinase activity, and a significant decrease was observed in muscle creatine phosphate concentration. Muscle glycogen content fell to 72% of the resting value after 1 h, 55% after 3 h and 37% after 5 h. During a 3-d recovery period, the glycogen concentration increased to 84% of the value at rest. The results show that glycogen breakdown was most rapid during the early part of the stress period and that recovery of glycogen poststress was a comparatively slow process and was associated with a significant reduction of muscle glucose-6-phosphate concentration. In propranolol-treated animals, the rate of glycogen breakdown during the first hour of mixing was reduced (P>.02). Nevertheless, propranolol was ineffective in preventing muscle glycogen depletion, which fell to 45% of the resting value at the end of the stress period. The results suggest that muscle glycogen depletion during social regrouping of cattle is not mediated predominantly by catecholamines.

¹ The authors wish to thank P. Ward and D. Troy for skilled technical assistance: J. Sherington for statistical analysis of results and T. Kendrick for art work.

² Meat Res. Dept.