SKELETAL-MUSCLE GROWTH, ENZYME-ACTIVITIES, AND THE DEVELOPMENT OF THERMOGENESIS - A COMPARISON BETWEEN ALTRICIAL AND PRECOCIAL BIRDS

We examined activities of enzymes indicative of aerobic capacity (citr ate synthase [CS]) and glycolytic capacity (pyruvate kinase [PK]) in p ectoral and leg muscles of chicks of the European starling, northern b obwhite, and Japanese quail. The starling exhibits altricial developme nt; the nestlings are highly dependent on their parents for food and m aintenance of a high body temperature. In contrast, the quail species exhibit precocial development; at hatching, the chicks can feed themse lves and thermoregulate under mild cold stress. In all three species, the proportion of muscle in the body increased with age; in addition, the proportion of pectoral muscle increased relative to that of leg mu scle. Pyruvate kinase activity increased in all muscles, but especiall y in the pectoral muscles of the quail (from approximately 50 to 800 m umol/min . g wet mass [IU/g] through day 21). In contrast, CS increase d rapidly in the pectoral muscle of the starling (from 10 to 150 IU/g between hatching and 16 d) but not in quail. Citrate synthase activity initially was high in the leg muscle of quail (40-70 IU/g) but decrea sed with age; in starling leg muscle, CS activity increased through da y 8 and then remained constant at about 45-50 IU/g. In the bobwhite, l actate dehydrogenase activity, which is associated with glycolytic met abolism, closely paralleled that of PK. Muscle-mass-specific metabolic scope, estimated from peak oxygen consumption under cold stress, was higher in neonates of both quail species than in starling neonates, bu t within 2 wk it increased to a higher level in the starling (30-38 mL /b . g muscle). In general, total metabolic scope per individual varie d in proportion to the total CS activity, not PK activity, of the pect oral and leg muscles, regardless of species and age. However, increase s in the activities of both CS and PK activities in the starling durin g the first week were not accompanied by development of thermogenic ca pacity.