Changes in Protein and Electrolyte Concentrations in the Pectoral and Leg Muscles during Avian Development

Abstract

-The development of functional capacity of skeletal muscle proceeds more rapidly in precocial than in altricial birds and more rapidly in leg than in pectoral muscles. To determine whether protein accumulation and electrolyte differentiation proceed in parallel with functional development, we examined changes in protein (lipid-free dry mass, LFDM), K+ concentrations, and Na+ concentrations in pectoral and leg muscles during postnatal development in altricial European Starlings (Sturnus vulgaris) and precocial Japanese Quail (Coturnix c. japonica) and Northern Bobwhites (Colinus virginianus). Proportion of LFDM in pectoral muscle had a similar pattern of increase in the three species during development, whereas the proportion in leg muscle was slightly higher in bobwhites than in the other two species during the first two weeks. Concentrations of K+ increased and Na+ decreased during development. [K+] exceeded [Na+] for the first time at 4 to 8 days for pectoral muscle and 2 to 8 days for leg muscle in starlings, and at about 5 days for pectoral muscle and 1 to 2 days for leg muscle in quail. Electrolyte concentrations approached adult levels at 13 to 16 days in starlings and 8 to 11 days in both quail. These ages preceded fledging in starlings and first flight in quail by 4 to 6 days. In neonates of each species, the proportion of LFDM was 3 to 5% higher in leg muscle than in pectoral muscle; adult levels of LFDM were achieved 3 to 7 days earlier in leg muscle. Differentiation of the two electrolytes (i.e. crossing of [K+] and [Na+] curves) occurred 2 to 4 days earlier in leg muscle than in pectoral muscle, especially for quail. These results indicate that compositional development of muscle parallels functional development of precocity during postnatal growth. Because the production and maintenance of proteins and the differentiation of electrolytes require metabolic energy expenditure, maturity may compete with growth rate for allocation of energy and nutrients. Received 16 September 1996, accepted 13 May 1997. PROTEINS AND ELECTROLYTES are important bases for the contractile and conductive apparatus of skeletal muscle. Because animals exhibit significant functional development during their postnatal growth, concentrations of these chemicals must change with increasing functional capacity. Previous studies have revealed that protein contents and excitability of skeletal muscles increase dramatically during the early growth period in mammals and birds (Moulton 1923, Dickerson 1960, Hazlewood and Nichols 1969). For instance, in the pectoral muscle of cockerels (domestic fowl), fibrillar and sarcoplasmic proteins increase 3E-mail: ichoi@dragon.yonsei.ac.kr 4Present address: Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, Missouri 63121, USA. more than 2-fold during the first month of growth (Dickerson 1960). Excitability of muscle fibers, indicated by resting membrane potential, increases more than 3-fold in the rat gastrocnemius (Hazlewood and Nichols 1969) and is known to be established by differential partitioning of electrolytes (e.g. potassium and sodium) across the cell membrane (Vernadakis and Woodbury 1964, Hazlewood and Nichols 1969, Park et al. 1981, Ward and Wareham 1985). The age at which concentrations of proteins and electrolytes reach mature levels has been called the point of chemical (Moulton 1923). Although previous studies have described ontogenic patterns of the composition of muscle tissue, few data relate variation in the processes of maturation to variation in the rate of development of functional capacity.