GROWTH-RATE AND FUNCTION OF SKELETAL-MUSCLES IN JAPANESE-QUAIL SELECTED FOR 4-WEEK BODY-MASS

We compared growth and physiological function in pectoral and leg musc les of Japanese quail chicks from a line selected for 4-wk body mass ( P line) and an unselected control (C) fine. Growth rates of pectoral a nd, especially leg muscles were higher in the P line than in the contr ol line during the first week posthasching but not thereafter. Citrate synthase (CS) activity in pectoral muscle increased early, in postnat al development but reached much higher levels in control chicks than i n selected chicks; metabolic response to cold stress also was greater in C-line chicks than in P-line chicks. Pyruvate kinase (PK) activity and potassium concentration increased rapidly during the early postnat al development period and did not differ between lines except that the increase in PK in leg muscle was delayed in P-line chicks compared wi th C-line chicks. Exponential growth rates (EGR) of leg and pectoral m uscles exhibited inverse relations to increasing PK activity that were indistinguishable between selected and control lines. Pyruvate kinase activity was normalized to the peak value for each muscle mass within each line to establish an index to functional maturity (M) extending from 0 to 1. The negative relation between EGR and M did not differ si gnificantly between type of muscle or between line. This result is con sistent with the presence of a growth rare-functional maturity trade-o ff that governs the rate of growth of skeletal muscle during postnatal development. Accordingly, the growth response of leg muscles of selec ted chicks may be related to delayed maturation, as indicated by PK ac tivity In addition, although the growth rate of leg muscle in unselect ed chicks during the first week posthatch is less than expected from M , EGR values for leg muscles of selected quail are close to predicted values. Thus, the growth rate of the leg muscles possibly is down regu lated in control fines but released in response to selection for rapid growth to realize more fully the growth potential of the muscle at a given state of maturity.