The postnatal development of mammalian skeletal muscle is characterized by an increased capacity for glycogen metabolism. The hormones, insulin and epinephrine, regulate glycogen synthesis and degradation via the control of cAMP levels in the muscle cell. The receptors and effector enzymes for insulin and epinephrine are located in the muscle cell plasma membrane (sarcolemma). This proposal seeks to determine the development of the insulin and Beta-adrenergic receptors in developing skeletal muscle sarcolemma. It is hypothesized that receptor development and/or coupling of receptor activation to effector enzyme stimulation may serve as a developmental signal for the metabolic pathways the hormone-receptor interaction modulates. Studies completed in the first year of the project indicate (1) skeletal muscle undergoes a 6- to 8-fold increase in glycogen metabolizing enzymes after 6 weeks of postnatal life and (2) isolated sarcolemma also shows a 6- to 8-fold increase in epinephrine stimulated adenylate cyclase activity to parallel increased capacity for glycogen metabolism. Work is now in progress to delineate Beta-adrenergic receptor and insulin receptor alterations during this developmental increase in physiological sensitivity to these hormones.