Expression of glycogen phosphorylase isozymes in developing rat lung

Glycogen accumulates to significant levels in epithelial cells of the developing respiratory tract. Mobilization of glycogen stores is regulated differentially along the respiratory epithelium such that glycogenolysis in the alveolar epithelium (the site of surfactant synthesis) precedes that in the bronchial and bronchiolar epithelium. The initial step in glycogen degradation is catalyzed by glycogen phosphorylase, which exists as three genetically distinct isozymes referred to as muscle, liver, and brain isoforms. The goal of this study was to characterize the temporal and spatial expression of each of the glycogen phosphorylase isozymes in developing lung to determine which isoform(s) was associated with glycogen mobilization in the fetal type II epithelial cell. RNA levels encoding glycogen phosphorylase were assessed by ribonuclease protection assay using isoform-specific antisense probes. RNAs encoding the brain and liver isozymes were detected in isolated day 20 fetal type II epithelial cells and at lower levels in adult type II cells. The muscle isoform RNA was barely detectable in fetal type II cells and was undetectable in adult type II cells. Expression of brain and liver isoform RNAs was higher in whole fetal lung than in fetal type II cells. Consistent with this result, in situ hybridization studies demonstrated widespread expression of the brain and liver isoforms in developing lung tissues; in contrast, expression of the muscle isoform was restricted to the pulmonary vein. Glycogen phosphorylase enzyme activity corresponding to the brain isoform was clearly detected in isolated fetal type II cells; however, the majority of enzyme activity migrated as two bands with distinct electrophoretic mobilities that may have been the result of isoform heterodimerization. Collectively, these results suggest that the brain and liver isoforms of glycogen phosphorylase may be involved in mobilization of type II cell glycogen during late fetal lung development.