The testis isoform of the phosphorylase kinase catalytic subunit (PhK-gammaT) plays a critical role in regulation of glycogen mobilization in developing lung.

In order to identify the form of phosphorylase kinase catalytic subunit expressed in developing lung, degenerate polymerase chain reaction primers were designed based on conserved domains of the two known catalytic subunits, expressed primarily in muscle and testis. Amplification of cDNA from day 19 fetal rat lung followed by cloning and sequence analyses indicated that only the testis isoform of phosphorylase kinase (PhK-gammaT) was detectable in fetal lung. In situ hybridization analyses indicated that expression of PhK-gammaT RNA in developing lung tissue was widespread and not restricted to Type II epithelial cells; PhK-gammaT protein expression was temporally and spatially correlated with expression of PhK-gammaT RNA. PhK-gammaT RNA and protein expression was also characterized in the PhK-deficient glycogen storage disease (gsd) rat. PhK-gammaT RNA levels were similar in Type II cells isolated from wild type and gsd/gsd fetuses; in contrast, PhK-gammaT protein was virtually undetectable in gsd/ gsd Type II cells and enzyme activity was very low. These results suggest that PhK-gammaT plays a critical role in mobilization of glycogen during fetal lung development and that failure to catabolize glycogen in the gsd/gsd rat is related to an untranslatable PhK-gammaT RNA or unstable protein.