ONTOGENIC REGULATION OF PHOSPHORYLASE KINASE (PK) IN RAT LUNG

Abstract

Pulmonary glycogen serves as substrate for surfactant lipid synthesis by Type II cells during the perinatal period. Glycogen is depleted from these cells in association with activation of phosphorylase (phos b). Phos b is phosphorylated to phos a by phosphorylase kinase (PK). The present study tests the hypothesis that known depletion of pulmonary glycogen is associated with ontogenic changes in PK. Lung PK was quantitated by phosphorylation of endogenous and exogeneous phos b assessed by SDS-PAGE and autoradiography. ³²P-Phos a was the major Ca²⁺-dep phospho-protein in fetal lung cytosol by 2-D, IEF-PAGE, co-migrating with purified muscle enzyme. PK increased from day 17 to 20-21 gestation declining to undetectable levels within a week of age. PK was Ca²⁺-dependent, EC₅₀ Ca²⁺ 10⁻⁷M. Exogenous phos b was readily phosphorylated by fetal but not adult cytosol. Added purified exogenous PK did not enhance ³²P-phos a in postnatal lung. PK was also demonstrated in purified Type II cells from rat. Activity was highest 16 hrs after isolation, decreasing during culture 3-4d. Conclusion: phosphorylase kinase activity was identified in cytosol from both Type II cells and fetal lung. PK increases prior to birth and decreases dramatically postnatally. Decreases in both phos b (substrate) and phosphorylase kinase may account for known postnatal decreases in phos a activity. High PK activity is associated with glycogen depletion and surfactant synthesis in the Type II cells at 20-21d gestation in the rat.

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