The effect of theophylline on adenosine 3′, 5′-monophosphate-dependent protein kinase and ACTH 1–24 stimulated steroidogenesis in bovine adrenal cortical cells

Abstract

Theophylline (theo), a known phosphodiesterase (PDE) inhibitor, was tested for its effects on ACTH 1–24 regulated steroidogenesis in isolated bovine adrenal cortical cells. Theo produced a dose related inhibition of ACTH 1–24 stimulated cortisol synthesis with half maximal inhibition occuring at 7 mM. Theo enhanced ACTH 1–24 stimulated cellular adenosine 3′, 5′-monophosphate (cAMP) levels above that produced by ACTH 1–24 alone confirming its inhibition of cAMP PDE. When tested on cAMP binding protein and cAMP-dependent protein kinase activities in cytosol prepared from bovine adrenal cortex, theo displaced 3H-cAMP binding to cAMP binding protein and inhibited cAMP-stimulated protein kinase activity. The half maximal inhibition of cAMP binding and protein kinase activity was observed at 10 and 5 mM, respectively. Inhibition of cAMP-dependent protein kinase by theo provides a possible explanation of its inhibitory effects on adrenal steroidogenesis and further implicates cAMP-dependent protein kinase in mediating ACTH stimulated steroidogenesis. Furthermore these studies suggest a novel mechanism of action for theo in addition to its known action on cAMP PDE.