Turnover of bis-diphosphoinositol tetrakisphosphate in a smooth muscle cell line is regulated by beta 2-adrenergic receptors through a cAMP-mediated, A-kinase-independent mechanism

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Abstract
Bis‐diphosphoinositol tetrakisphosphate ([PP]2‐InsP4 or ‘InsP8’) is a ‘high‐energy’ inositol phosphate; we report that its metabolism is receptor‐regulated in DDT1 MF‐2 smooth muscle cells. This conclusion arose by pursuing the mechanism by which F− decreased cellular levels of [PP]2‐InsP4 up to 70%. A similar effect was induced by elevating cyclic nucleotide levels, either with IBMX or by application of either Bt2cAMP (EC50 = 14.7 μM), Bt2cGMP (EC50 = 7.9 μM) or isoproterenol (EC50 = 0.4 nM). Isoproterenol (1 μM) decreased [PP]2‐InsP4 levels 25% by 5 min, and 71% by 60 min. This novel, agonist‐mediated regulation of [PP]2‐InsP4 turnover was very specific; isoproterenol did not decrease the cellular levels of either inositol pentakisphosphate, inositol hexakisphosphate or other diphosphorylated inositol polyphosphates. Bradykinin, which activated phospholipase C, did not affect [PP]2‐InsP4 levels. Regulation of [PP]2‐InsP4 turnover by both isoproterenol and cell‐permeant cyclic nucleotides was unaffected by inhibitors of protein kinases A and G. The effectiveness of the kinase inhibitors was confirmed by their ability to block phosphorylation of the cAMP response element‐binding protein. Our results indicate a new signaling action of cAMP, and furnish an important focus for future research into the roles of diphosphorylated inositol phosphates in signal transduction.