Regulation of cyclic AMP metabolism in bovine adrenal medullary cells.

The capacity of cultured bovine adrenal medullary cells to metabolize and export cyclic AMP has been studied. Basal cellular cyclic AMP levels were increased 50% by 100 microM 3-isobutyl-1-methylxanthine (IBMX) and rolipram, a class IV (cyclic AMP-specific) phosphodiesterase (PDE) inhibitor. They were not affected by inhibition of class I (Ca2+/calmodulin-dependent), class III (cyclic GMP-inhibited) or class V PDE (cyclic GMP-specific) with vinpocetine or 3-isobutyl-8-methoxymethyl-1-methylxanthine (8-methoxymethyl-IBMX), SK&F 94120, or MB 22,948, respectively, all at 100 microM. Furthermore, only IBMX and rolipram enhanced the cyclic AMP response to 0.3 microM forskolin. Rolipram had an EC50 of < or = 1 microM and was equally effective at 100 microM and 1 mM. IBMX enhanced cyclic AMP levels significantly more at 1 mM than at 100 microM. Neither vinpocetine nor 8-methoxymethyl-IBMX (100 microM) enhanced the Ca(2+)-dependent cyclic AMP response to K+ depolarization. Elevation of cyclic GMP levels with sodium nitroprusside (10 or 100 microM), to activate any cyclic GMP-stimulated class II PDE and to inhibit any cyclic GMP-inhibited class III PDE, also had no effect on basal or forskolin-stimulated cyclic AMP levels. In the presence of IBMX (1 mM), forskolin (5 microM) caused a rapid and large increase in cellular cyclic AMP levels which was maximal after about 5 min and declined slightly over 3 hr. Over this period, extracellular cyclic AMP levels rose almost linearly reaching levels 2-3 times those in the cells. The results indicate bovine adrenal medullary cells have a high capacity for sustained cyclic AMP export. Furthermore, two PDE isozymes appear to degrade cyclic AMP in these cells, a rolipram-sensitive, cyclic AMP-specific, class IV isozyme and a rolipram-insensitive isoform.