Regulation of cyclic AMP metabolism in bovine adrenal medullary cells

Abstract

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 μM 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 (Ca 2+/calmodulin-dependent), class III (cyclic GMP-inhibited) or class V PDE (cyclic GMP-specinc) with vinpocetine or 3-isobutyl-8-methoxymethyl-1-methylxanthine (8-methoxymethyl-IBMX), SK&F 94120, or MB 22,948, respectively, all at 100 μM. Furthermore, only IBMX and rolipram enhanced the cyclic AMP response to 0.3 μM forskolin. Rolipram had an EC 50 of ⩽ 1 μM and was equally effective at 100 μM and 1 mM. IBMX enhanced cyclic AMP levels significantly more at 1mM than at 100 μM. Neither vinpocetine nor 8-methoxymethyl-IBMX (100 μM) enhanced the Ca 2+-dependent cyclic AMP response to K + depolarization. Elevation of cyclic GMP levels with sodium nitroprusside (10 or 100 μM), 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 μM) 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.