Cyclic AMP metabolism by swine adipocyte microsomal and plasma membranes

Extracellular cyclic AMP is source of extracellular adenosine in brain and kidney. Whether this occurs in adipose tissue is unknown. The present study evaluated the capacity of swine adipocyte plasma membranes to metabolize c yclic AMP to AMP and adenosine, via phosphodiesterase (PDE) and 5'-nucleoti dase (5'-NT), respectively. Plasma membranes (PM) and microsomal membranes (MM) were isolated from over-the-shoulder subcutaneous adipose tissue of 3 month-old male miniature swine. The purity of the membrane fractions was de termined and PDE and 5'-NT activities in PM and MM fractions were corrected for cross-contamination. The maximal activity of MM-PDE was 7-fold greater than that of PM-PDE. MM-PDE was 100% inhibited by 5 mu M cilostamide, whil e PM-PDE was unaffected by this PDE3B inhibitor. Inhibitors of PDE1, PDE2, PDE4 and PDES also failed to inhibit PM-PDE. However, 1 mM DPSPX inhibited PM-PDE activity by 72%. When PM were incubated with 0.8 mu M cyclic AMP for 20 min, AMP accumulation was four times that of adenosine. These data demo nstrate that cyclic AMP can be converted to AMP and adenosine by the PM-bou nd enzymes 5'-NT and PDE, and suggest that the PM-PDE responsible for extra cellular cyclic AMP metabolism to AMP is distinct from the intracellular MM -PDE. (C) 1999 Elsevier Science Inc. All rights reserved.