Distribution of cyclic AMP phosphodiesterase in adipose tissue from trained rats

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 cyclic AMP to AMP and adenosine, via phosphodiesterase (PDE) and 5'-nucleotidase (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 determined 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 microM cilostamide, while PM-PDE was unaffected by this PDE3B inhibitor. Inhibitors of PDE1, PDE2, PDE4 and PDE5 also failed to inhibit PM-PDE. However, 1 mM DPSPX inhibited PM-PDE activity by 72%. When PM were incubated with 0.8 microM cyclic AMP for 20 min, AMP accumulation was four times that of adenosine. These data demonstrate that cyclic AMP can be converted to AMP and adenosine by the PM-bound enzymes 5'-NT and PDE, and suggest that the PM-PDE responsible for extracellular cyclic AMP metabolism to AMP is distinct from the intracellular MM-PDE.

Acute adaptation in adrenergic control of lipolysis during physical exercise in humans

During prolonged exercise, the free fatty acids derived from adipocyte lipolysis are the principal fuel utilized by muscles. In humans, the lipid mobilization from adipose tissue is mainly regulated by insulin and catecholamines: the latter hormones have both beta-adrenergic stimulatory and alpha 2-adrenergic inhibitory effects on lipolysis. The aim of this study was to determine whether rapid alterations in the peripheral action of the regulatory hormones occur during physical work and whether they are of importance for the enhanced lipid mobilization. The acute effects of exercise on the regulation of lipolysis were investigated in isolated adipocytes removed from the gluteal region of 14 healthy volunteers before and immediately after the exercise period. Exercise induced a 20-35% significant increase in the lipolytic response to noradrenaline alone and in combination with the selective alpha 2-antagonist yohimbine and to the pure beta-agonist isoproterenol in isolated adipocytes. The antilipolytic effects of both the alpha 2-agonist clonidine and insulin were unaffected by exercise. Exercise did not influence the specific adipocyte receptor binding of 125I-cyanopindolol (beta-adrenergic receptor), [3H]yohimbine (alpha-adrenergic receptor), and mono-125I-[Tyr A14]insulin (insulin receptor). In conclusion, a single period of submaximal exercise increases adipocyte lipolytic responsiveness to catecholamines through an increased beta-adrenoceptor-mediated effect at steps distal to the receptor binding. Thus the increased peripheral action of catecholamines may be of importance for the observed enhanced lipid mobilization during physical work.