Introduction: Phosphodiesterase-4 (PDE4) enzymes specifically break down the second messenger cAMP, thereby terminating the intracellular signaling cascade that plays an essential role in neurohormonal modulation of many physiological systems. PDE4 activity and expression are regulated by cAMP levels, suggesting that measurement of PDE4 provides an index of intracellular cAMP signaling.
Methods: Male Sprague-Dawley rats were administered (R)- or the less active enantiomer (S)-[11C]rolipram and sacrificed 30 min later with tracer retention measured in various tissues. Co-injections with saturating doses of unlabeled (R)-rolipram, (S)-rolipram and Ro 20-1724, as well as subtype-selective PDE inhibitors vinpocetine, Bay 60-7550, cilostazol and zaprinast were used to establish binding selectivity for PDE4 over PDE1, PDE2, PDE3 and PDE5 subtypes, respectively. Autoradiography was performed to substantiate results of biodistribution studies in the myocardium.
Results: In vivo (R)-[11C]rolipram retention was dose-dependently reduced by co-injections of (R)-rolipram and (S)-rolipram (ED50 values of 0.03 mg/kg and 0.2 mg/kg, respectively). Vinpocetine, Bay 60-7550, cilostazol and zaprinast had no effect on (R)-[11C]rolipram binding, while (R)-rolipram and Ro 20-1724 reduced the tracer uptake to nonspecific levels in PDE4-rich tissues.
Conclusions: In addition to the brain, (R)-[11C]rolipram binds selectively to PDE4 across all cardiac regions, skeletal muscle, lungs and pancreas, but not in the adipose tissues. In vivo findings were confirmed by in vitro autoradiography studies, suggesting that (R)-[11C]rolipram can be applied to evaluate alterations in central and peripheral PDE4 levels and cAMP-mediated signaling.