<Original Article> Effects of Sildenafil (Viagra TM) on the Cyclic AMP Hydrolyzing Phosphodiesterase Activity in the Canine Heart Assessed Using a Modified Enzymatic Fluorometric Assay Technique

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Sildenafil is a phosphodiesterase type-5(PDE5) inhibitor, but has stimulatory effects on heart in vivo and in vitro. We evaluated the effects of sildenafil on the cyclic AMP hydrolyzing PDE activity in a crude enzyme preparation made from the intact canine heart using the modified enzymatic fluorometric assay technique. The basal PDE activity was 485±47pmol/min/mg protein (n=6).Sildenafil as well as a non-specific PDE inhibitor, 3-isobutyl-1-methylxanthine (IBMX) inhibited the PDE activity in a concentration-related manner. Significant inhibition at 1-10μmol/L of sildenafil was almost equivalent to that of the same concentration of IBMX. These results suggest that supra-therapeutic concentration of sildenafil may directly inhibit cyclic AMP hydrolyzing PDEs in the intact heart.

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