The effect of isoenzyme‐selective PDE inhibitors on methacholine‐induced contraction of guinea‐pig and rat ileum

1 We have examined the effects of the isoenzyme‐selective phosphodiesterase (PDE) inhibitors, vinpocetine (type 1), siguazodan (type 3), rolipram (type 4) and zaprinast (type 5) and the non‐selective PDE inhibitor enprofylline on methacholine (MCh) contractile concentration‐response curves on guinea‐pig and rat isolated ileum. 2 In guinea‐pig ileum, vinpocetine (10–300 μ m ), zaprinast (1–300 μ m ) and enprofylline (100–1000 μ m ) produced a concentration‐dependent depression of the maximum response (E max ) to MCh only without effect on the MCh EC 50 values (rank order of potency: zaprinast > vinpocetine > enprofylline). In contrast, siguazodan (10–300 μ m ) and rolipram (10–300 μ m ) produced a rightward displacement of the MCh concentration‐response curve (increase in EC 50 : rank order: rolipram > siguazodan), with effects on the MCh maximum seen only at higher concentrations. 3 In the rat ileum, vinpocetine (10–300 μ m ), zaprinast (0.1–300 μ m ) and enprofylline (100–1000 μ m ) caused depression of the MCh maximum contraction (rank order: zaprinast > vinpocetine > enprofylline). Low concentrations of rolipram and siguazodan had no significant effect on the MCh maximum. In the presence of higher concentrations (> 100 μ m ) of rolipram and siguazodan, a maximum response was not achieved at the highest concentration of MCh tested. As in the guinea‐pig ileum, only rolipram (10–300 μ m ) and siguazodan (10–300 μ m ) produced a significant, concentration‐dependent, rightward displacement of the MCh concentration‐response curve (increase in EC 50 : rank order: rolipram > siguazodan). 4 In the guinea‐pig ileum, isoprenaline (0.1 μ m ) produced a rightward displacement (∼3 fold) of the MCh concentration‐response curve, accompanied by a significant depression of the maximum response. Increasing the isoprenaline concentration (1 μ m ) had no further effect on either parameter. Sodium nitroprusside (SNP, ≥ 10 μ m ) produced a concentration‐dependent depression of the MCh maximum without an effect on the EC 50 . 5 In the rat ileum, isoprenaline (1 μ m ) produced a concentration‐dependent rightward displacment (∼2.8 fold) of the MCh concentration‐response curve with depression of the MCh maximum at higher (≥ 100 μ m ) concentrations. SNP produced depression of the MCh maximum at a concentration of 10 μ m and above. Effects on the MCh EC 50 were seen only at 100 and 300 μ m . 6 In guinea‐pig ileum, isoprenaline (0.1 μ m ) in combination with rolipram (10 μ m ) further increased the MCh EC 50 and reduced the MCh maximum. The combination of SNP (10 μ m ) with zaprinast (0.1 μ m ) produced no further significant effect than SNP alone. 7 In rat ileum, isoprenaline (1 μ m ) in combination with rolipram (10 μ m ) further increased the EC 50 and reduced the maximum. SNP (10 μ m ) had no significant effect on either the MCh maximum or EC 50 . A combination with zaprinast (1 μ m ) had no further effect. 8 In conclusion, all the PDE inhibitors tested produced a concentration‐dependent inhibition of the MCh concentration‐response curve, indicating a modulator role for the PDE isoenzymes in gastrointestinal smooth muscle contractility. The PDE inhibitors that elevate cyclic GMP produced a depression of the MCh maximum response only, whilst those that elevate cyclic AMP produced a rightward displacement of the MCh concentration‐response curve. This was confirmed by the use of isoprenaline and SNP. This difference in the type of inhibition produced by these PDE isoenzyme inhibitors may reflect a different intracellular site/mechanism by which the cyclic AMP‐ and cyclic GMP‐activated kinases act functionally to antagonize the contractile response.