In Vitro Pharmacology of R 80122, a Novel Phosphodiesterase Inhibitor

The cardiac in vitro effects of R 80122, a novel phosphodiesterase (PDE) inhibitor, were investigated and compared with those of the reference compound milrinone and of the calcium-sensitizer adibendan. In guinea pig left atria, both milrinone and R 80122 increased contractile force; 10 muM milrinone was equieffective to 1 muM R 80122. The rate of spontaneously beating atria was not altered by R 80122 in the concentration range of 0.01-0.3 muM. Higher concentrations (1-10 muM) led to a statistically insignificant increase of 20%. Milrinone's effect on frequency was more pronounced and amounted to 21% at 10 muM and to 40% at 100 muM. Adibendan increased heart rate (HR) by 10% at a concentration of only 0.03 muM. This effect was not enhanced any further by increasing the concentration. In papillary muscle, the positive inotropic effects of both milrinone and R 80122 were inhibited by carbachol, indicating involvement of cyclic AMP. Further indications for a cyclic AMP-dependent action were obtained by induction of slow action potentials and synergism with isoprenaline. In electrophysiologic measurements, milrinone reduced action potential duration (APD) in a high concentration whereas R 80122 had no effect. Action potential changes elicited by a toxic concentration of ouabain were reduced by R 80122. Relaxation of rat aortic rings contracted by KCl and relaxation of guineapig aortic rings contracted by norepinephrine (NE) was comparable for both milrinone and R 80122. R 80122 also caused relaxation of canine coronary arteries constricted with prostaglandin F2alpha (PGF2alpha) both with and without endothelium. NE-induced contractions in canine gastrosplenic arteries were not affected by R 80122. Cardiac contractility that had been impaired to various degrees by pentobarbital or by aging was restored to control values by both milrinone and R 80122. R80122 enhanced cardiac contractility at lower concentrations than milrinone with no concomitant increase in frequency or shortening of the action potential, which may be advantageous for treatment of heart failure.