An analysis of the mechanism of the inotropic action of some milrinone analogues in guinea-pig isolated atria.

1 It has been reported previously that the milrinone analogues, ethyl 5-cyano-1,6-dihydro-2-methyl-6-oxo-3 pyridine carboxylate (I) and ethyl 5-cyano-1,6-dihydro-2-ethyl-6-oxo-3 pyridine carboxilate (II) exert a positive inotropic effect (EC50 = 15.6 ± 0.2 μm and 40.3 ± 0.1 μm) both on spontaneously beating and on electrically driven atria from reserpine-treated guinea-pigs. In the present study the mechanism of the inotropic action of these two agents was investigated. 2 In electrically driven left atrium from resperpine-treated guinea-pigs the EC50 values for inotropic activity for compounds (I) and (II) corresponded to that of milrinone (EC50 = 25 ± 0.1 μm) but compound (I) induced a greater maximum effect. This corresponded to a percentage increase in developed tension over control of 63 ± 0.3 whereas the maximum inotropic effect of milrinone was 48 ± 0.3 and that of compound (II) was 47 ± 0.2. 3 The inotropic activity of compounds (I) and (II) (10–100 μm) was resistant to propranolol (0.1 μm), thus excluding the involvement of β-adrenoceptors. 4 Since the inotropism induced by compounds (I) and (II) was not reduced by carbachol (1 nm-0.5 μm), an action involving changes in adenosine 3′:5′-cyclic monophosphate (cyclic AMP) can be excluded. 5 The inotropic action of compounds (I) and (II) was blocked selectively by 8-phenyltheophyline (10 μm) or adenosine deaminase (2 u ml−1). 6 Both (I) and (II) inhibited, in an apparently competitive manner, the negative inotropic effect induced by N6-(l-phenylisopropyl) adenosine (l-PIA), a stable adenosine agonist. The pA2 values for (I) and (II) were 4.79 and 4.36, respectively. 7 In rat brain compounds (I) and (II) inhibited the specific binding of N6-cyclohexyl[3H]-adenosine-([3H]-CHA) with an IC50 of 0.18 ± 0.01 mm and 0.25 ± 0.02 mm, respectively, which were similar to their IC50 values for blocking the PIA-induced negative inotropic effect and which are also in the range of concentrations that are effective in inducing positive inotropism in guinea-pig atria. 8 The results from the present study suggest that antagonism of endogenous purines causes positive inotropism without affecting intracellular cyclic AMP levels.