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 carboxylate (II) exert a positive inotropic effect (EC50 = 15.6 +/- 0.2 microM and 40.3 +/- 0.1 microM) 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 reserpine-treated guinea-pigs the EC50 values for inotropic activity for compounds (I) and (II) corresponded to that of milrinone (EC50 = 25 +/- 0.1 microM) 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 microM) was resistant to propranolol (0.1 microM), thus excluding the involvement of beta-adrenoceptors. 4. Since the inotropism induced by compounds (I) and (II) was not reduced by carbachol (1 nM-0.5 microM), 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 microM) 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.
Dorigo
P
P
Department of Pharmacology, Padua University, Italy.
Gaion
R M
RM
Belluco
P
P
Mosti
L
L
Borea
P A
PA
Maragno
I
I
D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D058892:Adenosine Deaminase Inhibitors; D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D002316:Cardiotonic Agents / Q000494:pharmacology*; D004558:Electric Stimulation; D006168:Guinea Pigs; D006325:Heart Atria / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D020105:Milrinone; D009200:Myocardial Contraction / Q000187:drug effects*; D010660:Phenylisopropyladenosine / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011728:Pyridones / Q000494:pharmacology*; D012110:Reserpine / Q000494:pharmacology; D013806:Theophylline / Q000031:analogs & derivatives / Q000494:pharmacology
D058892:Adenosine Deaminase Inhibitors; D002316:Cardiotonic Agents; D011728:Pyridones; C074247:ethyl 5-cyano-2-ethyl-1,6-dihydro-6-oxo-3-pyridinecarboxylate; D010660:Phenylisopropyladenosine; C027513:N(6)-cyclohexyladenosine; C074330:ethyl 5-cyano-1,6-dihydro-2-methyl-6-oxo-3-pyridinecarboxylate; D012110:Reserpine; D002217:Carbachol; D011433:Propranolol; D013806:Theophylline; C028322:8-phenyltheophylline; D020105:Milrinone; D000241:Adenosine