The effects of adenosine-und guanosine 3?,5?-phosphoric acid benzyl esters on guinea-pig ventricular myocardium

Abstract

1. The inotropic effects of benzyl-and substituted benzyl triesters of cyclic AMP and cyclic GMP were studied in isometrically contracting guineapig papillary muscles driven at a rate of 0.2 Hz. 2. The triesters of cyclic AMP produced marked concentration-dependent positive inotropic effects. The maximum increase of contraction force (ΔFc) was 2.2 g (n=5) for p-MeBncAMP and 1.8 g for BncAMP and o-NO2BncAMP, respectively (n=5 for each ester). Total contraction time (t1+t2) was thereby shortened by 44 ms (n=15). Threshold and maximally effective concentrations for the positive inotropic effects were 1×10−4 M and 6×10−4M (BncAMP), 3×10−4 M and 3×10−3 M (p-MeBncAMP) and 1×10−5 M and 2×10−4 M (o-NO2BncAMP), respectively. 3. The phosphodiesterase inhibitor 1-methyl,3-isobutyxanthine (IBMX) (2×10−5 M) shifted the concentration-effect curves of all 3 triesters to the left, decreasing the concentrations required to produce a half-maximal increase of the force of contraction by a factor of 6. 4. The time necessary to produce a half-maximum inotropic effect was 2 min for p-MeBncAMP, 10 min for BncAMP and 66 min for o-NO2BncAMP. The inotropic effects could not be reversed upon washing the preparations for 1 h with a drug-free solution. 5. The concentration-effect curves of the triesters of cyclic AMP were not affected by previous blockade of the β-adrenoceptors with 5×10−6 M (±)-propranolol. 6. BncGMP at concentrations ranging from 1×10−5 to 2×10−3 M, produced no inotropic effects on 3 guinea-pig papillary muscles, while a decrease of the contraction force was detectable 30 min after the addition of 2×10−3 M p-MeBncGMP, reaching 75% of the control value within 1 h (n=3). At 3×10−3 M, both esters induced a positive inotropic effect (ΔFc) of 0.2 g which was completed in 3–4 min. The negative inotropic effect of 2×10−3 M p-MeBncAMP was immediately reversed upon washing the preparation while the positive inotropic effect was not affected. 7. Spontaneous hydrolysis of the benzyl triesters of cyclic AMP or cyclic GMP in Krebs-Henseleit buffer gave rise to cyclic AMP or cyclic GMP and to the respective benzyl alcohols. The electron donating substituent p-CH3 of the p-MeBncAMP or the p-MeBncGMP increased the hydrolytic reactivity by a factor of 14 when compared to the unsubstituted benzyl esters which in turn were 5 times more reactive than the triesters which contained the electron withdrawing substituent o-NO2. 8. Triesters of cyclic AMP and cyclic GMP (3×10−3 M) inhibited beef heart phosphodiesterase (PDE) by 17–29% and by 58–65%, respectively (n=4 for each ester). The specific PDE-inhibitor IBMX (2×10−5 M) blocked PDE by 52% (n=4). 9. The data presented in this study are in agreement with the hypothesis that benzyl triesters of cyclic AMP are simply a transport form of cyclic AMP across the cell membrane and, once inside, they are hydrolyzed with a characteristic time course to cyclic AMP which is responsible for the positive inotropic effects. Assuming that the benzyl triesters of cyclic GMP are also intracellularly converted to cyclic GMP, its postulated mediator role of a negative inotropic effect is not in accordance with the present results.