Notes: Summary Pertussis toxin, which specifically inactivates guanine nucleotide-binding proteins (N-proteins) involved in the signal transduction in various receptor systems, did not influence the positive inotropic effect of the alpha1-adrenoceptor agonist phenylephrine in rat isolated left auricles. This indicates that the alpha1-adrenoceptor-mediated positive inotropic effect does not involve a pertussis toxin-sensitive N-protein.

Notes: Summary In atrial cardiac preparations adenosine exerts a receptor-mediated negative inotropic effect due to an increased potassium conductance. Pretreatment of guinea pigs with pertussis toxin abolished the negative inotropic and action potential shortening effect of adenosine and the adenosine analogue (−)-N6-phenylisopropyladenosine (PIA). As pertussis toxin specifically inactivates guanine nucleotide-binding proteins involved in the signal transfer from receptor binding to specific cell functions, it is concluded that a guanine nucleotide-binding protein is involved in the regulation of the receptor-mediated change in potassium conductance and force of contraction.

Notes: Summary The influence of pertussis toxin on the effects of adenosine, the adenosine receptor agonist (−)-N6-phenylisopropyladenosine (PIA) and the m-cholinoceptor agonist carbachol on heart rate and atrioventricular (AV) conduction was investigated in spontaneously beating isolated perfused guinea-pig hearts. In addition, the effects of the agents on the electrocardiogram recorded from anesthetized guinea pigs were studied. Adenosine (0.1–100 μmol/l) and PIA (0.001–100 μmol/l) had concentration-dependent negative chronotropic and negative dromotropic effects. These effects were prevented by pretreatment of the animals with pertussis toxin (150 μg/kg; i.v.). Carbachol (0.001–100 μmol/l) had similar cardiac depressant effects. These effects were also abolished by pertussis toxin. In contrast, the negative chronotropic and negative dromotropic effects of the calcium antagonist verapamil which was investigated for comparison were not influenced by pretreatment with pertussis toxin. Since the cardiac depressant effects mediated via adenosine receptors or via m-cholinoceptors are most probably due to an activation of a K+ conductance, it is concluded that both receptors in the sinus node and in the AV node may be coupled via a common pertussis toxin-sensitive guanine nucleotide-binding protein to the K+ channel. It remains to be elucidated whether an additional inhibitory coupling to Ca2+ channels also plays a role.

Notes: Summary (1) This study was performed to elucidate the relation between positive inotropy and phosphodiesterase inhibition in the heart. Therefore, the influence on the activity of guinea-pig cardiac phosphodiesterase (PDE) I–III separated by DEAE-cellulose anion exchange chromatography was investigated for the new cardiotonic agents pimobendan, its metabolite UD-CG 212 Cl and milrinone. These effects were compared with those of various other PDE inhibitiors such as IBMX, zaprinast, rolipram and AR-L 57 Cl. A selectivity factor (SF, mean of the IC50 values for PDE I and II inhibition divided by the IC50 for PDE III) was calculated for each drug. The greater this value the more selective was the PDE III inhibition. (2) UD-CG 212 Cl was the most potent (IC50 = 0.19 μmol/l) and most selective inhibitor of PDE III with a SF of 869. Also, selective PDE III inhibitors were pimobendan (SF = 50.5) and milrinone (SF = 70.0) with slightly smaller potencies (IC50 = 2.40 and 1.52 μmol/l, respectively). Zaprinast and rolipram preferentially inhibited PDE I and II, respectively. IBMX and AR-L 57 Cl inhibited PDE I-111 unselectively with similar potencies for all isoenzymes. (3) The PDE inhibitory effects of all substances were compared with their influence on force of contraction (electrically driven papillary muscles) and on frequency of beating (spontaneously beating right auricles) in guinea-pig hearts, thus in preparations of the same species. UD-CG 212 Cl and pimobendan resembled each other in their maximal positive inotropic effects with potencies (EC50) of 1.8 μmol/l and 6.0 μmol/l, respectively. Milrinone exerted a biphasic increase in force of contraction with EC5o values of 0.2 and 695 μmol/l. These substances which selectively inhibited PDE III increased force of contraction maximally by 2.04 mN (UD-CG 212 Cl), 2.77 mN (pimobendan) and 3.29 mN (milrinone). Milrinone increased the frequency of beating by 40%, pimobendan by 22% and UD-CG 212 Cl by 16% at maximal positive inotropic concentrations. (4) The unselective PDE I-III inhibitors IBMX and AR-L 57 Cl increased force of contraction with the highest maximal effects (5.9 mN and 4.23 mN, respectively). There was no correlation between EC5o values for the positive inotropic effects and IC50 values for the inhibition of PDE I–III. Zaprinast exerted only weak (0.87 mN) and rolipram no effects on force of contraction. The latter substances had no positive chronotropic effects. (5) In conclusion, the positive inotropic effects of the new cardiotonic agents pimobendan, UD-CG 212 Cl and milrinone may be at least in part due to selective cardiac PDE III inhibition. For UD-CG 212 Cl and pimobendan the IC50 for PDE III inhibition and the EC50 for the positive inotropic effects were of the same order of magnitude. For milrinone, only the lower EC50 value is similar to the IC50 for the inhibition of PDE III. In the case of the unselective PDE III inhibitors IBMX and AR-1. 57 Cl a factor of 20 was found between potencies for positive inotropy and PDE III inhibition. Thus, PDE III inhibition seems to be a prerequisite for PDE inhibition as a mechanism of action for increasing force of contraction. However, the pronounced positive inotropic effects of IBMX and AR-L 57 Cl provide evidence that a selectivity for PDE III inhibition seems not to be essential for increasing force of contraction. Finally, there is no indication that inhibition of PDE I or II plays a major role in increasing force of contraction.

Notes: Summary 1. The effects of adibendan (BM 14.478; 7,7-dimethyl-2-(4-pyridyl)-6,7-dihydro-3H,5H-pyrrolo[2,3-fl benzimidazole-6-one) on force of contraction and beating frequency were analysed in guinea-pig electrically driven papillary muscles and spontaneously beating right auricles, respectively. The effects of 3-isobutyl-l-methylxanthine (IBMX) and milrinone were studied for comparison. 2. Adibendan exerted a concentration-dependent (0.03 –300 μol/l) positive inotropic effect in papillary muscles (EC50 = 1.3 μmol/l) which was only partially reversible. The efficiency of adibendan was less than that of milrinone, but adibendan was about two orders of magnitude more potent and had only slight positive chronotropic effects (113% of pre-drug values) at most. Milrinone increased the frequency of beating maximally to 140% of pre-drug values. The positive inotropic effect of adibendan is probably at least partially mediated by cAMP since carbachol reduced the increase in force of contraction by about 75%. 3. To elucidate the mechanism of action of adibendan we investigated its effects on phosphodiesterase I–III and adenylate cyclase activity in isolated preparations from guinea-pig hearts. 4. Adibendan selectively inhibited phosphodiesterase III (PDE III) activity concentration-dependently (IC50 = 2.0 μmol/l). The IC50 values for the inhibition of PDEI or II were more than 60-fold higher. Since adibendan did not affect adenylate cyclase activity a stimulation of the cAMP synthesis as a mechanism of action can be ruled out. 5. The results provide evidence that the positive inotropic action of adibendan is at least in part due to an inhibition of cAMP-PDE III. However, other mechanisms of action like increased sensitivity of the contractile proteins to Ca2+ may be also involved.