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Negative chronotropic and inotropic effects exerted by diadenosine hexaphosphate (AP6A) via A1-adenosine receptors
U Vahlensieck1, P Bokník, J Knapp
1Institut für Pharmakologie und Toxikologie, Universität Münster, Germany.
Insights
Diadenosine hexaphosphate (AP6A) negatively impacts cardiac function, reducing heart rate and contractility in guinea-pig and human models. These effects are mediated by A1-adenosine receptors, influencing calcium currents and protein phosphorylation.
Area of Science:
- Cardiovascular Pharmacology
- Purinergic Signaling
- Cardiac Electrophysiology
Background:
- Diadenosine hexaphosphate (AP6A) is known to cause vasoconstriction.
- The specific effects of AP6A on cardiac function remain largely unexplored.
- Understanding AP6A's cardiac actions is crucial for its potential therapeutic or toxicological implications.
Purpose of the Study:
- To investigate the effects of diadenosine hexaphosphate (AP6A) on cardiac contractility and frequency.
- To elucidate the underlying mechanisms, including effects on ion currents, cyclic AMP, and protein phosphorylation.
- To determine the receptor subtype involved in AP6A's cardiac actions.
Main Methods:
- Isolated spontaneously beating right atria and electrically driven left atria and papillary muscles from guinea-pigs.
- Electrically driven human ventricular preparations.
- Measurement of cardiac contractility, rate, L-type calcium current, cyclic AMP levels, and protein phosphorylation (phospholamban, troponin inhibitor) in ventricular myocytes.
- Pharmacological blockade using selective A1-adenosine receptor antagonist DPCPX, M-cholinoceptor antagonist atropine, and P2-purinoceptor antagonist suramin.
Main Results:
- AP6A induced negative chronotropic and inotropic effects in guinea-pig and human cardiac preparations.
- AP6A attenuated isoprenaline-stimulated cardiac function and L-type calcium current.
- All observed effects were abolished by DPCPX, indicating mediation via A1-adenosine receptors, independent of cyclic AMP levels.
Conclusions:
- Diadenosine hexaphosphate (AP6A) exerts significant negative chronotropic and inotropic effects on cardiac tissue.
- These cardiac effects are primarily mediated through the activation of A1-adenosine receptors.
- AP6A modulates cardiac function by influencing L-type calcium current and associated protein phosphorylation pathways.
Abstract:
1. Diadenosine hexaphosphate (AP6A) exerts vasoconstrictive effects. The purpose of this study was to investigate whether AP6A has any effect on cardiac function. 2. The effects of AP6A (0.1-100 microM) on cardiac contractility and frequency were studied in guinea-pig and human isolated cardiac preparations. Furthermore, the effects of AP6A on the amplitude of the L-type calcium current, on the adenosine 3':5'-cyclic monophosphate (cyclic AMP) content and on the phosphorylation of regulatory phosphoproteins, i.e. phospholamban and troponin inhibitor, were investigated in guinea-pig isolated ventricular myocytes. 3. In isolated spontaneously beating right atria of the guinea-pig AP6A exerted a negative chronotropic effect and reduced the rate of contraction maximally by 35% (IC20 = 35 microM). 4. In isolated electrically driven left atria of the guinea-pig AP6A exerted a negative inotropic effect and reduced force of contraction maximally by 23% (IC20 = 70 microM). 5. In isolated electrically driven papillary muscles of the guinea-pig AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction maximally by 23% (IC20 = 60 microM). Furthermore, AP6A attenuated the relaxant effect of isoprenaline. 6. In human isolated electrically driven ventricular preparations AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction by maximally 42% (IC20 = 18 microM). Moreover, AP6A attenuated the relaxant effect of isoprenaline. 7. All these effects of AP6A were abolished by the selective A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX, 0.3 microM), whereas the M-cholinoceptor antagonist atropine (10 microM) and the P2-purinoceptor antagonist suramin (300 microM) failed to abolish the effects of AP6A. 8. AP6A 100 microM had no effect on the amplitude of the L-type calcium current, but attenuated isoprenaline-stimulated L-type calcium current. The maximum of the current-voltage relationship (I-V curve) was shifted to the left by isoprenaline and additional application of AP6A shifted the I-V curve back to the right to the control value. The phosphorylation state of phospholamban and the troponin inhibitor was unchanged by AP6A alone, but was markedly attenuated by AP6A in the presence of isoprenaline. Cyclic AMP levels remained unchanged by AP6A, even after stimulation with isoprenaline. 9. In summary, AP6A exerts negative chronotropic and inotropic effects in guinea-pig and human cardiac preparations. These effects are mediated via A1-adenosine receptors as all effects were sensitive to the selective A1-adenosine receptor antagonist DPCPX. Furthermore, the effects of AP6A on cyclic AMP levels, protein phosphorylation and the L-type calcium current are in accordance with stimulation of A1-adenosine receptors.