The role of nitric oxide (NO) in the pathophysiology of atrial fibrillation (AF) is controversially discussed. Here we studied signal transduction pathway of NO on human atrial myocytes from patients in sinus rhythm (SR) and in chronic AF (cAF).Methods
Experiments were performed in right atrial biopsies from 70 patients in SR and 40 patients in cAF. In all experiments S-nitroso-N-acetylpenicillamine (SNAP) was used as NO donor. L-type calcium current (ICa,L) and force of contraction (Fc) were measured as read-out parameters for cardiac function.Results
In accordance with the literature basal ICa,L in SR was larger than in cAF (5.87 ± 0.26 pA/pF, n = 71/40 vs. 3.45 ± 0.24 pA/pF, n = 141/71 [myocytes/patients]; p < 0.001). Basal Fc in trabeculae from patients in SR (9.49 ± 1.32 mN, n = 9/7) was greater than in cAF (4.57 ± 1.27 mN, n = 9/4; p < 0.001). SNAP (100 μM) increased basal ICa,L in SR by 2.86 ± 0.41 pA/pF (n = 141/71; p < 0.001), that was accompanied by a small positive inotropic effect at SNAP concentrations 300 μM and 1 mM. In cAF SNAP failed to increase ICa,L and had no effect on force of contraction.Results
The effect of SNAP on ICa,L in SR could be prevented by inhibition of soluble guanylate cyclase (sGC) with ODQ, 30 μM and PKA with Rp-8-Br-cAMPs, 1mM. The results obtained indicate that signal transduction of NO involves sGC and PKA.Results
In SR, SNAP increased ICa,L to the same extent as a high concentration of the PDE3 inhibitor cilostamide (1μM), which should inhibit almost all PDE3s. Interestingly, SNAP reduced cilostamide-stimulated ICa,L. Such inhibitory effect of SNAP was reversed by inhibition of PDE2 with EHNA (10 μM). In cAF ICa,L was increased with cilostamide, suggesting preserved function of PDE3. Moreover, in cAF SNAP also inhibited cilostamide-stimulated ICa,L. Thus, increase in cAMP via inhibition of PDE3 may sensitize PDE2 for cGMP activation resulting in greater cAMP breakdown and decreased ICa,L. These results suggest that also in cAF NO stimulates cGMP production. The same was true when cAMP level was increased during β-adrenoreceptor-stimulation with isoprenaline 1μM. SNAP reduced the isoprenaline effect to 78% in SR and 67% in cAF.Conclusions
NO exerts dual effects on ICa,L i.e. increase of basal current and inhibition of cAMP-stimulated current. We conclude that in cAF, interaction of cGMP with PDE3 is lost whereas interaction with PDE2 is preserved.