Negative chronotropic actions of endothelin‐1 on rabbit sinoatrial node pacemaker cells

Abstract

1 The eects of endothelin-1 (ET-1) on sinoatrial (SA) node preparations of the rabbit heart were studied by means of whole-cell clamp techniques. 2 ET-1 at 1 nM slowed the spontaneous beating activity and rendered half of the cells quiescent. At a higher concentration of 10 nM, the slowing and cessation of spontaneous activity were accompanied by hyperpolarization. 3 In voltage-clamp experiments, ET-1 decreased the basal L-type Ca 2+ current (I Ca(L) ) dose-dependently with a half-maximal inhibitory concentration (EC 50 ) of 0.42 nM and maximal inhibitory response (E max ) of 49.5%. The delayed rectifying K + current (I K ) was also reduced by 33.2+11.1% at 1 nM. In addition, an inwardly rectifying K + current was activated by ET-1 at higher concentrations (EC 50 =4.8 nM). These ET-1-induced changes in membrane currents were abolished by BQ485 (0.3 mM), a highly selective ET A receptor antagonist. 4 When I Ca(L) was inhibited by ET-1 (1 nM), subsequent application of 10 mM ACh showed no additional decrease in I Ca(L) , suggesting the involvement of cyclic AMP in the eects of ET-1 on I Ca(L) . In contrast, 1 nM ET-1 further decreased I Ca(L) in the presence of 10 mM ACh, suggesting that ET-1 activates some additional mechanism(s) which inhibit I Ca(L) . The ET-1-induced I Ca(L) inhibition was abolished by protein kinase A inhibitory peptide (PKI, 20 mM) or H-89 (5 mM). However, the I Ca(L) inhibition was not aected by methylene blue (10 mM), suggesting a minor role for cyclic GMP in the eect of ET-1 under basal conditions. 5 ET-1 failed to inhibit I Ca(L) when the pipette contained GDPbS (200 mM). However, incubation of the cells with pertussis toxin (PTX, 5 mg ml 71 , 46 h) only reduced the ET-1-induced inhibition to 21.5+9.5%, whereas it abolished the inhibitory eect of ACh on I Ca(L) . 6 Intracellular perfusion of 8-bromo cyclicAMP (8-Br cyclicAMP, 500 mM) attenuated, but did not abolish the inhibitory eect of ET-1 on I Ca(L) . This 8-Br cyclicAMP-resistant component (17.5+14.4%, n=20) was not aected by combined application of 8-Br cyclicAMP with 8-bromo cyclicGMP (500 mM), ryanodine (1 mM) or phorbol-12-myristate-13-acetate (TPA; 50 nM). 7 In summary, ET-1 exerts negative chronotropic eects on the SA node via ET A -receptors. ET-1 inhibits both I Ca(L) and I K , and increases background K + current. The inhibition of I Ca(L) by ET-1 is mainly due to reduction of the cyclicAMP levels via PTX-sensitive G protein, but some other mechanism(s) also seems to be operative.

Modulation by Endothelin-1 of Spontaneous Activity and Membrane Currents of Atrioventricular Node Myocytes from the Rabbit Heart

“…[56]–[58]). Tanaka and colleagues demonstrated activation by ET-1 in rabbit SAN cells of an inwardly rectifying K + current similar to that seen here, also showing this to be Ba 2+ -sensitive and to carry outward current over potentials relevant to the diastolic potential range [21]. Drici and colleagues have demonstrated that ACh-induced AV block in guinea-pig hearts is prevented by tertiapin at concentrations including that used here, without non-selective effects of tertiapin on other important cardiac ionic currents [47].…”

“…Ono and colleagues also reported a modest reduction in delayed rectifier (I K ) tails across a wide range of voltages for spindle-shaped cells and up for voltages up to ∼0 mV for rod-shaped cells, accompanied in both SAN types by positively shifted (∼+5–+14 mV) I K activation [23]. Tanaka and colleagues also showed suppression of SAN I K by ET-1, though they did not study the voltage-dependence of the effect [21]. The activation V 0.5 values under control conditions for the tail currents investigated in this study are within the range reported previously for AVN I Kr [33], [41].…”

“…Data from patients with angina pectoris have shown left bundle-branch block to be associated with raised ET-1 levels, suggesting that the hormone may be involved in conduction abnormalities [20]. Consistent with an ability of ET-1 to exert a direct effect on the pacemaker-conduction system, experiments on cells isolated from the rabbit SAN have demonstrated that ET-1 produces a negative chronotropic effect that is associated with direct ion channel modulation [21]–[23]. By contrast, to our knowledge, there is no current information available concerning direct effects of this peptide hormone on the AVN.…”

Endothelin‐1 Modulates the Arrhythmogenic Activity of Pulmonary Veins

“…However, in the PV cardiomyocytes with pacemaker activity, endothelin‐1 was demonstrated to have an antiarrhythmic potential due to decreasing the PV beating rates and triggered activity. Similar to that in this study, endothelin‐1 also has been shown to decrease the beating rates in rabbit sinoatrial pacemaker cells with the inhibition of the I Ca‐L 25 . These results suggest the complex effects of endothelin‐1 on PV cardiomyocytes with and without pacemaker activity.…”

“…Endothelin‐1 has negative chronotropic effects on guinea pig atria 24 and also on rabbit sinoatrial nodal cells 25 . In tissue experiments, we found a negative chronotropic effect in the PVs.…”