Effects of endothelin-1 on epicardial coronary tone, coronary blood flow, ECG-ST change and regional wall motion in anesthetized dogs

Modulation by endothelin-1 of spontaneous activity and membrane currents of atrioventricular node myocytes from the rabbit heart

Background

The atrioventricular node (AVN) is a key component of the cardiac pacemaker-conduction system. Although it is known that receptors for the peptide hormone endothelin-1 (ET-1) are expressed in the AVN, there is very little information available on the modulatory effects of ET-1 on AVN electrophysiology. This study characterises for the first time acute modulatory effects of ET-1 on AVN cellular electrophysiology.

Methods

Electrophysiological experiments were conducted in which recordings were made from rabbit isolated AVN cells at 35–37°C using the whole-cell patch clamp recording technique.

Results

Application of ET-1 (10 nM) to spontaneously active AVN cells led rapidly (within ∼13 s) to membrane potential hyperpolarisation and cessation of spontaneous action potentials (APs). This effect was prevented by pre-application of the ET_A receptor inhibitor BQ-123 (1 µM) and was not mimicked by the ET_B receptor agonist IRL-1620 (300 nM). In whole-cell voltage-clamp experiments, ET-1 partially inhibited L-type calcium current (I_Ca,L) and rapid delayed rectifier K⁺ current (I_Kr), whilst it transiently activated the hyperpolarisation-activated current (I_f) at voltages negative to the pacemaking range, and activated an inwardly rectifying current that was inhibited by both tertiapin-Q (300 nM) and Ba²⁺ ions (2 mM); each of these effects was sensitive to ET_A receptor inhibition. In cells exposed to tertiapin-Q, ET-1 application did not produce membrane potential hyperpolarisation or immediate cessation of spontaneous activity; instead, there was a progressive decline in AP amplitude and depolarisation of maximum diastolic potential.

Conclusions

Acutely applied ET-1 exerts a direct modulatory effect on AVN cell electrophysiology. The dominant effect of ET-1 in this study was activation of a tertiapin-Q sensitive inwardly rectifying K⁺ current via ET_A receptors, which led rapidly to cell quiescence.

Flow-independent myocardial ischemia induced by endothelin-1: an NADH fluorescence analysis

The endothelin-1 (ET-1) is known to cause myocardial ischemia; however, whether this effect is entirely dependent on vasoconstriction is uncertain. The aim of this study was to characterize the myocardial ischemia after the intracoronary administration of endothelin-1, and compare it with that induced by coronary stenosis. In the left anterior descending coronary artery of 15 dogs, a mild inflow reduction (30%) was produced for 10 minutes using intracoronary endothelin-1 (46 +/- 33 pmol/min) or coronary stenosis. The hearts were rapidly cross-sectioned at short axial plane and freeze-clamped within 120 milliseconds using a specially developed device to visualize and quantify the area of ischemia (%IA) with NADH fluorescence photography. The %IA was larger in the endothelin-1 group than in the stenosis group (66 +/- 23 versus 18 +/- 18, P = 0.0005); furthermore, the ischemia was transmural in the ET-1 group, but limited to subendocardium in the stenosis group. ET-1 increased the coronary arterial resistance especially in subepicardial region and produced smaller ischemic foci in microcirculation. The mechanism of larger ischemia produced by ET-1 might depend on pro-ischemic effects on myocytes and vasoconstriction of the coronary microcirculation, predominantly in the subepicardium in vivo.

Endothelin and myocardial ischemia

Endothelin is a potent vasoconstrictor with a wide range of effects on the heart. Changes in myocardial and circulating levels of endothelin have been described in various experimental models of myocardial ischemia, and in humans with acute myocardial infarction and different forms of angina pectoris. The role played by endothelin in the different states of myocardial ischemia is unclear. However, myocardial damage has been shown to be reduced in several experimental models of myocardial infarction by administering agents that block the action of endothelin. The aim of this review article is to present the current literature concerning the interaction between endothelin and the various forms of myocardial ischemia, and to explore the significance of such interactions.