Role of endothelin receptor subtype B (ET-B) in myocardial ischemia

Previous work indicated that endothelin (ET) may be involved in the pathogenesis of myocardial ischemia, although the relative importance of the ET receptor subtypes is presently not clear. The purpose of this study was to determine the role of myocardial ET-B receptors in mediating ischemic/reperfusion damage in isolated rat hearts. Saturation binding analyses were conducted with [125I]ET-1 and [125I]IRL-1620 to assess changes in ET-A and ET-B receptor binding. Total ET receptor density (Bmax) was greater in atrial versus ventricular tissue. ET-A Bmax was 8 to 10-fold greater than ET-B Bmax. In ischemic and ischemic/reperfused atrial tissue neither the equilibrium dissociation constant (Kd) nor Bmax for ET-B receptors was changed. The ET-B receptor Kd in ischemic or ischemic/reperfused ventricular tissue was also unchanged. In ischemic ventricular tissue there was a trend towards an increased ET-B Bmax, which was accentuated after ischemia/reperfusion. No changes were found in ET-A Bmax or Kd in ischemic ventricular or atrial tissue. The physiological importance of this receptor subtype in ischemic myocardium was determined using the selective ET-B agonist, sarafotoxin S6c. In non-ischemic tissue no effect on coronary flow or function were observed with sarafotoxin S6c. Furthermore, no changes were seen in ischemic time to contracture or any of the reperfusion indexes of myocardial damage. The sarafotoxin S6c utilized was active as it inhibited [125I]ET-3 binding to ET-B receptors (Ki = 0.1 nM). Thus, the pro-ischemic effect of ET-1 seems to be mediated by ET-A receptors. ET-B receptors do not appear to play a role in the pathogenesis of myocardial ischemia.