Distribution of endothelin-1-receptor subtypes in rat portal vein.

Endothelin-1 (ET-1), a potent vasoactive peptide, was first isolated from cultured porcine endothelial cells. Subsequent studies revealed the existence of two additional related peptides, ET-2 and ET-3, and at least two distinct ET-receptor subtypes, ETA (selective for ET-1) and ETB (nonselective for ET isopeptides). These isopeptides and receptors are widely distributed in many tissues and are involved in numerous biological responses. The aim of this study was to identify the eventual distribution of the two distinct endothelin-receptor subtypes in isolated endothelium-denuded rat portal vein rings (PVRs) and strips (PVSs). BQ-123 (0.6, 1, and 6 microM) and PD-145065 (0.06, 0.1, 0.6, and 6 microM) were used to differentiate the subtypes because they are selective antagonists for ETA and nonselective antagonists for ETA-ETB receptors, respectively. To characterize the ET receptors further, sarafotoxin-S6c (a selective ETB-receptor agonist) and IRL-1038 (a selective ETB-receptor antagonist) were used. In PVRs, cumulative additions of ET-1 (0.1-100 nM) caused graded and slow contractions and potentiated spontaneous rhythmic contractions. The EC50 values and maximal response to 100 nM of ET-1 were 2.72 nM and 0.75 g, respectively (n = 7). PVSs showed ET-1 EC50 values very similar to those of PVRs, but Emax values to 100 nM of ET-1 were significantly lower (Emax = 0.33 g; n = 7). Moreover, ET-1 clearly increased the amplitude and frequency of spontaneous contractions in both types of specimens, although these were greater in the PVSs. Thirty-minute incubation with the selective ETA-receptor antagonist BQ-123 blunted ET-1-induced effects in PVS specimens but only weakly antagonized ET-1-induced contractions in PVRs. In contrast, the nonselective ETA-ETB-receptor antagonist PD-145065 significantly shifted the ET-1 concentration-response curve to the right in PVRs and partially inhibited ET-1 effects in PVSs. Moreover, sarafotoxin-S6c (0.1-100 nM) contracted PVRs and PVSs in a similar manner to ET-1; its effects were antagonized by IRL-1038 only at the PVR level. The differences observed in PVR and PVS specimens in response to agonists and antagonists of ET confirmed the great heterogeneity of endothelin-sarafotoxin receptors. In our experimental models, functionally ETB-like (or non-ETA) receptors seem mostly to mediate vasoconstriction.