Abstract
Endothelins (ETs) and sarafotoxins (SRTXs) are active isopeptides that have very similar structures and functions. All isoforms interact with two specific G-protein–coupled receptors, ETA and ETB. To characterize functional vascular ET receptors in the poisonous snake, Bothrops jararaca, cumulative concentration-response curves to ETs and SRTXs were performed in isolated aortic rings, in the absence and presence of selective ET receptor antagonists. Vascular expression of ET receptor messenger RNA (mRNA) was evaluated by reverse transcriptase (RT) polymerase chain reaction (PCR) analysis, and a fragment of the ETA receptor was cloned and sequenced. In vivo, ET-1 induced a dose-dependent biphasic response on anesthetized B. jararaca snakes. In vitro, ET-1, SRTX-b, ET-3, SRTX-c, and IRL-1620 induced concentration-dependent vasoconstriction, with a potency order suggesting the presence of typical ETA receptors. BQ-123, a selective ETA antagonist, inhibited contractions induced by ET-1 and SRTX-b with expected negative log of the dissociation constant, KB, (pKB) values for mixed ETA/ETB receptor populations. The nonselective ETA/ETB receptors antagonist, PD-142893, produced similar inhibition. The ETB antagonist, IRL-1038, potentiated contractile responses to SRTX-c. ET-1 and SRTX-c responses were also potentiated when aortic rings were pretreated with Nω-nitro-L-arginine methyl ester (L-NAME) plus indomethacin. Processing of the B. jararaca aortic first-strand complementary DNA, by RT-PCR with primers designed from the Gallus gallus ETA receptor sequence, enabled isolation, purification, cloning, and sequencing of a single band. The partial sequence of the B. jararaca ETA receptor showed a very high sequence similarity with ETA receptor sequences from chicken, rat, human, and Xenopus. In conclusion, vascular responses to SRTXs/ETs in the B. jararaca aorta are mediated predominantly, but not exclusively, by typical ETA receptors.
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