Endothelin-1 causes systemic vasodilatation in anaesthetised turtles(Trachemys scripta) through activation of ETB-receptors

Abstract

The effects of endothelin-1 (ET-1) on systemic and pulmonary circulation were investigated in anaesthetised freshwater turtles (Trachemys scripta) instrumented with arterial catheters and blood flow probes. Bolus intra-arterial injections of ET-1 (0.4-400 pmol kg(-1)) caused a dose-dependent systemic vasodilatation that was associated with a decrease in systemic pressure (P(sys)) and a rise in systemic blood flow (Q(sys)), causing systemic conductance (G(sys)) to increase. ET-1 had no significant effects on the pulmonary vasculature, heart rate (fh) or total stroke volume (Vs(tot)). This response differs markedly from mammals, where ET-1 causes an initial vasodilatation that is followed by a pronounced pressor response. In mammals, the initial dilatation is caused by stimulation of ET(B)-receptors, while the subsequent constriction is mediated by ET(A)-receptors. In the turtles, infusion of the ET(B)-receptor agonist BQ-3020 (150 pmol kg(-1)) elicited haemodynamic changes that were similar to those of ET-1, and the effects of ET-1 were not affected by the ET(A)-antagonist BQ-610 (0.15 micromol kg(-1)). Conversely, all effects of ET-1 were virtually abolished after specific ET(B)-receptor blockade with the ET(B)-antagonist BQ-788 (0.15 micromol kg(-1)). The subsequent treatment with the general ET-receptor antagonist tezosentan (15.4 micromol kg(-1)) did not produce effects that differed from the treatment with ET(B)-antagonist, and the blockade of ET-1 responses persisted. This present study indicates, therefore, that ET(B)-receptors are responsible for the majority of the cardiovascular responses to ET-1 in Trachemys.