Agonist- and System-Dependent Arterial Effects of Endothelin ETA-Receptor Stimulation

MG Compeer1, JGR De Mey1,2. 1Dept. of Pharmacology, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands, 2Dept. of Cardiovascular and Renal Research, Institute of Molecular Medicine, University of Southern Denmark, Odense, Denmark

The 21 amino acid bicyclic endothelin peptides (ET) activate the 7 transmembrane domain ETA- and ETB-receptors and are proposed to be involved in several diseases. ET1 and ET2 differ in 2 amino acids in their N-terminal loop, bind to ETA and ETB with equal affinity and have been proposed to display comparable pharmacological properties. We recently observed significant differences in the apparent affinity of low molecular weight ETA-antagonists against arterial effects of ET1 and ET2 (Br J Pharmacol 2012, 166, 1833). Here we tested the hypothesis that these agonists stimulate different signal-transduction pathways in arteries from different vascular beds. We recorded vasomotor responses to 0.25 - 16 nM ET in isolated rat mesenteric resistance (MrA) and -basilar arteries (BA) pretreated with capsaicin and studied in the continuous presence of L-NAME and indomethacin in order to concentrate on arterial smooth muscle responses. Pharmacological tools were tested at concentrations that prevent arterial contractile responses to 40 mM K+ or 10 μM phenylephrine. In MrA and BA, ET1 and ET2 caused contractions, but Sarafotoxin 6c (ETB-agonist) failed to elicit a vasomotor response. In both tissues the potency and maximal effects of ET1 and ET2 did not differ but their potency was somewhat larger in BA than MrA. 1 nM felodipine (calcium antagonist) did not alter sensitivity to ETs and caused full and partial relaxation of ET1-induced contractions in MrA and BA, respectively. 10 μM OH-fasudil (inhibitor of Rho kinase, ROCK) did not alter sensitivity to ETs, relaxed only ET2-induced responses in MrA but relaxed ET1- and ET2-induced responses in BA. 10 μM U73122 (inhibitor of phopholipase C-β, PLC-β) on the other hand, relaxed ET1- and ET2-induced responses in MrA but only relaxed ET1-induced responses in BA. We conclude that arterial smooth muscle ETA-receptor activation displays agonist- and system-dependent properties. This should stimulate development of selective negative allosteric modulators for treatment of ET-related diseases.

Work performed in the frame of Top Institute Pharma project T2-301.