Pharmacological endothelin receptor interaction does not occur in veins from ETB receptor deficient rats

Heterodimerization of G-protein coupled receptors can alter receptor pharmacology. ETA and ETB receptors heterodimerize when co-expressed in heterologous expression lines. We hypothesized that ETA and ETB receptors heterodimerize and pharmacologically interact in vena cava from wild-type (WT) but not ETB receptor deficient (sl/sl) rats. Pharmacological endothelin receptor interaction was assessed by comparing ET-1-induced contraction in rings of rat thoracic aorta and thoracic vena cava from male Sprague Dawley rats under control conditions, ETA receptor blockade (atrasentan, 10 nM), ETB receptor blockade (BQ-788, 100 nM) or ETB receptor desensitization (Sarafotoxin 6c, 100 nM) and ETA plus ETB receptor blockade or ETA receptor blockade plus ETB receptor desensitization. In addition, similar pharmacological ET receptor antagonism experiments were performed in rat thoracic aorta and vena cava from WT and sl/sl rats. ETA but not ETB receptor blockade or ETB receptor desensitization inhibited aortic and venous ET-1-induced contraction. In vena cava but not aorta, when ETB receptors were blocked (BQ-788, 100 nM) or desensitized (S6c, 100 nM), atrasentan caused a greater inhibition of ET-1-induced contraction. Vena cava from WT but not sl/sl rats exhibited similar pharmacological ET receptor interaction. Immunocytochemistry was performed on freshly dissociated aortic and venous vascular smooth muscle cells to determine localization of ETA and ETB receptors. ETA and ETB receptors qualitatively co-localized more strongly to the plasma membrane of aortic compared to venous vascular smooth muscle cells. Our data suggest that pharmacological ETA and ETB receptor interaction may be dependent on the presence of functional ETB receptors and independent of receptor location.