Orn5]URP acts as a pure antagonist of urotensinergic receptors in rat cortical astrocytes

Cultured rat astrocytes, which express functional urotensin II (UII)/UII-related peptide (URP) receptors (UT), represent a very suitable model to investigate the pharmacological profile of UII and URP analogs towards native UT. We have recently designed three URP analogs D-Trp4]URP, Orn5]URP and D-Tyr6]URP, that act as UT antagonists in the rat aortic ring bioassay. However, it has been previously reported that UII/URP analogs capable of inhibiting the contractile activity of UII possess agonistic activity on UT-transfected cells. In the present study, we have compared the ability of URP analogs to compete for 125 I]URP binding and to modulate cytosolic calcium concentration (Ca2+]c) in cultured rat astrocytes. All three analogs displaced radioligand binding: D-Trp4]URP and D-Tyr6]URP interacted with high- and low-affinity sites whereas Orn5]URP only bound high-affinity sites. D-Trp4]URP and D-Tyr6]URP both induced a robust increase in Ca2+]c in astrocytes while Orn5]URP was totally devoid of activity. Orn5]URP provoked a concentration-dependent inhibition of URP- and UII-evoked Ca2+]c increase and a rightward shift of the URP and UII dose-response curves. The present data indicate that D-Trp4]URP and D-Tyr6]URP, which act as UII antagonists in the rat aortic ring assay, behave as agonists in the Ca2+]c mobilization assay in cultured astrocytes, whereas Orn5]URP is a pure selective antagonist in both rat aortic ring contraction and astrocyte Ca2+]c mobilization assays.