Functional receptors for fish neuropeptide urotensin II in major rat arteries

Receptor binding of fish neuropeptide urotensin II (UII) was characterized in membranes isolated from major rat arteries. Monoiodinated UII radioligand (125I-UII) was prepared and purified using high-pressure liquid chromatography (HPLC). The contractile potency of iodinated UII (I-UII) on rat thoracic aorta strips was somewhat lower than that of native UII. The binding of 125I-UII to the membrane preparations of rat thoracic aorta was saturable, specific and time-dependent. Scatchard analysis indicated a single population of binding sites with an apparent dissociation constant of 5.9 x 10(-9) M. The calculated maximal number of binding sites was about 155 fmol/mg protein. The specific binding to the membrane preparations from the abdominal aorta and mesenteric artery was about 27 and 8%, respectively, of those in the thoracic aorta, which corresponds to the order of contractile potency of UII on rat blood vessels: thoracic aorta greater than abdominal aorta greater than mesenteric artery. The displacement of 125I-UII binding by the UII peptide or its fragments (UII-(5-12), UII-(6-12) and UII-(6-11] were also comparable to their contractile effects on rat thoracic aorta strips (UII greater than UII-(5-12) greater than UII-(6-12) much greater than UII-(6-11]. These results suggest that the fish neuropeptide, UII, can induce contraction of rat vascular tissue by interacting with its functional receptors.