CARDIOVASCULAR ACTIONS OF FROG UROTENSIN-II IN THE FROG, RANA-CATESBEIANA

The effects of synthetic frog urotensin II on cardiac output and arter ial blood pressure and on the motility of isolated vascular smooth mus cle were investigated in the bullfrog, Rana catesbeiana. Bolus injecti ons of frog urotensin II (100 nmol/kg) into the left systemic arch pro duced a rapid and sustained fall in blood flow through the right branc h of the truncus arteriosus (to 62 +/- 5% of preinjection values; n = 8). The response exhibited strong tachyphylaxis. There were no signifi cant effects on heart rate and central arterial blood pressure but the fact that a fall in cardiac output was not accompanied by a fall in p ressure suggests that the peptide produced an increase in peripheral v ascular resistance. Rings of vascular smooth muscle from the proximal and distal regions of the left and right systemic arches responded to urotensin II with sustained and concentration-dependent contractions. The tissues from the different regions did not significantly differ in their maximum response and sensitivity to the peptide (EC(50) values from 4.6 x 10(-9) to 6.5 x 10(-9) M; n = 6). Acetylcholine (3 x 10(-8) to 3 x 10(-6) M) significantly (P < 0.05) relaxed the rings in an end othelium-dependent manner but urotensin II did not produce relaxation at any concentration tested. The contractile effect of urotensin II (1 0(-7) M) was not affected by preincubation of the rings with atropine, tetrodotoxin, and somatostatin-14 but indomethacin produced a signifi cant (P < 0.05) decrease in the amplitude of the contractions of the s ystemic arch (to 18 +/- 2% of control values; n = 6). Arachidonic acid (10(-5) M) also produced a sustained contraction of the rings. The da ta suggest an involvement of prostaglandin synthesis in the contractil e action of urotensin II on isolated frog vascular smooth muscle but, unlike in mammals, urotensin II does not stimulate nitric oxide synthe sis in this tissue. (C) 1995 Academic Press, Inc.