Lack of involvement of endothelin-1 in angiotensin II-induced contraction of the isolated rat tail artery
1. The contribution of endothelin-1 (ET-1) to angiotensin II (Ang II)-mediated contraction of the isolated rat tail artery was assessed with measurements of tension, and cytosolic calcium ([Ca(2+)](i)). The distribution of the AT(1) receptor was studied with RT - PCR and immunohistochemistry. 2. Ang II induced an endothelium-independent contraction (pEC(50) 7.95+/-0.06 and E(max): 0.46 g+/-0.05 with endothelium vs 7.81+/-0.02 and 0.41 g+/-0.07 without endothelium; P>0.05). Ang II (0.003 - 0.3 microM)-induced a non-sustained contraction of endothelium-intact preparations which was not antagonized by BQ-123 (1 microM), but was inhibited by losartan (10 nM). In addition, the maximal contraction induced by ET-1 (0.1 microM) could be further increased by the addition of 0.1 microM Ang II. 3. Ang II (0.001 - 0.3 microM) elevated [Ca(2+)](i) in single vascular smooth muscle cells (VSMCs) in a dose-dependent manner (pEC(50) 9.12+/-0.26) and the Ang II-induced increases in [Ca(2+)](i) were not affected by a Ca(2+)-free solution, but were abolished by pretreatment with caffeine (5 mM). Ang II did not increase [Ca(2+)](i) in endothelial cells. ET-1 (0.1 microM) increased [Ca(2+)](i) in single VSMCs in a normal Ca(2+) containing physiological saline solution (PSS), but not in a Ca(2+)-free solution. 4. Ang II-induced contraction was insensitive to inhibition by nifedipine (0.1 microM), an antagonist of L-type voltage-gated Ca(2+) channels, and SK&F96365 (10 microM), which blocks non-selective cation channels, whereas that to ET-1 was inhibited by SK&F69365. 5. RT - PCR data indicate the expression of AT(1A) and AT(1B) on both VSMCs and endothelial cells, but immunohistochemical evidence illustrates that the AT(1) is located primarily on VSMCs. 6. These results indicate that endothelium-derived ET-1 is not involved in the Ang II-mediated vasoconstriction of the rat tail artery and that Ang II- and ET-1-mediated VSM contractions utilize distinct pathways.