The Role of Endothelin and Oxidative Stress in the Slow Pressor Responses to Angiotensin II (ANG II).
Chronic infusion of subpressor doses of Ang II causes blood pressure to increase progressively over several days. The mechanisms underlying this response are unknown but may involve Ang II-induced generation of additional vasoconstrictor processes. In this study, we tested whether endothelin and/or oxidative stress are implicated in the slow pressor responses to Ang II. We infused either vehicle (group 1; n=6) or Ang II (group 2; n=5) intravenously at 5 ng/kg/min via osmotic pumps for 15 days into Sprague Dawley rats. In addition to the Ang II infusion, groups 3 and 4 (n=6 each) received 30 mg/kg/day of either losartan (an angiotensin AT1 receptor blocker) or bosentan (a blocker of both endothelin receptors, ETA and ETB) in their drinking water. We measured systolic blood pressure (SBP) during the infusion, and the levels of circulating Ang II and isoprostanes (a marker of oxidative stress) at the conclusion of the experiments. Rats infused with vehicle had no change in SBP (from 138±13 to 138±2 mmHg) and normal levels of Ang II (34.5±9 pg/ml) and isoprostanes (111±10 pg/ml). Ang II infusion increased SBP from 133±10 to 158±8 mmHg, as well as circulating levels of Ang II (144±65 pg/ml) and isoprostanes (156±19 pg/ml). Losartan treatment abolished Ang II induced increases in SBP (SBP went from 137±5 to 120±4 mmHg), and isoprostanes (115±15 pg/ml), without altering Ang II levels (101±30 pg/ml). Bosentan also blocked Ang II-induced increases in SBP (from 135±4 to 139±3) but did not alter the increased isoprostane levels (146±14 pg/ml). Surprisingly, bosentan blunted the increase in Ang II levels (51±10 pg/ml). In conclusion, low dose Ang II-induced increases in SBP and oxidant stress depend on the AT1 receptor. Endothelin receptor blockade also reduces SBP, but it does so independently of reducing oxidative stress (as measured by isoprostanes).