A comparison of angiotensin II and angiotensin III as vasoconstrictors in the mesenteric circulation of dogs

Angiotensin II increases portosystemic collateral resistance in portal hypertension

The decreased vasoconstrictive response of the splanchnic vasculature in portal hypertension (PHT) to angiotensin II (ANGII) is newly established. This could explain the limited ability of PHT patients to compensate in hypovolemic shock. However, the effect of ANGII upon portosystemic collateral resistance (Rc) is not known. We hypothesized that ANGII could directly effect Rc and thus change portal venous pressure (PVP). Chronic PHT was induced in New Zealand white rabbits by partial portal vein ligation 3 weeks prior to study. Splanchnic blood flow and portosystemic shunt (PSS) were measured simultaneously in six normal rabbits and then in six PHT rabbits at baseline and during intravenous ANGII infusion at 1.0 microgram/kg/min. Flow and resistance were standardized to 100 g small intestine weight. Superior mesenteric artery flow (Qsma) in normal rabbits was 64.9 +/- 3.6 ml/min, increased to 134.6 +/- 13.5 ml/min in the PHT animals (P less than 0.001) and was reduced 30% (P less than 0.05 vs PHT baseline) with ANGII. Concomitantly, PVP in the PHT animals was twice normal, 7.0 +/- 0.32 vs 14.4 +/- 0.55 mm Hg (P less than 0.001) and rose slightly with ANGII. The high PSS in PHT (84 +/- 6.0%, P less than 0.001 vs normal) was not affected significantly by ANGII infusion. Rc in the PHT rabbits rose 50% from 0.06 +/- 0.001 to 0.09 +/- 0.01 mm Hg/ml/min (P less than 0.001) with ANGII. This is the first evidence for a vasoconstrictive response in portosystemic collaterals during ANGII infusion. This change in collateral resistance causes both PVP and PSS to remain pathologically elevated in PHT despite a fall in portal inflow, thus predisposing to repeat variceal bleeding.

Decreased mesenteric vascular response to angiotensin II in portal hypertension

We studied the mesenteric and systemic vascular response to angiotensin II in normotensive and portal hypertensive (PHT) rabbits, because of the documented poor tolerance of hemorrhagic shock in PHT. Normally, the hemodynamic response to angiotensin II (AII) is characterized by selective and disproportionate splanchnic vasoconstriction. We postulated that the response to AII could be diminished in PHT. Chronic PHT was induced by partial portal vein ligation 3 weeks prior to graded angiotensin II infusion. Baseline hemodynamic measurements showed markedly elevated portal pressure (PPV) and superior mesenteric artery blood flow (QSMA) compared with those of normotensive animals (P less than 0.01). Superior mesenteric artery resistance (RSMA) was markedly reduced in PHT compared to that in controls (P less than 0.05). Angiotensin II infusion in normals resulted in a marked selective rise in RSMA compared with the rise in systemic resistance (RSYS) (P less than 0.01). AII infusion in PHT resulted in a rise in RSMA and RSYS, but the disproportionate in RSMA was attenuated. Furthermore, in both normal and PHT, AII caused a significant rise in PPV (P less than 0.01). We conclude that the findings indicate that the splanchnic vasoconstrictive response to AII is substantially impaired in PHT, and that AII will paradoxically cause a rise in PPV, possibly aggravating the tendency to hemorrhage in PHT.

Classification of angiotensin receptors in rat isolated uterus, portal vein, and aorta using a slowly dissociating antagonist [Sar1,Ile8]ANG II for receptor blockade

Receptor blockade with the slowly dissociating angiotensin (ANG) antagonist, [Sar1,Ile8]ANG II, was used to determine dissociation constants (Kd) for angiotensins II and III at receptors in rat isolated uterus, portal vein and aorta. The Kd values obtained were 1) ANG II: uterus, 2.2 +/- 1.2 X 10(-8)M; portal vein, 8.5 +/- 2.5 X 10(-9)M; aorta, 7.8 +/- 1.7 X 10(-9)M; and 2) ANG III: uterus, 5.7 +/- 1.7 X 10(-7)M; portal vein, 4.5 +/- 3.1 X 10(-7)M; aorta, 1.4 +/- 0.5 X 10(-7)M. For either ANG II or ANG III, there were no clearly significant differences in the Kd values obtained for the smooth muscle tissues investigated. The derived Kd values provided a relationship between receptor occupancy and response that illustrated that the two peptides had similar efficacies in all three tissues. The method provides a quantitative description of receptors and suggests that the angiotensin receptor is of the same type in all three tissues.

Distinctive effect of angiotensin II on prostaglandin production in dog renal and femoral arteries

The effect of angiotensin II (Ang II) on prostaglandin (PG) production in dog renal and femoral vasculature was examined in vivo and in vitro. In pentobarbital anesthetized dogs, the reduction of blood flow induced by intra-arterial infusion of Ang II was potentiated by pre-treatment with indomethacin (5 mg/kg) in the renal but not the femoral vasculature. Isolated renal and femoral arterial strips were incubated and the release of PGE2 and PGI2 (as 6-keto-PGF1 alpha) into the medium was measured by radioimmunoassay. Basal PGE2 and PGI2 production by renal and femoral arterial strips was approximately the same. PGI2 production was predominant for both strips. Ang II stimulated PG production in renal but not femoral arteries. In the renal artery, Ang II-induced PG production was inhibited by indomethacin (10(-6) M), mepacrine (10(-4) M) and saralasin (10(-6) M). These results suggest that Ang II stimulates PG production by the renal artery per se and the Ang II receptor is linked to phospholipase A2 in the renal but not the femoral artery.