Candesartan improves survival following severe hypovolemia in pigs; a role for the angiotensin II type 2 receptor?

Pre-treatment with the angiotensin receptor 1 blocker losartan protects renal blood flow and oxygen delivery after propofol-induced hypotension in pigs

Hypotensive events are strongly correlated to the occurrence of perioperative acute kidney injury, but the underlying mechanisms for this are not completely elucidated. We hypothesised that anaesthesia-induced hypotension causes renal vasoconstriction and decreased oxygen delivery via angiotensin II-mediated renal vasoconstriction. Pigs were anaesthetised, surgically prepared and randomised to vehicle/losartan treatment (0.15 mg*kg⁻¹). A deliberate reduction in arterial blood pressure was caused by infusion of propofol (30 mg*kg⁻¹) for 10 min. Renal function and haemodynamics were recorded 60 min before and after hypotension. Propofol induced hypotension in all animals (p < 0.001). Renal blood flow (RBF) and renal oxygen delivery (RDO₂) decreased significantly regardless of treatment but more so in vehicle-treated compared to losartan-treated (p = 0.001, p = 0.02, respectively). During recovery RBF and RDO₂ improved to a greater extent in the losartan-treated compared to vehicle-treated (+ 28 ml*min⁻¹, 95%CI 8–50 ml*min⁻¹, p = 0.01 and + 3.1 ml*min⁻¹, 95%CI 0.3–5.8 ml*min⁻¹, p = 0.03, respectively). Sixty minutes after hypotension RBF and RDO₂ remained depressed in vehicle-treated, as renal vascular resistance was still increased (p < 0.001). In losartan-treated animals RBF and RDO₂ had normalised. Pre-treatment with losartan improved recovery of renal blood flow and renal oxygen delivery after propofol-induced hypotension, suggesting pronounced angiotensin II-mediated renal vasoconstriction during blood pressure reductions caused by anaesthesia.

Angiotensin II during Experimentally Simulated Central Hypovolemia

Central hypovolemia, defined as diminished blood volume in the heart and pulmonary vascular bed, is still an unresolved problem from a therapeutic point of view. The development of pharmaceutical agents targeted at specific angiotensin II receptors, such as the non-peptidergic AT₂-receptor agonist compound 21, is yielding many opportunities to uncover more knowledge about angiotensin II receptor profiles and possible therapeutic use. Cardiovascular, anti-inflammatory, and neuroprotective therapeutic use of compound 21 have been suggested. However, there has not yet been a focus on the use of these agents in a hypovolemic setting. We argue that the latest debates on the effect of angiotensin II during hypovolemia might guide for future studies, investigating the effect of such agents during experimentally simulated central hypovolemia. The purpose of this review is to examine the role of angiotensin II during episodes of central hypovolemia. To examine this, we reviewed results from studies with three experimental models of simulated hypovolemia: head up tilt table test, lower body negative pressure, and hemorrhage of animals. A systemic literature search was made with the use of PubMed/MEDLINE for studies that measured variables of the renin–angiotensin system or its effect during simulated hypovolemia. Twelve articles, using one of the three models, were included and showed a possible organ-protective effect and an effect on the sympathetic system of angiotensin II during hypovolemia. The results support the possible organ-protective vasodilatory role for the AT₂-receptor during hypovolemia on both the kidney and the splanchnic tissue.

The angiotensin II type 2 receptor and the gastrointestinal tract

The renin-angiotensin system (RAS) is well known for its vital involvement in body fluid homeostasis and circulation. However, very little research has been devoted to the impact of this regulatory system on the gastrointestinal (GI) system. This is surprising because the GI tract is fundamental for the intake and excretion of fluid and electrolytes (and nutrients), and it accommodates a large proportion of bodily haemodynamics and host defence systems. The RAS is well expressed and active in the GI tract, although the exact roles for the key mediator angiotensin II (Ang II) and its receptors in general, and the type 2 (AT 2) receptor in particular, are not completely settled. There are several reports showing Ang II regulation of intestinal fluid and electrolyte transport. For example, mucosaprotective duodenal bicarbonate-rich secretion is inhibited by Ang II via type 1 (AT1) receptor-mediated facilitation of sympathoadrenergic activity, but this secretory process can also be stimulated by Ang II via AT2 receptors. Novel data from human oesophagus and jejunum suggest that the AT1 receptor mediates muscular contractions and that the AT2 receptor regulates epithelial functions. Data are accumulating suggesting involvement of AT1 and AT2 receptors in GI inflammation and carcinogenesis. The picture of the RAS and AT 2 receptor in the GI tract is, however, far from complete. Much more basic research is needed with regard to GI pathophysiology before concluding clinical significance and potential applicability of pharmacological interferences with the RAS.

Angiotensin II Receptor Expression Following Intestinal Transplantation in Mice

BACKGROUND

To further improve the success rate of intestinal transplantation there is a need to find early appearing indicators of rejection. The specific aim of this study was to compare Angiotensin (Ang) II type 1 receptor and Ang II type 2 receptor expression in relation to histological signs of rejection.

METHODS

Mice of the C57BL6 strain with syngeneic intestinal grafts were compared to mice subjected to allogeneic intestinal transplantation with BalbC strain as donors. Local expression of Ang II type 1 and 2 receptor was evaluated using rt-PCR and Western blot and compared to histological picture in grafts and native intestine.

RESULTS

The Ang II type 2 receptor protein expression was markedly up-regulated in the allogeneically transplanted graft from day 1 postoperatively. Histological signs of rejection were not seen until day 6.

CONCLUSION

Intestinal allograft transplantation in mice is associated with a marked up-regulation of the Ang II type 2 receptor. However, the detailed role of the renin-angiotensin system in the immune rejection following intestinal transplantation remains to be clarified.