Angiotensin II mesenteric and renal vasoregulation: dissimilar modulatory effects with nitroprusside

Comparison Between Two Pharmacologic Strategies to Alleviate Rewarming Shock: Vasodilation vs. Inodilation

Rewarming from hypothermia is often challenged by coexisting cardiac dysfunction, depressed organ blood flow (OBF), and increased systemic vascular resistance. Previous research shows cardiovascular inotropic support and vasodilation during rewarming to elevate cardiac output (CO). The present study aims to compare the effects of inodilatation by levosimendan (LS) and vasodilation by nitroprusside (SNP) on OBF and global oxygen transport during rewarming from hypothermia. We used an in vivo experimental rat model of 4 h 15°C hypothermia and rewarming. A stable isotope-labeled microsphere technique was used to determine OBF. Cardiac and arterial pressures were monitored with fluid-filled pressure catheters, and CO was measured by thermodilution. Two groups were treated with either LS (n = 7) or SNP (n = 7) during the last hour of hypothermia and throughout rewarming. Two groups served as hypothermic (n = 7) and normothermic (n = 6) controls. All hypothermia groups had significantly reduced CO, oxygen delivery, and OBF after rewarming compared to their baseline values. After rewarming, LS had elevated CO significantly more than SNP (66.57 ± 5.6/+30% vs. 54.48 ± 5.2/+14%) compared to the control group (47.22 ± 3.9), but their ability to cause elevation of brain blood flow (BBF) was the same (0.554 ± 0.180/+81 vs. 0.535 ± 0.208/+75%) compared to the control group (0.305 ± 0.101). We interpret the vasodilator properties of LS and SNP to be the primary source to increase organ blood flow, superior to the increase in CO.

Altered systemic hemodynamic and baroreflex response to angiotensin II in postural tachycardia syndrome

BACKGROUND

Postural tachycardia syndrome (POTS) is characterized by excessive orthostatic tachycardia and significant functional disability. We have previously found that patients with POTS have increases in plasma angiotensin II (Ang II) that are twice as high as healthy subjects despite normal blood pressures (BPs). In this study, we assess systemic and renal hemodynamic and functional responses to Ang II infusion in patients with POTS compared with healthy controls.

METHODS AND RESULTS

Following a 3-day sodium-controlled diet, we infused Ang II (3 ng/kg per minute) for 1 hour in patients with POTS (n=15) and healthy controls (n=13) in the supine position. All study subjects were women with normal BP. Ages were similar for patients with POTS and controls (mean±SEM, 30±2 versus 26±1 years; P=0.11). We measured the changes from baseline mean arterial pressure, renal plasma flow, plasma renin activity, aldosterone, urine sodium, and baroreflex sensitivity in both groups. In response to Ang II infusion, patients with POTS had a blunted increase compared with controls in mean arterial pressure (10±1 versus 14±1 mm Hg, P=0.01) and diastolic BP (9±1 versus 13±1 mm Hg, P=0.01) but not systolic BP (13±2 versus 15±2 mm Hg, P=0.40). Renal plasma flow decreased similarly with Ang II infusion in patients with POTS versus controls (-166±20 versus -181±17 mL/min per 1.73 kg/m(2), P=0.58). Postinfusion, the decrease in plasma renin activity (-0.9±0.2 versus -0.6±0.2 ng/mL per hour, P=0.43) and the increase in aldosterone (17±1 versus 15±2 pg/mL, P=0.34) were similar in both groups. The decrease in urine sodium excretion was similar in patients with POTS and controls (-49±12 versus -60±16 mEq/g creatinine, P=0.55). The spontaneous baroreflex sensitivity at baseline was significantly lower in patients with POTS compared with controls (10.1±1.2 versus 16.8±1.5 ms/mm Hg, P=0.003), and it was further reduced with Ang II infusion.

CONCLUSIONS

Patients with POTS have blunted vasopressor response to Ang II and impaired baroreflex function. This impaired vasoconstrictive response might be exaggerated with upright posture and may contribute to the subsequent orthostatic tachycardia that is the hallmark of this disorder. Clinical Trial Registration- URL: http://www.clinicaltrials.gov. Unique identifier: NCT00962949.

Vasoconstriction and blood flow responses in visceral arteries to mental task in humans

The vascular responses to mental task in visceral arteries in humans have not been elucidated. We observed the responses in the renal (RA) and superior mesenteric (SMA) arteries to mental stress, using simultaneous pulsed and echo Doppler ultrasound flowmetry. Nine healthy females performed a computerized colour word conflict test (CWT) for 3 min. The mean blood velocity (MBV) in the right RA and SMA, heart rate (HR) and blood pressure were measured. The mean arterial pressure (MAP) was divided by the flow velocity to assess the vascular resistance (VR). The CWT significantly increased the MAP, HR and VR in both arteries from the first minute. During the CWT, flow in the RA decreased significantly at the third minute relative to baseline, while flow in the SMA showed no significant change from the first to the third minute. The degree of vasoconstriction in the RA bed was greater than that in the SMA bed. These results suggest that the mental task causes vasoconstriction in visceral arteries, and imply that it induces differential blood flow and vascular responses in visceral arteries.

Splanchnic blood flow in low-flow states

IMPLICATIONS

Insufficient splanchnic blood flow in critically ill patients is the result of a multitude of different diseases, treatment modalities and their interplay, and is associated with increased morbidity and mortality. A combination of diminished and heterogeneous mesenteric blood flow, impaired or exhausted regulatory mechanisms and adverse drug effects may coexist with normal systemic hemodynamics.

Effect of extracellular volume expansion and surgical stress on splanchnic blood flow and cardiac output in anesthetized rats: role of nitric oxide

In a normal volume state, surgical stress decreases rather than increases nitric oxide (NO) production in the vascular system. In our studies, the effect of minor and major surgical stress and three different degrees of volume expansion on systemic and splanchnic circulatory parameters and on the NO dependence of the circulation have been investigated. When the degree of volume expansion was increased, cardiac output and organ blood flow increased without significant change in vascular resistances. Major surgical stress reduced the increase in cardiac output and organ blood flow elicited by the volume expansion. NO synthase (NOS) inhibition significantly increased blood pressure and total peripheral resistance (TPR) and decreased cardiac output in all groups of animals. As the degree of volume expansion was increased, the NO dependence of the circulation in the surgically less- and more-stressed animals was inversely influenced in some cases. With the three degrees of volume expansion (20, 40, and 60 ml/kg), the NOS inhibition increased the TPR from 30.7 R/kg +/- 1.90 to 73.6 R/kg +/- 5.00, from 20.7 R/kg +/- 1.43 to 66.7 R/kg +/- 3.88, and from 19.9 R/kg +/- 1.25 to 49.1 R/kg +/- 3.84 in the surgically less-stressed animals and from 38.6 R/kg +/- 2.14 to 59.8 R/kg +/- 5.62, from 31.9 R/kg +/- 2.70 to 81.7 R/kg +/- 9.89, and from 29.1 R/kg +/- 2.49 to 91.1 R/kg +/- 6.36 in the surgically more-stressed animals. Volume expansion increases the NO dependence of the vascular resistance in the surgically more-stressed animals but decreases it in the surgically less-stressed animals.

Splanchnic vasoconstriction by angiotensin II is arterial pressure dependent

BACKGROUND

Our hypothesis was that splanchnic vasoconstriction by exogenous angiotensin II (Ang II) is significantly potentiated by local mechanisms increasing vasomotor tone and that splanchnic tissue oxygenation during administration of Ang II is perfusion pressure dependent. The aim was to study local splanchnic circulatory effects and tissue oxygenation during intravenous infusion of Ang II at different levels of regional arterial driving pressure in a whole-body large animal model.

METHODS

Ang II was infused in incremental doses (0-200 microg x h-1) in anaesthetised instrumented pigs (n=8). Mean superior mesenteric arterial pressure (PSMA) was adjusted by a local variable perivascular occluder. Perivascular ultrasound and laser-Doppler flowmetry were used for measurements of mesenteric venous blood flow and superficial intestinal blood flow, respectively. Intestinal oxygenation was evaluated by oxygen tissue tension (PtiO2) and lactate fluxes.

RESULTS

Ang II produced prominent and dose-dependent increases in mesenteric vascular resistance (RSMA) when the intestine was exposed to systemic arterial pressure, but Ang II increased RSMA only minimally when PSMA was artificially kept constant at a lower level (50 mmHg) by the occluder. Although Ang II decreased PtiO2 at a PSMA of 50 mmHg, splanchnic lactate production was not observed.

CONCLUSION

We demonstrate that splanchnic vasoconstriction by exogenous Ang II is dependent on arterial driving pressure, suggesting significant potentiation through autoregulatory increases in vasomotor tone. Intestinal hypoxaemia does not seem to occur during short-term infusion of Ang II in doses that significantly increases systemic arterial pressure.