Vasoactive agents and the mesenteric microcirculation

Disparity in osmolarity-induced vascular reactivity

Conventional peritoneal dialysis solutions (PDS) are vasoactive. This study was conducted to identify vasoactive components of PDS and to describe quantitatively such vasoactivity. Anesthetized nonheparinized rats were monitored continuously for hemodynamics while the microvasculature of the jejunum was studied with in vivo intravital microscopy. In separate experiments, vascular reactivity of rat endothelium-intact and -denuded aortic rings (2 mm) was studied ex vivo in a standard tissue bath. In both studies, suffusion of the vessels was performed with filter-sterilized isotonic and hypertonic solutions that contained glucose or mannitol as osmotic agents. PDS served as a control (Delflex 2.25%). Hypertonic glucose and mannitol solutions produced a significant vascular reactivity in aortic rings and instantaneous and sustained vascular relaxation at all levels of the intestinal microvasculature. Similarly, lactate that was dissolved in a low-pH isotonic physiologic salt solution produced significant force generation in aortic rings. Whereas isotonic glucose and mannitol solutions had no vasoactivity in aortic rings, isotonic glucose produced a selective, insidious, and time-dependent vasodilation in the intestinal premucosal arterioles (18 +/- 0.2% of baseline), which was not observed in the larger inflow arterioles (100 mum). This isotonic glucose-mediated vascular relaxation can be attenuated by approximately 50% with combined adenosine A(2a) and A(2b) receptor antagonists and completely abolished by adenosine A(1) receptor inhibition. By using two different experimental techniques, this study demonstrates that hyperosmolality and lactate are the major vasoactive components of clinical peritoneal dialysis solutions. The pattern and the magnitude of such reactivity are dependent on vessel size and on the solutes' metabolic activity. Low pH of conventional PDS is not a vasoactive component by itself but renders lactate vasoactive. Energy-dependent transport of glucose into cells mediates vasodilation of small visceral arterioles by an adenosine receptor-mediated mechanism and constitutes a significant fraction of PDS-mediated vascular reactivity in the visceral microvasculature.

Arginine vasopressin reduces intestinal oxygen supply and mucosal tissue oxygen tension

We investigated intestinal oxygen supply and mucosal tissue PO2 during administration of increasing dosages of continuously infused arginine vasopressin (AVP) in an autoperfused, innervated jejunal segments in anesthetized pigs. Mucosal tissue PO2 was measured by employing two Clark-type surface oxygen electrodes. Oxygen saturation of jejunal microvascular hemoglobin was determined by tissue reflectance spectrophotometry. Microvascular blood flow was assessed by laser-Doppler velocimetry. Systemic hemodynamic variables, mesenteric venous and systemic acid-base and blood gas variables, and lactate measurements were recorded. Measurements were performed at baseline and at 20-min intervals during incremental AVP infusion (n = 8; 0.007, 0.014, 0.029, 0.057, 0.114, and 0.229 IU.kg(-1).h(-1), respectively) or infusion of saline (n=8). AVP infusion led to a significant (P < .05), dose-dependent decrease in cardiac index (from 121 +/- 31 to 77 +/- 27 ml.kg(-1).min(-1) at 0.229 IU.kg(-1).h(-1)) and systemic oxygen delivery (from 14 +/- 3 to 9 +/- 3 ml.kg(-1).min(-1) at 0.229 IU.kg(-1).h(-1)) concomitant with an increase in systemic oxygen extraction ratio (from 31 +/- 4 to 48 +/- 10%). AVP decreased microvascular blood flow (from 133 +/- 47 to 82 +/- 35 perfusion units at 0.114 IU.kg(-1).h(-1)), mucosal tissue PO2 (from 26 +/- 7 to 7 +/- 2 mmHg at 0.229 IU.kg(-1).h(-1)), and microvascular hemoglobin oxygen saturation (from 51 +/- 9 to 26 +/- 12% at 0.229 IU.kg(-1).h(-1)) without a significant increase in mesenteric venous lactate concentration (2.3 +/- 0.8 vs. 3.4 +/- 0.7 mmol/l). We conclude that continuously infused AVP decreases intestinal oxygen supply and mucosal tissue PO2 due to a reduction in microvascular blood flow and due to the special vascular supply in the jejunal mucosa in a dose-dependent manner in pigs.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.