Adenosine regulation of mesenteric vasodilation

Adenosine Deaminase (ADA)-Deficient Severe Combined Immune Deficiency (SCID): Molecular Pathogenesis and Clinical Manifestations

Deficiency of adenosine deaminase (ADA, EC3.5.4.4), a housekeeping enzyme of purine metabolism encoded by the Ada gene, is a cause of human severe combined immune deficiency (SCID). Numerous deleterious mutations occurring in the ADA gene have been found in patients with profound lymphopenia (T^- B^- NK^-), thus underscoring the importance of functional purine metabolism for the development of the immune defense. While untreated ADA SCID is a fatal disorder, there are multiple life-saving therapeutic modalities to restore ADA activity and reconstitute protective immunity, including enzyme replacement therapy (ERT), allogeneic hematopoietic stem cell transplantation (HSCT) and gene therapy (GT) with autologous gene-corrected hematopoietic stem cells (HSC). We review the pathogenic mechanisms and clinical manifestations of ADA SCID.

Connexin 36 mediates blood cell flow in mouse pancreatic islets

The insulin-secreting β-cells are contained within islets of Langerhans, which are highly vascularized. Blood cell flow rates through islets are glucose-dependent, even though there are no changes in blood cell flow within in the surrounding exocrine pancreas. This suggests a specific mechanism of glucose-regulated blood flow in the islet. Pancreatic islets respond to elevated glucose with synchronous pulses of electrical activity and insulin secretion across all β-cells in the islet. Connexin 36 (Cx36) gap junctions between islet β-cells mediate this synchronization, which is lost in Cx36 knockout mice (Cx36(-/-)). This leads to glucose intolerance in these mice, despite normal plasma insulin levels and insulin sensitivity. Thus, we sought to investigate whether the glucose-dependent changes in intraislet blood cell flow are also dependent on coordinated pulsatile electrical activity. We visualized and quantified blood cell flow using high-speed in vivo fluorescence imaging of labeled red blood cells and plasma. With the use of a live animal glucose clamp, blood cell flow was measured during either hypoglycemia (∼50 mg/dl) or hyperglycemia (∼300 mg/dl). In contrast to the large glucose-dependent islet blood velocity changes observed in wild-type mice, only minimal differences are observed in both Cx36(+/-) and Cx36(-/-) mice. This observation supports a novel model where intraislet blood cell flow is regulated by the coordinated electrical activity in the islet β-cells. Because Cx36 expression and function is reduced in type 2 diabetes, the resulting defect in intraislet blood cell flow regulation may also play a significant role in diabetic pathology.

Use of rapid sampling microdialysis for intraoperative monitoring of bowel ischemia

PURPOSE

Intestinal ischemia is a major cause of anastomotic leak and death and remains a clinical challenge as the physician relies on several nonspecific signs, biologic markers, and radiologic studies to make the diagnosis. This study used rapid sampling online microdialysis to evaluate the biochemical changes occurring in a segment of human bowel during and after resection, and assessed for the feasibility and reproducibility of this technique in monitoring intestinal ischemia.

METHODS

A custom made, rapid sampling online microdialysis analyzer was used to monitor the changes in the bowel wall of specimens being resected intraoperatively. Two patients were recruited for the pilot study to optimize the analyzer and seven patients undergoing colonic resections were recruited for the data collection and analysis.

RESULTS

The concentration of glucose in the extracellular bowel wall fluid decreased transiently after division of individual feeding arteries followed by a rebound increase in the concentration back to baseline concentrations. After completion of resection, glucose concentrations continued to decrease while lactate concentrations increased constantly.

CONCLUSION

Rapid sampling microdialysis was feasible in the clinical environment. These results suggest that tissue responds to ischemic insult by mobilizing glucose stores which later decrease again, whereas lactate concentrations constantly increased.

Mesenteric hemodynamic response to circulatory shock

PURPOSE OF REVIEW

The mesenteric hemodynamic response to circulatory shock is substantial and asymmetrical; the vasoconstrictive response disproportionately affects the mesenteric organs. The cardiac output is sustained partially, at no cost in nutrient flow to the mesenteric organs, by vasoconstriction of the mesenteric veins, resulting in the "autotransfusion" of up to 30% of the circulating blood volume into the systemic circulation.

RECENT FINDINGS

Hemorrhagic or cardiogenic shock also results in decreased perfusion pressure, prompting selective vasoconstriction of the mesenteric arterioles to maintain perfusion pressure of the vital organs, here at the selective expense of the mesenteric organs. Septic shock may be associated with increased or decreased mesenteric blood flow but is characterized by increased oxygen consumption, exceeding the capability of mesenteric oxygen delivery.

SUMMARY

The response to any of these conditions can, variably and unpredictably, cause hemorrhagic gastric stress erosions, nonocclusive mesenteric ischemia of the small bowel, ischemic colitis, ischemic hepatitis, acalculous cholecystitis, and/or ischemic pancreatitis. Injury to the mesenteric organs can also initiate the systemic inflammatory response syndrome and, consequently, multiple organ failure.

Glucose-induced islet blood flow increase in rats: interaction between nervous and metabolic mediators

This study investigated the mechanisms for glucose-induced islet blood flow increase in rats. The effects of adenosine, adenosine receptor antagonists, and vagotomy on islet blood flow were evaluated with a microsphere technique. Vagotomy prevented the islet blood flow increase expected 3, 10, and 20 min after injection of glucose, whereas theophylline (a nonspecific adenosine receptor antagonist) prevented the islet blood flow increase from occurring 10 and 20 min after glucose administration. Administration of selective adenosine receptor antagonists suggested that the response to theophylline was mediated by A1 receptors. Exogenous administration of adenosine did not affect islet blood flow, but local accumulation of adenosine, induced by the adenosine uptake inhibitor dipyridamole, caused a doubling of islet blood flow. In conclusion, the increased islet blood flow seen 3 min after induction of hyperglycemia is caused by the vagal nerve, whereas the increase in islet blood perfusion seen at 10 and 20 min after glucose administration is caused by both the vagal nerve and adenosine.

Adenosine triphosphate-magnesium dichloride during hyperdynamic porcine endotoxemia: effects on hepatosplanchnic oxygen exchange and metabolism

OBJECTIVE

To assess the effects of adenosine triphosphate-magnesium dichloride (ATP-MgCl2) on systemic and hepatosplanchnic hemodynamics, oxygen exchange, and energy metabolism over 24 hrs of hyperdynamic normotensive porcine endotoxemia.

DESIGN

Prospective, randomized, controlled experimental study with repeated measures.

SETTING

Investigational animal laboratory.

SUBJECTS

Seventeen pigs were divided into two groups: eight animals receiving endotoxin served as a control group and nine animals received endotoxin (lipopolysaccharide) and ATP-MgCl2.

INTERVENTIONS

Pigs were anesthetized, mechanically ventilated, and instrumented. Endotoxemia was achieved by continuous intravenous infusion of Escherichia coli lipopolysaccharide. Animals were resuscitated by hetastarch targeted to maintain mean arterial pressure of >75 mm Hg. Twelve hours after the start of the endotoxin infusion, ATP-MgCl2, or its vehicle, were administered for 12 hrs.

MEASUREMENTS AND MAIN RESULTS

Mean arterial pressure was maintained in the control group because of a sustained increase in cardiac output achieved by fluid resuscitation, whereas ATP-MgCl2 significantly decreased mean arterial pressure because of further systemic vasodilatation. ATP-MgCl2 markedly increased portal venous flow. In contrast to the controls, hepatic arterial flow remained unchanged until the end of the experiment, despite the further increase in cardiac output. The ileal mucosal-arterial PCO2 gap (Delta PCO2) progressively increased (p <.05) in control animals, whereas it was restored to prelipopolysaccharide levels during ATP-MgCl2 infusion. Changes in Delta PCO2 correlated with those of portal vein blood flow in these animals (r = -.68, p <.05). Moreover, ATP-MgCl2 blunted the lipopolysaccharide-induced decrease in hepatic lactate balance but did not affect portal venous pH, hepatosplanchnic oxygen exchange, splanchnic lactate/pyruvate ratios, isoprostane, NO2- + NO3-, cytokine concentrations, or tissue nucleotide content.

CONCLUSION

During long-term hyperdynamic porcine endotoxemia, ATP-MgCl2 normalized the otherwise progressive rise of the ileal mucosal-arterial Delta PCO2. Furthermore, it allowed blunting of the continuous decrease in hepatic lactate clearance, thus preserving the metabolic coupling between lactate release from the intestine and lactate utilization by the liver.

Combined use of adenosine and amrinone inhibits reperfusion injury of rat liver

Although intraportal infusion of adenosine suppressed the oxidative stress caused by activated neutrophils and attenuated ischemia-reperfusion injury of canine liver, high doses of adenosine elicit systemic hypotension. The present work demonstrates that combined use of low doses of adenosine and amrinone, a phosphodiesterase inhibitor, strongly inhibited reperfusion injury of the liver without eliciting hypotension. After 45 min ischemia followed by 60 min reperfusion of rat liver, low doses of adenosine and amrinone were administrated intraportally, resulting in significantly increased hepatic levels of cGMP, cAMP, nitrite plus nitrate in plasma, and decreased alanine aminotransferase in plasma without changing hemodynamics. Thus, intraportal administration of low doses of adenosine and amrinone increased the cyclic nucleotides, thereby improved microcirculation and attenuated reperfusion injury of the liver.

Plasma and rectal adenosine in inflammatory bowel disease: effect of methotrexate

In animal models, the antiinflammatory mechanism of action of methotrexate has been attributed to elevation of the extracellular concentration of the endogenous autocoid, adenosine. Our goal was to determine if methotrexate elevates adenosine concentrations in plasma and at the site of disease in patients with inflammatory bowel disease. In 10 patients with Crohn's disease or ulcerative colitis, rectal adenosine and plasma adenosine concentrations were measured before and immediately after a subcutaneous injection of methotrexate, 15 or 25 mg. The mean predose rectal adenosine concentration of 2.4 mumol/l was not significantly different from the postdose concentration of 2.1 mumol/l, p = 0.17, (paired two-tailed t test). Rectal adenosine concentration tended to correlate with rectal endoscopic disease activity, r = 0.59, p = 0.067 (Spearman rank order correlation). After methotrexate injection, neither the mean daily plasma adenosine concentration, nor the plasma adenosine at any individual time point, were significantly different from preinjection values. In patients with inflammatory bowel disease, an injection of methotrexate in the clinically effective dose range does not raise rectal or plasma adenosine concentrations. A role for adenosine as the mediator of the antiinflammatory action of methotrexate is not supported.

Responses of mesenteric and renal blood flow dynamics to acute denervation in anesthetized rats

Previous studies have shown that renal autoregulation dynamically stabilizes renal blood flow (RBF). The role of renal nerves, particularly of a baroreflex component, in dynamic regulation of RBF remains unclear. The relative roles of autoregulation and mesenteric nerves in dynamic regulation of blood flow in the superior mesenteric artery (MBF) are similarly unclear. In this study, transfer function analysis was used to identify autoregulatory and baroreflex components in the dynamic regulation of RBF and MBF in Wistar rats and young spontaneously hypertensive rats (SHR) anesthetized with isoflurane or halothane. Wistar rats showed effective dynamic autoregulation of both MBF and RBF, as did SHR. Autoregulation was faster in the kidney (0.22 +/- 0.01 Hz) than in the gut (0.13 +/- 0.01 Hz). In the mesenteric, but not the renal bed, the admittance phase was significantly negative between 0.25 and 0. 7 Hz, and the negative phase was abrogated by mesenteric denervation, indicating the presence of an arterial baroreflex. The baroreflex was faster than autoregulation in either bed. The presence of sympathetic effects unrelated to blood pressure was inferred in both vascular beds and appeared to be stronger in the SHR than in the Wistar rats. It is concluded that a physiologically significant baroreflex operates on the mesenteric, but not the renal circulation and that blood flow in both beds is effectively stabilized by autoregulation.

The mesenteric circulation. Anatomy and physiology

The purpose of this article is to review the angiographic appearance of the major visceral arteries, the more common variants, their embryologic origins, and some of the most common sources of collateral flow. A brief review of the physiology of the mesenteric circulation is also provided, including a discussion of the intrinsic and extrinsic mechanisms of splanchnic blood flow control.