Alkaline Phosphatase, Soluble Extracellular Adenine Nucleotides, and Adenosine Production after Infant Cardiopulmonary Bypass

Abstract

RationaleDecreased alkaline phosphatase activity after infant cardiac surgery is associated with increased post-operative cardiovascular support requirements. In adults undergoing coronary artery bypass grafting, alkaline phosphatase infusion may reduce inflammation. Mechanisms underlying these effects have not been explored but may include decreased conversion of extracellular adenine nucleotides to adenosine.Objectives1) Evaluate the association between alkaline phosphatase activity and serum conversion of adenosine monophosphate to adenosine after infant cardiac surgery; 2) assess if inhibition/supplementation of serum alkaline phosphatase modulates this conversion.Methods and ResearchPre/post-bypass serum samples were obtained from 75 infants <4 months of age. Serum conversion of 13C5-adenosine monophosphate to 13C5-adenosine was assessed with/without selective inhibition of alkaline phosphatase and CD73. Low and high concentration 13C5-adenosine monophosphate (simulating normal/stress concentrations) were used. Effects of alkaline phosphatase supplementation on adenosine monophosphate clearance were also assessed. Changes in serum alkaline phosphatase activity were strongly correlated with changes in 13C5-adenosine production with or without CD73 inhibition (r = 0.83; p<0.0001). Serum with low alkaline phosphatase activity (≤80 U/L) generated significantly less 13C5-adenosine, particularly in the presence of high concentration 13C5-adenosine monophosphate (10.4μmol/L vs 12.9μmol/L; p = 0.0004). Inhibition of alkaline phosphatase led to a marked decrease in 13C5-adenosine production (11.9μmol/L vs 2.7μmol/L; p<0.0001). Supplementation with physiologic dose human tissue non-specific alkaline phosphatase or high dose bovine intestinal alkaline phosphatase doubled 13C5-adenosine monophosphate conversion to 13C5-adenosine (p<0.0001).ConclusionsAlkaline phosphatase represents the primary serum ectonucleotidase after infant cardiac surgery and low post-operative alkaline phosphatase activity leads to impaired capacity to clear adenosine monophosphate. AP supplementation improves serum clearance of adenosine monophosphate to adenosine. These findings represent a potential therapeutic mechanism for alkaline phosphatase infusion during cardiac surgery.New and NoteworthyWe identify alkaline phosphatase (AP) as the primary soluble ectonucleotidase in infants undergoing cardiopulmonary bypass and show decreased capacity to clear AMP when AP activity decreases post-bypass. Supplementation of AP ex vivo improves this capacity and may represent the beneficial therapeutic mechanism of AP infusion seen in phase 2 studies.

Alkaline Phosphatase in Infant Cardiopulmonary Bypass: Kinetics and Relationship to Organ Injury and Major Cardiovascular Events

“…We previously identified a decrease in serum capacity to convert extracellular adenosine monophosphate (AMP) to adenosine in the immediate post-operative period that is directly related to serum AP activity. 21 Ex-vivo addition of liver AP markedly improved this capacity. It is possible that decreased AP activity could result in poor clearance of endotoxin and/or extracellular adenine nucleotides leading to subsequent inflammation and cardiovascular instability.…”

“…These activities include dephosphorylation of endotoxin to a less toxic monophosphoryl byproduct and conversion of harmful extracellular adenine nucleotides to adenosine. 19-21 Preclinical and phase 2 adult studies of AP therapy for sepsis and inflammatory colitis have shown a reduction in inflammation and organ injury. 22-24 …”

Alkaline Phosphatase, an Unconventional Immune Protein

“…In contrast, degradation of extracellular ATP and ADP to AMP and adenine causes cessation of inflammatory signaling, and induction through adenine receptors of an anti-inflammation response ( 36 , 37 ). TNAP has been implicated in protection against inflammation in multiple diseases and promotion of intestinal microbial populations through hydrolysis of extracellular ATP/ADP to AMP and adenosine ( 38 – 40 ).…”

Tissue alkaline phosphatase activity and expression in an experimental infant swine model of cardiopulmonary bypass with deep hypothermic circulatory arrest

“…Clearance could be improved by ex vivo addition of either exogenous bovine intestinal AP (BiAP) or human liver AP. [47,48] These clinical studies, however, are limited in their ability to comprehensively detail changes in native AP after cardiac surgery with CPB as they examine only circulating AP and are unable to measure tissue-level changes in AP activity or expression. Our group recently demonstrated that kidney AP activity decreases after CPB in a piglet model of infant CPB with DHCA and that decreased levels of renal tissue AP were associated with increased histologic acute kidney injury.…”