Our work in Tanzanian children and Indonesian adults demonstrated impaired production of nitric oxide (NO), impaired mononuclear cell NO synthase 2 (NOS2) expression, and very low levels of the NOS substrate arginine. We concluded that NO and NOS2 are protective against development of severe malaria. NO has antimicrobial effects in vitro against a wide variety of organisms including Plasmodium falciparum, and it can modulate endothelial function (reduction of cell adhesion molecule expression and adherence of parasitized erythrocytes). Arginine can be converted to citrulline and NO by NOS, and to ornithine and urea by arginase. In vivo NO bioavailability can be decreased because of both reduced NO production and hemolysis (hemoglobin and arginase released from erythrocytes can quench NO and consume arginine, respectively). Our studies in Indonesian adults and Tanzanian children confirmed that arginine is low in clinical malaria. We have also shown that adults with moderately severe malaria have endothelial dysfunction as determined by abnormal reactive hyperemia-peripheral arterial tonometry measurements. This reactive hyperemia is primarily dependent on NO. We found that endothelial dysfunction is associated with elevated blood lactate levels, measures of hemolysis, and NO bioavailability. Our studies showed that administration of arginine intravenously to these adults is safe and results in reversion of hypoargininemia, improved endothelial function, and increased exhaled NO. No one has determined the precise cause of hypoargininemia in adults and children who have malaria. Furthermore, no one has evaluated children with malaria for endothelial dysfunction and its relationship to arginine levels and NO production. We hypothesize that hypoargininemia results from a combination of increased arginine degradation and decreased arginine synthesis;that hypoargininemia contributes to the malaria-associated reduced NO bioavailability and endothelial dysfunction;and that hypoargininemia is important in the overall pathophysiology of severe malaria. In addition, we hypothesize that arginine administered in conjunction with parasiticidal antimalarial chemotherapy will be useful in the treatment of severe malaria.
The specific aims are to (1) determine the mechanisms of hypoargininemia in clinical malaria in Tanzanian children;(2) to assess endothelial function in children with malaria with a special emphasis on relationships to arginine levels and NO production;and (3) to perform a dose-ranging study of intravenous arginine administration in hospitalized children with moderately severe malaria. Malaria is the most common cause of hospitalization and death in African children, and there is a high rate of death even in those who receive antimalarial chemotherapy. Our studies will answer important questions regarding the pathophysiology of severe malaria, and the work may lead to new adjunctive treatments for this frequently deadly disease.
Malaria causes more global mortality than any other parasitic disease. Plasmodium falciparum is responsible for severe malaria and the estimated 1.5 to 2.7 million deaths that occur from malaria each year. The majority of these deaths occur in African children less than five years of age. Case fatality rates in severe malaria remain high despite the use of rapidly parasiticidal antimalarial chemotherapy. Adjunctive therapies (treatments added to antimalarial chemotherapy) have not been beneficial. Our earlier studies indicate that arginine (through nitric oxide pathways) may be a useful adjunctive therapy. The studies we propose here will answer important questions relative to the pathophysiology of severe malaria regarding arginine and nitric oxide, and the work may lead to new adjunctive treatments for this frequently deadly disease.
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