645 Clinical Science (2002) 102, 645–650 (Printed in Great Britain) Increased cationic amino acid flux through a newly expressed transporter in cells overproducing nitric oxide from patients with septic shock Michael C. READE*, Megan F. CLARK†, J. Duncan YOUNG* and C. A. R. BOYD† *Nuffield Department of Anaesthetics, University of Oxford, Radcliffe Infirmary, Oxford OX2 6HE, U.K., and †Department of Human Anatomy and Genetics, University of Oxford, South Parks Road, Oxford OX1 3QX, U.K. A B S T R A C T Increased production of nitric oxide (NO) is thought to be a factor in the pathogenesis of many human diseases – among them the hypotension that often accompanies sepsis. The supply of the cationic amino acid arginine is known to be rate-limiting for NO production. We hypothesized that cationic amino acid transport might be increased in cells producing excess NO from patients with septic shock. Peripheral blood mononuclear cells were isolated from patients with sepsis and from healthy control subjects. The rates of both NO production and cationic amino acid uptake were increased in cells from patients with septic shock. The increased transport was due almost entirely to an increase in the activity of one transporter, subtype y + . The activity of the other major cationic amino acid transporter ( y + L) was unchanged. The expression of CAT2 mRNA, which encodes a y + transporter protein, was also increased in these cells. We suggest that CAT2 might be a therapeutic target to prevent excess NO production in sepsis and possibly other human disease states, while leaving basal production unchanged. INTRODUCTION Overproduction of nitric oxide (NO) is thought to be central to the pathogenesis of many diseases, among them septic shock. Septic shock is the leading cause of death in non-coronary intensive care, with a mortality rate of around 50 % [1]. While there is increasing evidence that the vasodilator NO is produced in increased quantities in septic shock [2], endothelium-derived NO is now thought to be paradoxically reduced [3,4]. The cellular source of this extra NO in sepsis is thus not known. A decrease in NO production would be expected to improve the blood pressure of patients with septic shock, and presumably decrease mortality. The only known potent inhibitors of the NO biosynthetic pathway act Key words : arginine, CAT2, nitric oxide, sepsis, septic shock, y + ,y + L. Abbreviations : DAF-2, 4,5-diaminofluorescein ; DAF-2 DA, 4,5-diaminofluorescein diacetate ; DAF-2 T, a fluorescent product of DAF-2 ; L-NAME, N G -nitro-L-arginine methyl ester ; PBMC, peripheral blood mononuclear cell ; RT-PCR, reverse transcription– PCR. Correspondence : Dr Michael Reade (e-mail michael.readebrasenose.oxford.ac.uk). non-specifically on all NO synthase enzymes. While initial case reports using these drugs appeared promising [5,6], it now appears their therapeutic ratio is too narrow to be of clinical benefit. A number of other strategies to normalize NO production are under investigation. Among these, inhibition of the transport into the cell of the precursor to NO, L-arginine, may show particular promise. The supply of arginine is known to be rate-limiting to the production of NO, in vitro [7] and in vivo [8]. Arginine enters mammalian cells through functionally distinct transport systems, principally systems y + and y + L [9]. Families of proteins with these functions have now been sequenced, and in humans are designated CAT1, CAT2 and CAT4 (encoding y + activity) and  2002 The Biochemical Society and the Medical Research Society