Aortic and renal venous levels for creatinine, serine, arginine, and tyrosine, reported as peak areas detected by the mass spectrometer. The uniform decrease in creatinine is a positive control, consistent with renal clearance. Serine and arginine increased from aorta to renal vein in all nine individuals, whereas tyrosine increased in seven individuals (P < 0.001 for all three metabolites).