Furosemide and Creatinine Tubular Stress Test in Order to Measure Proximal Tubule Residual Function

Residual kidney function (RKF) its a critical tool in the CKD evolution and prognosis. It's well known that preserving RKF increases survival in patients with CKD. Traditionally Glomerular Filtration Rate (GFR), and proteinuria have been the variables used to asses RKF, wich are glomerular residual function (GRF) variables. This glomerulocentric assestment of RKF has been challenged recently. It is knows that urea is not the principal uremic toxine, the protein boud uremic toxins are particullary important because it's elimination depends of the proximal tubular organic anionic transporters (OAT), and the organic cationic transporters (OCT). The functional integrity of this transporters are lost in advanced CKD stages, and it´s accumulation increases cardiovascular and renal damage because they activate proinflammatory citokines, increasing mortality, this was shown specially with the accumulation and clearance of hippurate and p-cresol independently of GFR. The purpose of this study is to asses the functional capacity of two AOT (hOAT1, and hOAT3, that can be blocked by furosemide), and one OCT (OCT2 blocked by creatinine) using an stress test. The stress test will be performed in healthy subjects as well as in CKD from different stages. Protein bound uremic toxins such as indoxyl sulphate, p-cresol, and hippurate will be measured. In the stress the typical furosemide stress test described previously by Mehta will be implemented, in addition to an oral load of 5 grams of creatinine oral load. This levels should be efficient enough to assess the functionality of the OCT2. A comparison between the tubular creatinine secretine to filtrated creatinine measuring GFR using Iohexol will be performed. In conclusion this is a pilot study aimed to establish a practical methodology for the assesment of proximal tubular function.