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Evaluation of tubular function is not usually done in kidney donors because of the assumption that normal glomerular filtration rate (GFR) is a surrogate of normal tubular function. This assumption is unwarranted. We evaluated the tubular secretion of creatinine (TScr) response to an oral creatinine load, a response that depends on the proximal tubule organic cationic transporters and its relation to GFR, in kidney transplant donors before and after nephrectomy
Nine kidney donors were studied before and 3 months after nephrectomy. The participants were studied after 12-hour fasting and were taking no medications in the preceding 7 days. After an oral load of 5g of creatinine and induced aqueous diuresis, hourly serum samples were obtained and hourly urine was collected for 4 hours. Complete bladder was confirmed by ultrasonography. Urinary creatinine excretion (Ucr x Uvol, mg/min) and mean serum creatinine in the corresponding period (Scr, mg/dl), were used to determine filtered creatinine (Fcr = Scr x eGFR, mg/min) and TScr (UcrV – Fcr, mg/min).
(mean ± SE). TScr had peak values in the first hour (Figure 1). The stimulated TScr in the 4 hours of the study ranged from 1 to 6.5 mg/min and from 0 to 3 mg/min, before and after nephrectomy, respectively (Figure 2). There was no relation between stimulated TScr and eGFR before or after nephrectomy. Three months after nephrectomy the remaining kidney recovered 77±7% of the pre-nephrectomy eGFR and the recovery of the tubular secretory response ranged from none to 100% of the response obtained with 2 kidneys (figure 3).
The response of the organic cationic transporters to an exogenous creatinine load is unrelated to GFR in kidney donors before and after nephrectomy. Tubular stimulation tests may be used to explore determinants of tubular function and to uncover subclinical kidney injury that may play a role in long-term prognosis of kidney disease.