The Role of Renal Response to Amino Acid Infusion and Oral Protein Load in Normal Kidneys and Kidney With Acute and Chronic Disease

Francis B. Gabbai


Curr Opin Nephrol Hypertens. 2018;27(1):23-29. 

In This Article

New Methodologies to Measure or Estimate Renal Functional Reserve

Assessment of Maximum Renal Functional Reserve Response

Investigators have used different approaches to evaluate RFR including infusion of different doses of amino acid solutions or oral protein loads (OPLs), the latter either as cooked red meat or casein in variable amounts of protein expressed as g/kg body weight.[2] Evaluation of the optimal doses of protein load was recently tested by Sharma et al.[20] in a group of 18 normal volunteers who measured the increment in GFR after a 1 and 2 g/kg body weight protein load administered as cooked beef. These investigators found that a protein load of 1 or 2 g/KG body weight led to a similar increase in GFR (33.8 vs. 34.4 ml/min/1.73 m2 suggesting the presence of a ceiling or maximal sGFR response; however, large differences were observed between normal individuals. The presence of a potential ceiling, with respect to the maximal increase in GFR after a protein load, is suggested by the study of RFR in living kidney donors and recipients by Spinelli et al..[21] These investigators compared bGFR and sGFR after a 1 g/kg body weight OPL in donors and recipients after kidney transplantation. After transplantation, sGFR was similar to bGFR for both donors and recipients with a limited or nonexistent RFR. Of interest, the sum of the donor's and recipient's posttransplant sGFR was similar to the pretransplant donor's sGFR.

Noninvasive Techniques to Evaluate Renal Functional Reserve

In addition to the use of creatinine clearance or other specific GFR markers, researchers have focused on establishing alternative methods to measure RFR, which utilize Doppler ultrasound measurements, including resistive index, pulsatility index, and intraparenchymal renal resistive index variation. Pekkafali and Kara measured both resistive and pulsatility indices starting at 30, 75, and 120 min after an OPL in 40 healthy volunteers and demonstrated a 22.2% resistive index and 25.4% pulsatility index reduction 75 min after OPL.[22]

Samoni et al.[23] have tested whether the measurement of the renal resistive index before and during mechanical abdominal pressure (intraparenchymal renal resistive index variation) equal to 10% body weight, provides information regarding renal vasodilatory capacity similar to testing renal response with an OPL. The basis for this comparison is the fact that compression of renal vessels and protein-induced renal vasodilation share afferent arterial vasodilation as a common mechanism, the former by a decrease in blood flow and activation of autoregulation, the latter through several humoral mechanisms as previously described (Table 1). The results of this study in 30 healthy volunteers demonstrated a good correlation between intraparenchymal renal resistive index variation and RFR (Pearson's coefficient 74.16%, P < 0.001). These results, if reproducible in a larger patient population, may provide a quick assessment of renal vasodilatory capacity accessible to patients in various clinical settings.