Effect of amino acid infusion on renal hemodynamics in humans

The effects of amino acid (AA) infusion on renal hemodynamics was examined in 19 healthy subjects. Thirteen subjects participated in the first protocol (normal protein intake) and six in the second protocol (low protein intake). The first protocol consisted of three studies: study 1 (n=13), AA were infused over 3 h to increase plasma amino acid levels two- to threefold; study 2 (n=7), AA were infused with somatostatin and peripheral replacement of insulin, glucagon, and growth hormone; study 3 (n=6), somatostatin was infused with basal hormonal replacement as in study 2. During study 1, glomerular filtration rate (GFR) rose by 20% (from 107 ± 5 to 128 ± 4 ml·173 m^-2·min^-1, P<0.001). Renal plasma flow (RPF) increased by a similar percentage (599 ± 35 to 704 ± 33 ml·173 m^-2·min^-1, P<0.001). When somatostatin was infused with AA (study 2), neither GFR nor RPF changed from base line. Somatostatin infusion alone (study 3) had no effect on GFR or RPF. During protocol 2, six subjects received a low-protein diet (40 g/day) for 7 days and AA were infused as per study 1. Base-line GFR (104 ± 5 to 96 ± 4 ml·1.73 m^-2·min^-1 and RPF (593 ± 32 to 507 ± 23 ml·1.73 m^-2·min^-1) both decreased (P<0.02) after the low-protein diet. When AA were infused, both GFR (114 ± 5 ml/min) and RPF (584 ± 32 ml/min) increased (P<0.01), but the increment was of similar magnitude to that observed in subjects consuming a normal protein intake. These results indicate that 1) hyperaminoacidemia causes a modest increase in GFR and RPF; 2) the increase is most likely mediated by some hormone(s) whose secretion is inhibited by somatostatin; 3) protein restriction causes a reduction in RPF and GFR without change in fasting amino acid levels but fails to alter the renal hemodynamic response to AA infusion.