Amino acids enhance renal tubular absorption of the los-molecular-weight proteins insulin and growth hormone.

The effect of amino acids(AA) on the tubular absorption of low-molecular-weight (LMW) proteins was studied in isolated rat kidneys. Kidneys were perfused with an albumin-electrolyte solution that contained insulin or human growth hormone (hGH) and, unless otherwise stated, the following L-amino acids: glycine, isoleucine, serine, alanine, methionine, proline, arginine, and aspartic acid. In kidneys perfused without AA, fractional urinary insulin clearance (FCi) averaged 7.4 +/- 1.54%, whereas in the presence of multiple AA the FCi was significantly lower (0.68 +/- 0.2%, P less than 0.01). Addition of glycine or alpha-aminoisobutyric acid (AIB) alone also reduced the FCi significantly (1.79 +/- 0.66 and 1.59 +/-1.06%, respectively). By contrast, perfusion with the other AA individually did not alter the FCi. The fractional urinary hGH clearance was also significantly lower in kidneys perfused with multiple AA (0.94 +/- 0.47%) than in those perfused without AA (9.07 +/- 1.2%). We conclude that tubular absorption of filtered insulin and hGH is enhanced by the presence of AA. The mechanism is unclear, but enhancement of insulin absorption can be produced by glycine and AIB alone. This raises the possibility of a link between the absorption of insulin and the glycine and AIB shared transport system, but excludes a primary metabolic effect because AIB is nonmetabolizable.