638 THE RENAL HANDLING OF 3-METHYLHISTIDINE

Abstract

The urinary excretion of the unique amino acid, 3-methylhistidine (3MH), has been proposed as a marker of endogenous protein catabolism and an index of a patient's metabolic state. Yet, the mechanisms of the renal handling of 3MH remain unclear. Consequently, we determined the simultaneous clearance of 3MH (C3MH) and inulin (Cin) in rats. Net reabsorption of 3MH was a consistent finding. In ten euvolemic animals, C3MH (313.2 ± 37.5 μl/min/100g BW) was less than Cin (1017.1 ± 21.2) and the percent 3MH reabsorbed was 72.6 ± 3.7%. In five volume expanded animals, the percent 3MH reabsorbed was not significantly altered (80 ± 5.0%). To confirm net reabsorption, 3MH combined with inulin was microinjected into proximal tubules of surface nephror Inulin recovery from the microinjected kidney averaged 102 ± 5% while the recovery of 3MH was only 74 ± 6% (P < 0.01). The percent of 3MH reabsorbed had no relationship to the quantity of 3MH injected, suggesting that there is no Tm for 3MH. This study clearly demonstrates that: 1) the renal clearance of 3MH is less than that of inulin, 2) net tubular reabsorption occurs over a wide range of filtered loads of 3MH and 3) a Tm for reabsorption of this amino acid metabolite could not be documented. These factors must be considered in using urinary excretion of 3MH as a marker of nutritional status.