Effect of urine metabolites from healthy and uremic subjects on gluconeogenesis in slices of rat kidney cortex and liver.

A high molecular weight fraction was obtained by extended dialysis of urine of healthy and uremic subjects. After addition to the incubation medium, this fraction inhibited gluconeogenesis by rat kidney cortex slices. From the six subfractions extracted by gel chromatography (Sephadex G 100) fraction IV caused a decrease of glucose formation. The activity of PEP-carboxykinase but not of pyruvate carboxylase was reduced, indicating a decreased formation of phosphoenol pyruvate. The total high molecular weight fraction stimulated glucose release by liver slices from fed but not from starved rats. In the absence of amino acids, urea formation was not stimulated. The activity of pyruvate carboxylase was reduced in both groups, PEP-carboxykinase activity was, however, only reduced in the starved group. The addition of uremic serum caused increased glucose release. Inhibition of PEP-carboxykinase activity by quinolinic acid (15 mM) resulted in inhibition of glucose formation by 35% in the uremic group and 54% in the control group in livers of 24 hr starved rats. Thus in uremia there may be incorporation of serine carbon skeletons into glucose via hydroxypyruvate, not via pyruvate. Chromatography on calibrated columns indicated that about 40% of the urinary fractions had molecular weights in the upper range of the "middle molecules" category. The positive correlation between toxicity and the total amount of high molecular weight substances excreted do not confirm the hypothesis of augmented retention of "toxins" in uremic patients. It must be appreciated that these results refer only to the undialyzable fraction of urine which contains only 0.5% by weight of the total urine solids.