Glutamine and glutamic acid uptake by rat renal brushborder membrane vesicles

Abstract

Glutamine uptake by rat renal brushborder vesicles occurred via two distinct saturable processes with Km values of 0.145 and 8.5 mM which were stimulated by both ionic and sodium gradients with a pH optimum of 6.8--7.1. Glutamic acid uptake also occurred by a two-component system with Km values of 0.016 and 3.60 mM. Both components were stimulated specifically by a sodium gradient. The low Km system for glutamic acid had a pH optimum of 7.2--7.4. Glutamine entry at 0.06 mM was inhibited by a variety of amino acids at 3 mM, including dibasic amino acids, glycine, valine, and phenylalanine. Glutamic acid entry at 0.06 mM was inhibited 20--30% by 3 mM phenylalanine, valine, alpha-aminoisobutyric acid, and glutamine. No metabolic alteration of glutamic acid was observed on incubation with membrane vesicles, but glutamine was significantly hydrolyzed to glutamic acid upon prolonged incubation. Hydrolysis of glutamine was negligible at 15 sec incubation which was employed for measurement of initial rate of entry. These studies provide support for the existence of an uptake system in the brushborder of the renal proximal tubule cell capable of handling the reabsorption of glutamine normally present in glomerular filtrate.

Tubular CO₂ Production from Glutamine in the Rat: Segmental Profile and Modulation

“…oxidation of the carbon skeleton by mitochondria may be inhibited [4,26]. Our results with isolated tubule segments confirm the findings with cor tical suspensions [12], and renal cortical slices [7], The decline in oxidation at alkalotic pH can also be due to reduced uptake [18], but one must consider the same limitations on the relationship between oxidation and uptake as discussed for acidosis. The restriction of the effect of pH to the proximal tubule is similar to the observations of Good and Burg[l] regard ing ammonia production during metabolic acidosis.…”

“…We have observed an increased oxidation rate at moderate acidotic levels that was not sustained during severe acidosis. The latter change may be due to a relative decline in glutamine uptake at pH levels below the optimal value [18]. Such a decline, however, would have to be extreme as the saturation of oxidation is significantly below the Km for up take.…”

“…can be used to derive a qualitative idea about the utilization of the substrate, as the modulation studies done in this paper indi cate. The higher rates of oxidation observed when glutamine was the only substrate in the incubation medium compared to the supple mented medium may be related in part to the decline in the uptake of glutamine when other amino acids are present [ 17,18].…”

Sodium-dependent Transport of Amino Acids in Renal Brush Border Membrane Vesicles and Urinary Free Amino Acid Excretion in Rats

“…With a lower concentration of glutamic acid, however, Weiss et al 9 ) found that glutamic acid (0.06 mM) uptake exhibited a Na + -dependent overshoot phenomenon of The Na + -dependent uptake of each amino acid was obtained by subtracting the uptake in the KCI medium from that in the NaCI medium. For the urinary excretion of free amino acids, the values represent the means for four rats and the vertical bars indicate the standard error.…”

Transport of β-Hydroxy-β-Methyl-Glutarate and β-Hydroxbutyrate by Renal Brushborder Membrane Vesicles

“…Cortical slices were made with a Stadie-Riggs microtome and the tissue weighed. Brushborder membrane vesicles were then prepared using the method of Booth and Kenny (2) modified as described by Weiss et al (21). The final membrane pellet was suspended in T H M buffer, pH 7.4 (2 mM Tris/HEPES + 100 mM mannitol) to a protein concentration of 0.3-0.4 mg/ml as determined by the method of Lowry et al (13).…”