Role of System Gly in glycine transport in monolayer cultures of liver cells

The high-affinity component of glycine uptake by the hepatoma cell line HTC and by the ordinary rat hepatocyte corresponds to System Gly, the agency serving for glycine uptake by pigeon red blood cells and rabbit reticulocytes, and at most to only a minor extent to System ASC. This component was identified in HTC by its sensitivity to inhibition by sarcosine but scarcely by 2-(methylamino) isobutyric acid, by its insensitivity to lowering of the pH, and by the unique relation of its rate to the square of the Na+ concentration. The identity of the low-affinity component with System A was confirmed by opposite properties, and by its stimulation by insulin or amino acid starvation. Both components differed sharply from the System ASC uptake as measured with threonine.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24480/1/0000755.pd

Features of amino acid structure enhancing or obstructing cosubstrate reactivity of Na+ in transport

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32883/1/0000261.pd

Reactions of neutral amino acids plus Na+ with a cationic amino acid transport system

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32985/1/0000369.pd

Cellular uptake of lithium via amino acid transport system A

We now add to the agencies by which cells take up lithium the process of cotransport with neutral amino acids via System A. In the Ehrlich cell various natural and synthetic amino acids, depending on their structure, can cause substantial accelerations of Li+ uptake over a considerable range of levels of Na+, Li+ and H+. Half the maximal augmentation of uptake, namely 1.2 mequiv. Li/kg cell water per 15 min, was obtained for 5.4 mM alanine in a double-reciprocal plot. Alanine also stimulated the exodus of Li+ from the Ehrlich cell. The human red blood cell, lacking System A as it does, becomes an imperfect model for studying cellular uptake of Li+. Until the Li+ dependence of amino acid uptake in the reticulocyte is known, reticulocytosis can be suspected of contributing to the interpersonal variations seen in Li+-for-Na+ exchange.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22511/1/0000055.pd

Does the non-saturable cell entry apply to the charge-free form of amino acids?

The slow cellular entry shown by neutral [alpha]-amino acids at very high concentrations appears not to arise from diffusion of the totally uncharged species through the plasma membrane of the Ehrlich cell, judging from a similarity of the rates observed for the two conformational isomers of 1-amino-2-hydroxy-cyclohexane-carboxylic acid. One of these isomers provides in neutral solution 4 times as large a proportion of the charge-free species as the other, and 5 times the proportion calculated for alanine.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22845/1/0000405.pd

MODIFIED TRANSPORT SUBSTRATES AS PROBES FOR INTRAMEMBRANE GRADIENTS *

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71522/1/j.1749-6632.1974.tb14400.x.pd

Collecting duct-specific Rh C glycoprotein deletion alters basal and acidosis-stimulated renal ammonia excretion

NH3 movement across plasma membranes has traditionally been ascribed to passive, lipid-phase diffusion. However, ammonia-specific transporters, Mep/Amt proteins, are present in primitive organisms and mammals express orthologs of Mep/Amt proteins, the Rh glycoproteins. These findings suggest that the mechanisms of NH3 movement in mammalian tissues should be reexamined. Rh C glycoprotein (Rhcg) is expressed in the collecting duct, where NH3 secretion is necessary for both basal and acidosis-stimulated ammonia transport. To determine whether the collecting duct secretes NH3 via Rhcg or via lipid-phase diffusion, we generated mice with collecting duct-specific Rhcg deletion (CD-KO). CD-KO mice had loxP sites flanking exons 5 and 9 of the Rhcg gene (Rhcgfl/fl) and expressed Cre-recombinase under control of the Ksp-cadherin promoter (Ksp-Cre). Control (C) mice were Rhcgfl/fl but Ksp-Cre negative. We confirmed kidney-specific genomic recombination using PCR analysis and collecting duct-specific Rhcg deletion using immunohistochemistry. Under basal conditions, urinary ammonia excretion was less in KO vs. C mice; urine pH was unchanged. After acid-loading for 7 days, CD-KO mice developed more severe metabolic acidosis than did C mice. Urinary ammonia excretion did not increase significantly on the first day of acidosis in CD-KO mice, despite an intact ability to increase urine acidification, whereas it increased significantly in C mice. On subsequent days, urinary ammonia excretion slowly increased in CD-KO mice, but was always significantly less than in C mice. We conclude that collecting duct Rhcg expression contributes to both basal and acidosis-stimulated renal ammonia excretion, indicating that collecting duct ammonia secretion is, at least in part, mediated by Rhcg and not solely by lipid diffusion

Proximal tubule-specific glutamine synthetase deletion alters basal and acidosis-stimulated ammonia metabolism

Glutamine synthetase (GS) catalyzes the recycling of NH4 (+) with glutamate to form glutamine. GS is highly expressed in the renal proximal tubule (PT), suggesting ammonia recycling via GS could decrease net ammoniagenesis and thereby limit ammonia available for net acid excretion. The purpose of the present study was to determine the role of PT GS in ammonia metabolism under basal conditions and during metabolic acidosis. We generated mice with PT-specific GS deletion (PT-GS-KO) using Cre-loxP techniques. Under basal conditions, PT-GS-KO increased urinary ammonia excretion significantly. Increased ammonia excretion occurred despite decreased expression of key proteins involved in renal ammonia generation. After the induction of metabolic acidosis, the ability to increase ammonia excretion was impaired significantly by PT-GS-KO. The blunted increase in ammonia excretion occurred despite greater expression of multiple components of ammonia generation, including SN1 (Slc38a3), phosphate-dependent glutaminase, phosphoenolpyruvate carboxykinase, and Na(+)-coupled electrogenic bicarbonate cotransporter. We conclude that 1) GS-mediated ammonia recycling in the PT contributes to both basal and acidosis-stimulated ammonia metabolism and 2) adaptive changes in other proteins involved in ammonia metabolism occur in response to PT-GS-KO and cause an underestimation of the role of PT GS expressio