Effect of K+ and K+ gradients on accumulation of sugars by isolated intestinal epithelial cells

Randles

1976

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Characteristics of 2-deoxyglucose uptake (2DG) by intestinal epithelial cells isolated from chickens were evaluated as a means of discriminating between the concentrative transport system for monosaccharides, associated with the mucosal brush border, and other possible routes of monosaccharide entry. 2DG was chosen as it is not a substrate for the mucosal transport system. The deoxysugar enters via a saturable pathway which is not Na+-dependent, is not inhibited by K+, does not accumulate solute against a concentration gradient; exhibits a high sensitivity to inhibition by phloretin; is relatively insensitive to phlorizin inhibition; and has low affinity [but high capacity relative to Na+-dependent mucosal transport of 3-O-methylglucose (3-OMG) and other monosaccharides]. These characteristics confirm those established in an earlier report for Na+-independent uptake of 3-OMG. Complications encountered in the use of 2DG as a test sugar include significant rates of metabolic conversion to an anionic form which presumably is a phosphorylated species. Methods for distinguishing between transport and subsequent metabolism are described. Inhibition of 2DG entry by several other sugars is described and inhibitory constants (K's) given for each.

Section: /O Mannitol Minutesmentioning

confidence: 85%

2-Deoxyglucose transport by intestinal epithelial cells isolated from the chick

Randles

1976

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“…In most of the experiments described here, this possibility was ignored; and the following paper [12] is a detailed consideration of the role the potassium might play in sodium-dependent substrate transport. A detailed description of a modified model for a directly energized transport mechanism will be developed after consideration of the additional data.…”

Section: Discussionmentioning

confidence: 99%

“…The following two papers consider the role of K § and K § gradients in sugar or amino acid accumulation by the cells, and the significance of the interaction observed between the transport systems for sugars and amino acids [12,13].…”

Section: Summary Accumulation Of Neutral Amino Acids By Isolated Chimentioning

confidence: 99%

Characteristics of amino acid accumulation by isolated intestinal epithelial cells

Tucker

1973

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Summary. Accumulation of neutral amino acids by isolated chick epithelial cells has been studied and the results discussed in terms of the ion gradient model, and a model invoking a direct input of metabolic energy. The cells establish four-to eightfold concentration gradients of amino acids at an extracellular concentration of 1 mM. The accumulation is sodium-dependent, inhibited by high extracellular potassium concentrations, and is sensitive to a variety of metabolic inhibitors. Also, amino acid uptake is depressed by actively transported sugars, and certain other amino acids, and is stimulated by phloridzin.Cells equilibrated with valine and loaded with 30 to 40 mM intracellular sodium begin immediately to actively accumulate valine when suddenly introduced to media containing 20 mM sodium. The cells establish a threefold gradient of amino acid during the interval when intracellular sodium is higher than extracellular sodium.Amino acid accumulation and 22Na efflux were monitored simultaneously in cells treated with phloridzin. While phloridzin causes a 30% stimulation of amino acid uptake, no variation in the rate of 22Na efflux or the steady-state level of 2ZNa maintained by the cells can be detected. Similarly, either 2.5 mM glucose or 2.5 mM 3-O-methylglucose cause approximately a 50 % inhibition of 1 mM valine uptake, but no detectable change in steady-state cellular 22Na content. Several aspects of the data seem inconsistent with concepts embodied in the ion gradient hypothesis, and it is suggested that a directly energized transport mechanism can better accommodate all of the data.The mechanism by which the cell membrane of intestinal epithelial cells transduces metabolic energy and utilizes it for active accumulation of sugars and amino acids has been the subject of a wide variety of independent investigations. Basically, two different views have been presented with regard to the nature of the energy input for intestinal nonelectrolyte transport events. Models of the type first proposed by Riggs, Walker and Christensen [22] and Crane and his colleagues [3-5] envision a mechanism in which 1 J. Membrane Biol. 12

“…We have recently suggested an alternative to the Newey and Smyth proposal in which we considered the possibility that a basic set of energy transduction events associated with the cell membrane might function in support of a variety of energy-dependent transport processes [18,20,21]. It is tempting to speculate that membrane-bound energized intermediates might play a primary role in the events required for active transport in a manner similar to that described for the phosphotransferase system for sugar transport which operates in certain bacterial species [33].…”

Section: Discussionmentioning

confidence: 99%

Interaction between Na+-dependent transport systems for sugars and amino acids. Evidence against a role for the sodium gradient

Randles

1973

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Summary. The concept that interaction between sodium-dependent transport systems represents competition for energy inherent in the transmembrane sodium gradient was examined with the use of isolated intestinal epithelial cells. The isolated cells exhibit transport interactions which are more significant in magnitude than those which have been described for intact tissue preparations. Accumulation of 1 mu valine is inhibited 60% by 10 mM 3-OMG. Conversely, uptake of 1 mM 3-OMG is inhibited only 20% by 10 mM valine. These data suggest that 3-OMG must discharge the cellular Na + gradient more effectively than valine, if Na + gradient dissipation can be taken as a basis for the inhibitory interaction. However, entry of 10 mu 3-OMG is significantly slower than the entry of 10 mM valine. Even if appropriate corrections are made for passive substrate entry and for differences in Na/substrate entry stoichiometry, it appears that valine should be somewhat more effective than 3-OMG in discharging the Na + gradient. In light of these facts, it seems unlikely that the mechanistic basis for interaction between sugar and amino acid transport systems can be related to concomitant co-entry of Na +. It is suggested that the interaction results instead from competition for energized intermediates generated at limited rates by basic energy transduction events associated with the cell membrane which serve in support of a variety of active transport systems.