Proximity equilibration cells

Lang, K.W., McCune, T.D., and Steinberg, M.P. 1981. A proximity equilibration cell for rapid determination of sorption isotherms. J. Food Sci. 46, 936-938. 

A. Lenart and J.M. Flink, An improved proximity equilibration cell method for measuring water activity of food, LWT, 76 84 (1983). 

Figure 9.6 Nucleoside transporters in renal proximal tubule cells. Basolateral uptake proceeds by facilitated transport (bidirectional) via equilibrative nucleoside transporters (nENT 1, hENT2). At the luminal membrane, concentrative nucleoside transporters (CNT1-3) mediate nucleoside reabsorption via cotransport with sodium. Additionally, a role of ENTs at the brush border membrane has been proposed. Reprinted from Biochemistry and Cell Biology, Volume 84, A.N. EIwi, V.L. Damaraju, S.A. Baldwin, S.D. Young, M.B. Sawyer, and C.E. Cass, Renal nucleoside transporters Physiological and clinical implications, pp. 844 58, Copyright 2006, with permission from the NRC Research Press.

The membrane (or cell) preparation is added to the tubes to begin the binding reaction. The reagents are equilibrated for 30 to 90 minutes (time required for equilibration must be determined experimentally) and then the amount of bound ligand is quantified (either by separation or reading of scintillation proximity beads). The nsb and total binding are obtained from this experiment as shown (in bound pM). 

Apical membrane Na+/H+ exchange (via NHE3) and bicarbonate reabsorption in the proximal convoluted tubule cell. Na+/K+ ATPase is present in the basolateral membrane to maintain intracellular sodium and potassium levels within the normal range. Because of rapid equilibration, concentrations of the solutes are approximately equal in the interstitial fluid and the blood. Carbonic anhydrase (CA) is found in other locations in addition to the brush border of the luminal membrane. 

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