Background/Aims: Controversy exists regarding the nature of the hepatocyte membrane transport system for sulfobromophthalein and other organic anions and its driving forces. Most studies have been performed in the absence of albumin, the plasma sulfobromophthalein binding protein, or using very low albumin concentrations. We have shown that in the latter case uptake kinetics reflect dissociation/diffusion events and not membrane transport. In contrast, without albumin very high sulfobromophthalehr concentrations reach the cell surface and may overwhelm a high affinity/low capacity system operating in viva. The aim of this study was to test the latter hypothesis. Methods: Stdfobromophthalein uptake was measured by rapid filtration in isolated hepatocytes without albumin (up to 15 pM sulfobromophthalein) and with 600 pM albumin (sulfobromophthalehualbumin from 0.03:1 to l:l), a physiologic setting which greatly reduces the unbound BSP concentration. Unbound sulfobromophthalein concentration was estimated according to a three binding site model. XPENMENTS E dating back more than 20 years clearly indicate that the hepatic uptake of sulfobromophthalein (BSP) and other organic anions such as bilirubin, dibromosulfophthalein and indocyanine green is carrier-mediated (reviewed in ref. 1 and 2). Although there is general consensus that these compounds share the same system, exists regarding the Received 17 July, revised 15 Decentbe< accepted 19 December 1995 Correspondence: Dr. Dario Sorrentino, Medicina Intema, Policlinico Universitario, P.le S. Maria Misericordia 1, 33100, Udine, Italy. Tel. +39-432-559803; Fax +39-432- 42097. Results: In the absence of albumin, kinetic parameters for sulfobromophthalein uptake were similar to those reported in the literature (K,,,: 7.1k1.2 PM; V ,,,=: 452ti7 pmol/mht/5x104 cells). In the presence of albumin, stiobromophthaleht uptake displayed much greater affinity and much lower capacity (&: i!lO&ll nM; V_: fXl9 pmol/min/5x104 cells). Conclusions: These ikulings suggest that in the absence of albumin, resulting high sulfobromophthalein concentrations overload (and make undetectable) a high affinity/low capacity system operating at physiologic albumin concentrations (i.e. at low unbound sulfobromophthalein concentrations). Previously characterized transport systems may be operating only under defined conditions. These findings could explain the apparent controversy regarding the nature of the sulfobromophthalein transport system and its driving forces.

Hepatocellular sulfobromophthalein uptake at physiologic albumin concentrations: Kinetic evidence for a high affinity/low capacity sinusoidal membrane system

SORRENTINO, Dario Rosario;
1996-01-01

Abstract

Background/Aims: Controversy exists regarding the nature of the hepatocyte membrane transport system for sulfobromophthalein and other organic anions and its driving forces. Most studies have been performed in the absence of albumin, the plasma sulfobromophthalein binding protein, or using very low albumin concentrations. We have shown that in the latter case uptake kinetics reflect dissociation/diffusion events and not membrane transport. In contrast, without albumin very high sulfobromophthalehr concentrations reach the cell surface and may overwhelm a high affinity/low capacity system operating in viva. The aim of this study was to test the latter hypothesis. Methods: Stdfobromophthalein uptake was measured by rapid filtration in isolated hepatocytes without albumin (up to 15 pM sulfobromophthalein) and with 600 pM albumin (sulfobromophthalehualbumin from 0.03:1 to l:l), a physiologic setting which greatly reduces the unbound BSP concentration. Unbound sulfobromophthalein concentration was estimated according to a three binding site model. XPENMENTS E dating back more than 20 years clearly indicate that the hepatic uptake of sulfobromophthalein (BSP) and other organic anions such as bilirubin, dibromosulfophthalein and indocyanine green is carrier-mediated (reviewed in ref. 1 and 2). Although there is general consensus that these compounds share the same system, exists regarding the Received 17 July, revised 15 Decentbe< accepted 19 December 1995 Correspondence: Dr. Dario Sorrentino, Medicina Intema, Policlinico Universitario, P.le S. Maria Misericordia 1, 33100, Udine, Italy. Tel. +39-432-559803; Fax +39-432- 42097. Results: In the absence of albumin, kinetic parameters for sulfobromophthalein uptake were similar to those reported in the literature (K,,,: 7.1k1.2 PM; V ,,,=: 452ti7 pmol/mht/5x104 cells). In the presence of albumin, stiobromophthaleht uptake displayed much greater affinity and much lower capacity (&: i!lO&ll nM; V_: fXl9 pmol/min/5x104 cells). Conclusions: These ikulings suggest that in the absence of albumin, resulting high sulfobromophthalein concentrations overload (and make undetectable) a high affinity/low capacity system operating at physiologic albumin concentrations (i.e. at low unbound sulfobromophthalein concentrations). Previously characterized transport systems may be operating only under defined conditions. These findings could explain the apparent controversy regarding the nature of the sulfobromophthalein transport system and its driving forces.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11390/1092757
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