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Patrick R. Cammarata, Grant Schafer, Shiuh-Wei Chen, Zhen Guo, Rustin E. Reeves; Osmoregulatory Alterations in Taurine Uptake by Cultured Human and Bovine Lens Epithelial Cells. Invest. Ophthalmol. Vis. Sci. 2002;43(2):425-433.
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purpose. Comparative assessment of cultured human lens epithelial cells (HLECs)
and bovine lens epithelial cells (BLECs) established the nature of the
relationship between taurine-concentrating capability and intracellular
polyol accumulation or extracellular hypertonicity.
methods. The kinetic characteristics of active taurine accumulation based on the
measurement of in vitro [3H]-taurine uptake were resolved
by side-to-side review of cultured HLECs and BLECs pre-exposed to
either galactose-supplemented medium or extracellular hypertonicity.
Competitive RT-PCR was used to appraise variation in taurine
transporter (TauT) mRNA abundance from cells maintained in hyperosmotic
medium over a 72-hour exposure period.
results. The capacity to accumulate [3H]-taurine was significantly
lowered after prolonged (20-hour) incubation of cultured BLECs in 40 mM
galactose in contrast to HLECs, the latter cells’ velocity curve being
indistinguishable from control cells in physiological medium.
Inhibition of the intracellular taurine transport site appeared to be
noncompetitive, in that there was a marked reduction in the V max without significant alteration in the K m to a high-affinity transport site.
Galactitol content in BLECs exceeded five times that found in HLECs.
The coadministration of the aldose reductase inhibitor, sorbinil, with
40 mM galactose completely prevented the inhibitory effect of galactose
on [3H]-taurine uptake. Acute exposure (3 hours) of HLECs
and BLECs to a range of 10 to 40 mM galactitol or 10 to 40 mM galactose
plus sorbinil-supplemented medium suggested by Dixon plot that neither
galactitol nor galactose interacted with the extracellular taurine
transport site. In contrast, [3H]-taurine accumulation
was markedly elevated in both HLECs and BLECs after prolonged exposure
to galactose-free medium made hyperosmotic by supplementation with
sodium chloride. The enhanced taurine uptake capacity involved increase
in peak velocity (V max) without significant
change in Michaelis-Menten constant (K m).
Cultured HLECs and BLECs responded to hypertonicity with an inducible
but transitory upregulation of TauT mRNA.
conclusions. These results demonstrate that lens epithelial cells express a
high-affinity TauT protein capable of active uptake, but predisposed to
inhibition by intracellular galactitol when the sugar alcohol is
present in sufficiently high concentration to interfere with cell
metabolism. Furthermore, lens epithelial cells respond to hypertonic
stress by raising taurine transport activity. The increase in taurine
uptake is due to an increase in the number of high-affinity TauTs
expressed as a result of an increase in the manifestation of taurine
mRNA stemming from exposure to hypertonic
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