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Department of Surgery, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215
Adenosine release
from mucosal sources during inflammation and ischemia activates
intestinal epithelial Cl
secretion. Previous data suggest that
A2b receptor-mediated
Cl
secretory responses may
be dampened by epithelial cell nucleoside scavenging. The present study
utilizes isotopic flux analysis and nucleoside analog binding assays to
directly characterize the nucleoside transport system of cultured T84
human intestinal epithelial cells and to explore whether adenosine
transport is regulated by secretory agonists, metabolic inhibition, or
phorbol ester. Uptake of adenosine across the apical membrane displayed characteristics of simple diffusion. Kinetic analysis of basolateral uptake revealed a Na+-independent,
nitrobenzylthioinosine (NBTI)-sensitive facilitated-diffusion system
with low affinity but high capacity for adenosine. NBTI binding studies
indicated a single population of high-affinity binding sites
basolaterally. Neither forskolin,
5'-(N-ethylcarboxamido)-adenosine, nor metabolic inhibition significantly altered adenosine transport. However, phorbol 12-myristate 13-acetate significantly reduced both
adenosine transport and the number of specific NBTI binding sites,
suggesting that transporter number may be decreased through activation
of protein kinase C. This basolateral facilitated adenosine transporter
may serve a conventional function in nucleoside salvage and a novel
function as a regulator of adenosine-dependent
Cl
secretory responses and
hence diarrheal disorders.
purinergic receptors; chloride channels; inflammation; ischemia; intestinal secretion
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