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1 Department of Veterinary Biomedical Sciences and 2 Department of Medical Physiology, 3 Dalton Cardiovascular Research Center, University of Missouri-Columbia, Columbia, Missouri 65211
Adenosine (ADO), an endogenous regulator of coronary vascular
tone, enhances vasorelaxation in the presence of nucleoside transport
inhibitors such as dipyridamole. We tested the hypothesis that coronary
smooth muscle (CSM) contains a high-affinity transporter for ADO.
ADO-mediated relaxation of isolated large and small porcine coronary
artery rings was enhanced 12-fold and 3.4-fold, respectively, by the
transport inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI). Enhanced relaxation was independent of endothelium and was selective for ADO over synthetic analogs. Uptake of [3H]ADO into
freshly dissociated CSM cells or endothelium-denuded rings was linear
and concentration dependent. Kinetic analysis yielded a maximum uptake
(Vmax) of 67 ± 7.0 pmol · mg
protein
1 · min
1 and a Michaelis
constant (Km) of 10.5 ± 5.8 µM in
isolated cells and a Vmax of 5.1 ± 0.5 pmol · min
1 · mg wet wt
1
and a Km of 17.6 ± 2.6 µM in intact
rings. NBTI inhibited transport into small arteries
(IC50 = 42 nM) and cells. Analyses of extracellular space and diffusion kinetics using [3H]sucrose indicate
the Vmax and Km for ADO
transport are sufficient to clear a significant amount of extracellular
adenosine. These data indicate CSM possess a high-affinity nucleoside
transporter and that the activity of this transporter is sufficient to
modulate ADO sensitivity of large and small coronary arteries.
dipyridamole; S-(4-nitrobenzyl)-6-thioinosine; erythro-9-(2-hydroxy-3-nonyl)-adenine hydrochloride; 2-chloroadenosine
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