Blood-brain barrier transport and brain metabolism of adenosine and adenosine analogs
Pardridge, W.M.; Yoshikawa, T.; Kang, Y.S.; Miller, L.P.
Journal of Pharmacology and Experimental Therapeutics 268(1): 14-18
1994
ISSN/ISBN: 0022-3565 PMID: 8301550 Document Number: 437919
Adenosine transport through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo, is mediated by a saturable transport system that has not been characterized extensively. Moreover, the inability of adenosine to augment cerebral blood flow in most species after intracarotid adenosine administration suggests the presence of an enzymatic BBB to circulating adenosine. Therefore, the present studies investigate the Michaelis-Menten kinetics of BBB adenosine transport and the rate of cerebral metabolism of circulating adenosine after internal carotid artery perfusion in anesthetized rats. The studies also assess the ability of various adenosine analogues to inhibit (3 H)adenosine transport at the BBB in vivo. Initial rates of BBB transport of adenosine in vivo were observed for at least 15 sec of internal carotid artery perfusion. BBB adenosine transport was partially sodium-dependent and was saturable with the following kinetic parameters: K-m = 1.1 +- 0.2 mu-M; V-max = 202 +- 44 pmol/min/g; and K-D = 34 +- 6 mu-l/min/g. BBB transport of (3H)adenosine was not inhibited by cyclohexyladenosine or S-(4-nitrobenzyl)-6-thionosine, but was inhibited by dipyridamole (K-i = 2.2 +- 0.9 mu-M). Capillary depletion studies were performed, which demonstrated sequestration of (3H) radioactivity by the microvascular pellet after carotid arterial infusion of (3H)adenosine. Only 10 +- 3% of cerebral (3H) radioactivity resided in the free adenosine pool after 15 sec of internal carotid artery perfusion of (3H)adenosine and rapid termination of brain metabolism with microwave irraddiation. In conclusion, these studies demonstrate that: 1) the BBB adensine tansporter has an affinity higher than that demonstrated previously with the brain-uptake index technique; 2) circulating adensine is metabolized rapidly by brain, possibly at the microvasculate, to form and enzymatic BBB; and 3) the adenosine transporter on the luminal side of the BBB is S-(4-nitrobenzyl)-6-thioinosine-insensitive.