BLOOD-BRAIN-BARRIER TRANSPORT AND BRAIN METABOLISM OF ADENOSINE AND ADENOSINE-ANALOGS

Adenosine transport through the brain capillary endothelial wall, whic h makes up the blood-brain barrier (BBB) in vivo, is mediated by a sat urable transport system that has not been characterized extensively. M oreover, 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 adeno sine transport and the rate of cerebral metabolism of circulating aden osine after internal carotid artery perfusion in anesthetized rats. Th e studies also assess the ability of various adenosine analogues to in hibit [H-3]adenosine transport at the BBB in vivo. Initial rates of BB B transport of adenosine in vivo were observed for at least 15 sec of internal carotid artery perfusion. BBB adenosine transport was partial ly sodium-dependent and was saturable with the following kinetic param eters: 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 [H-3]adenosine was not inhibit ed by cyclohexyladenosine or S-(4-nitrobenzyl)-6-thioinosine, but was inhibited by dipyridamole (K, = 2.2 +/- 0.9 mu M). Capillary depletion studies were performed, which demonstrated sequestration of [H-3] rad ioactivity by the microvascular pellet after carotid arterial infusion of [H-3]adenosine. Only 10 +/- 3% of cerebral [H-3] radioactivity res ided in the free adenosine pool after 15 sec of internal carotid arter y perfusion of [H-3]adenosine and rapid termination of brain metabolis m with microwave irradiation. In conclusion, these studies demonstrate that: 1) the BBB adenosine transporter has an affinity higher than th at demonstrated previously with the brain-uptake index technique; 2) c irculating adenosine is metabolized rapidly by brain, possibly at the microvasculature, to form an enzymatic BBB; and 3) the adenosine trans porter on the luminal side of the BBB is S-(4-nitrobenzyl)-6-thioinosi ne-insensitive.