Release of Adenosine from Ischemic Brain

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Abstract
A threefold increase in tissue adenosine levels was produced by ischemia in the dog and the rat brain within 1 minute; adenosine levels increased further with longer periods of ischemia. Inosine and hypoxanthine were also increased by ischemia but to different degrees. The nucleosides and hypoxanthine appeared in cerebrospinal fluid during ischemia in the dog brain, and incubation of adenosine in normal cerebrospinal fluid failed to show the presence of degradative enzymes. Intra-arterially administered adenosine in the dog and the cat produced little or no increase in cerebral blood flow or the diameter of pial arterioles, respectively, when it was given in amounts that reduced arterial blood pressure. However, when it was applied topically to exposed pial arterioles of the cat, adenosine induced dilation that was roughly proportional to the dose used. When U-14C-adenosine was infused into the internal carotid arteries of the dog, no radioactivity was detectable in the cerebrospinal fluid and practically none appeared in the brain tissue. When labeled adenosine was added to dog cerebrospinal fluid, only a few counts appeared in the cerebral venous blood, whereas cerebral tissue was heavily labeled with 84-87% of the radioactivity in the form of adenine nucleotides. In the rat, after intravenous injection of labeled adenosine, the counts per gram of heart were twenty- to thirtyfold greater than those per gram of brain. These observations indicate that intra-arterially administered adenosine probably fails to cross the blood-brain barrier rapidly enough to influence cerebral blood flow but that it can be released from the ischemic brain into the cerebrospinal fluid and be reincorporated from the cerebrospinal fluid into brain nucleotides. Hence, adenosine can conceivably participate in the regulation of cerebral blood flow.