The density of adenosine receptors in membranes derived from rat hearts in 25 times lower than the density of receptors in rat brain membranes. Consequently, adenosine radioligands which are useful in brain such as l-[3H]phenylisopropyladenosine, [3H]cyclohexyladenosine, [3H]-2-chloroadenosine and l-[125I]hydroxyphenylisopropyladenosine are of limited usefulness in heart, due to a high ratio of nonspecific to specific binding. We have synthesized a new radioligand, [125I]-N6-4-aminobenzyladenosine, which binds to rat heart membranes with one-sixth the nonspecific binding of the other radioligands. [125I]-N6-4-aminobenzyladenosine bound to rat ventricle membranes with a KD equivalent to that of l-[125I]hydroxyphenylisopropyladenosine and a Bmax of 15.2 fmol/mg protein. [125I]-N6-4-aminobenzyladenosine bound with a higher affinity to brain (KD = 1.93 nM) than to heart membranes (KD = 11.6 nM). At the radioligand KD, 60% of the total [125I]-N6-4-aminobenzyladenosine bound to heart membranes was specifically bound. Iodination of aminobenzyladenosine increased its affinity for the adenosine receptor by 22-fold, possibly due to a steric or hydrophobic effect of iodine. The new ligand was found to be a full adenosine agonist based on its ability to inhibit cyclic adenosinemonophosphate accumulation in isolated embryonic chick heart cells and rat adipocytes. [125I]-N6-4-Aminobenzyladenosine bound to a single affinity site and was displaced from cardiac and brain adenosine receptors by other adenosine analogues with a potency order of l-phenylisopropyladenosine greater than 5'-N-ethylcarboxamide adenosine. These characteristics suggest that the radioligand binds to an Ri adenosine receptor.
Technetium-99m hexamethylpropyleneamine oxime (Tc-99m HM-PAO) was successfully used with single photon emission computed tomography (SPECT) on fourteen comatose patients who had acute head injuries. The SPECT scans were correlated with computed tomography (CT) scans obtained within 24 hours of the injury. Tc-99m HM-PAO SPECT was shown to have the following advantages: It reflected perfusion changes, was more sensitive than CT in demonstrating more lesions, and demonstrated lesions at an earlier stage than those demonstrated with CT. Different types of lesions, as evidenced by cerebral perfusion changes, have been described and categorized. The lesions that had a favorable prognosis could be separated from those with an unfavorable prognosis on the basis of the findings of Tc-99m HM-PAO SPECT.
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