Astrocytes as a predominant cellular site of ^99mTc-HMPAO retention

Technetium-99m-d,l-hexamethylpropylene amine oxime (^99mTc-HMPAO) retention in the brain monitored by single photon emission computed tomography (SPECT) is currently used as a marker of cerebral blood flow. The purported mechanism by which ^99mTc-HMPAO accumulates in the brain is through its intracellular conversion from a lipophilic form to more hydrophilic derivatives within the brain parenchyma. The issue of the contribution of different cell types on ^99mTc-HMPAO retention was investigated in vitro by studying the accumulation of ^99mTc-HMPAO in primary cultures of mouse cortical astrocytes and neurons. Results show that ^99mTc-HMPAO retention predominates in astrocytes over neurons by a factor of ∼2.5 (0.26 ± 0.05 vs. 0.095 ± 0.042 fmol/mg protein after 120 minutes, respectively). Diethyl maleate (60 μmol/L), ethacrynic acid (1 mmol/L) and buthionine sulfoximine (1 mmol/L), 3 agents which significantly reduced glutathione levels also decreased ^99mTc-HMPAO retention in both astrocytes (29%, 3%, and 46% of control, respectively) and neurons (69%, 11% and 63% of control). Decrease did not always correlate with glutathione levels, however, which suggests that other factors could be involved. The possibility that cell energy status determines ^99mTc-HMPAO retention was also assessed. Agents that activate (glutamate, azide) or inhibit (cytochalasin B) glucose utilization in astrocytes, as measured by the ³H-2-deoxyglucose method, were without effect on ^99mTc-HMPAO retention. In conclusion, the data presented indicate that astrocytes may constitute a prominent site of ^99mTc-HMPAO retention and most likely contribute significantly to the SPECT signal. In addition, the data also suggest that specific alterations in glial cell metabolism could explain flow-independent changes in ^99mTc-HMPAO retention in the brain as observed by SPECT in some pathologies.