σ1 and σ2 receptors have been shown to exist in a number of rodent and human tumor cell lines. Although their expression is heterogeneous and their function is unknown, σ receptors have been proposed as potential targets for diagnostic tumor-imaging agents. In this study, the density of σ2 receptors in proliferative (P) and quiescent (Q) cells of the mouse mammary adenocarcinoma, line 66, was examined. Scatchard analyses of σ2 receptors were performed on membrane preparations of 66 P cells from 3-day cultures and 66 Q cells from 7-, 10-, and 12-day cultures. The Scatchard studies revealed that 66 P cells had ∼10 times more σ2 receptors/cell than the 66 Q cells from 10-day cultures. Although >97% of the cells were quiescent after 7 days in culture, the maximum differential in the σ2 expression between 66 P and 66 Q cells was not attained until these cells had been in culture for 10 days. These data suggest that ligands labeled with positron-emitting or single photon-emitting radionuclides, which selectively bind σ2 receptors, have the potential to noninvasively assess the proliferative status of human breast tumors.

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This work was supported by NIH Grants NS-31907 (to R. H. M.) and CA-45156 (to K. T. W.).

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