Dopamine receptors in immunohistochemically characterized null cell adenomas and normal human pituitaries

Dopamine receptors were analyzed in plasma membranes from five null cell adenomas and five normal human pituitary tissues by [3H]spiperone binding. One prolactin (PRL)-producing, one growth hormone (GH)-producing, and an adrenocorticotropic (ACTH)-producing adenoma were also analyzed for dopamine receptors. Immunohistochemical staining showed that all null cell adenomas were positive for chromogranin A, while 20 to 30% of cells in each normal pituitary stained for this marker. The dissociation constant (Kd) and maximal binding capacity (Bmax) were 1.07 +/- 0.49 nM and 148 +/- 34 fmol/mg protein for null cell adenomas and 1.23 +/- 0.20 nM and 107 +/- 21 fmol/mg protein for normal pituitary tissues. The one PRL adenoma had a similar Kd but had a 5.6-fold higher Bmax than the mean Bmax for the null cell adenomas. These results indicate that immunohistochemically characterized null cell adenomas as well as normal pituitaries express dopamine receptors, but that the binding sites in null cell adenomas are much less those in PRL-secreting adenomas.