Objective and Patients: 24 pituitary adenomas from acromegalic patients (13 f, 11 m; age range 19–65 yrs) were characterized for somatostatin receptor subtype 2A (sst_2A), dopamine D₂ receptor (D₂R), GH and PRL expression by immunohistochemistry, and results correlated with the in vitro and in vivo hormone responses to octreotide and quinagolide. In 9 cases, GH and PRL content was further studied by immuno-electron microscopy (EM).

Results: Immunoreactivity was semiquantitatively scored as 2 (>50% stained cells), 1 (10% to 50% stained cells), 0 (<10% stained cells). Sst_2A was scored as 2 in 13 cases, 1 in 10, and 0 in 1; D₂R was scored as 2 in 13 cases, 1 in 9, and 0 in 2; GH was 2 in 15 cases and 1 in 9, PRL was 2 in 6 cases, 1 in 13, 0 in 5. Sst_2A was positively correlated with in vitro (p=0.003) and in vivo (p=0.006) percent GH suppression by octreotide, and with the chronic suppression of IGF-I by somatostatin analogs (p=0.008). D₂R was positively correlated with in vitro percent GH (p=0.000) and PRL (p=0.005) suppression by quinagolide. EM revealed 2 pure somatotroph adenomas, 5 somatomammotrophs with a variable co-expression of GH and PRL in the same cells, and 2 tumors consisting of mixed cell types which resulted less sensitive to quinagolide and octreotide.

Conclusion: Sst_2A and D₂R are frequently co-expressed in adenomas from acromegalic patients, and immunohistochemistry may be helpful in characterizing receptor expression in pituitary adenomas, in order to select patients responsive to different treatments.