Pituitary tumors diagnosed before surgery as "non-functioning" in fact represent a heterogenous group, the majority of which express
glycoprotein hormones or their free subunits. It is known that some of them expresses
somatostatin receptors, but the data available until now rarely refer to the receptor subtype. Five different subtypes of
somatostatin receptors (sst1-5) have been cloned. We studied 18
pituitary tumors diagnosed before surgery as "non-functioning." After the surgery the
tumors were immunostained with
antibodies against
pituitary hormones and alpha subunit as well as with
antibodies against the
somatostatin receptor proteins 1-5. Thirteen
adenomas expressed immunoreactivity for FSH, LH, and/or alpha subunit and were classified as gonadotroph
adenomas. The remaining five
adenomas were immunonegative for all the examined
pituitary hormones and were diagnosed as null cell
adenomas. All the
adenomas of both the groups showed immunopositivity for at least three receptor subtypes. The strongest immunopositivity was found in both groups with anti-sst1 and anti-sst5
antibodies. The marked immunopositivity was also revealed in both groups with anti-sst2B antibody. On the other hand, the sst2A immunopositivity was weak or absent in a majority of
tumors. The main difference between two groups was in the
sst4 receptor subtype which was absent in all but two gonadotroph
adenomas but present in all but one null cell
adenoma. These findings suggest that "non-functioning"
pituitary adenomas are potential candidates for
therapy with
somatostatin analogs targeted mainly to the receptor subtypes 1 and 5.