The effects of somatostatin or the somatostatin analog SMS 201-995 were studied on 4 densely granulated somatotroph adenomas and 4 sparsely granulated somatotroph adenomas in vitro. Release of growth hormone (GH) into culture media during incubation with somatostatin or SMS 201 -995 were measured by radioimmunoassay, and light-microscopical and ultrastructural morphometric parameters were compared with those of cultured control somatotroph adenoma cells of the same tumor. In all tumors except for 1 densely granulated somatotroph adenoma, somatostatin or SMS 201-995 decreased GH release into culture media in 24- and 2-hour incubations. After 48-hour incubation with somatostatin or SMS 201-995, there was no change in cell size or secretory granule diameter. One densely granulated adenoma showed decreased cytoplasmic volume density (CVD) of Golgi apparatus and secretory granules, and a sparsely granulated adenoma had reduced CVD of endoplasmic reticulum. All the tumors that responded with decreased GH release exhibited increased CVD of lysosomes after incubation with somatostatin or SMS 201-995. These results indicate that both densely and sparsely granulated somatotroph adenomas respond to somatostatin inhibition and, furthermore, that inhibition of hormone release is associated with accumulation of lysosomes, suggesting lysosomal degradation of stored hormone.
Keywords: Acromegaly; Adenoma; Growth Hormone; Pituitary Adenoma; Secretory Granule.