Objective: We studied the effects of long-term in-vitro exposure of human GH secreting pituitary adenoma cells to octreotide on GH release, intracellular GH concentrations and GH messenger ribonucleic acid (mRNA) levels.
Design: Human GH-secreting pituitary adenoma cells were cultured for periods from 4 days up to 3 weeks without or with octreotide (10 nM) and/or bromocriptine (10 nM). The effects of these drugs were measured on GH release, intracellular GH concentrations and intracellular GH mRNA levels.
Patients: Thirteen patients with GH-secreting pituitary adenomas were studied. Twelve patients were untreated, one had been pretreated with octreotide (12 weeks, 3 x 100 micrograms daily).
Measurements: GH, PRL, alpha-subunit and IGF-I concentrations in plasma, media and cell extracts were determined by immunoradiometric or radioimmuno-assays. GH mRNA levels were determined by automatic quantification of grain numbers in individual adenoma cells.
Results: Incubation of the adenoma cells for 4 days with 10 nM octreotide induced a dose-dependent inhibition of GH release and a parallel increase (increase varying between 124 and 617% of control) in the intracellular GH levels was observed in six of seven adenomas. In addition, bromocriptine, when effective in inhibiting GH release by the adenomas, also induced an increase in intracellular GH levels. Even after 3 weeks of exposure to 10 nM octreotide in vitro there was a statistically significant increase in intracellular GH levels (between 191 and 923% of control). Withdrawal of octreotide after 6 days of incubation resulted in a lowering of intracellular GH levels to control values, showing that the octreotide-induced increase in intracellular GH is reversible. In a 96-hour incubation with 10 nM octreotide, GH mRNA levels were increased in two, and slightly decreased in one of the three adenomas tested. This effect was time dependent in that there was no significant effect of 10 nM octreotide on GH mRNA levels in a 24-hour incubation.
Conclusions: (1) Long-term in-vitro exposure of GH-adenoma cells to octreotide causes an increase in intracellular GH levels in the majority of the adenomas, probably because of an increase in GH mRNA levels in the adenoma cells; and (2) this considerable increase in intracellular GH levels may be one of the explanations for the relatively poor effect of octreotide on tumour shrinkage in patients with GH-secreting pituitary adenomas.