The stimulatory effect of potassium depolarization upon somatostatin (SS) mRNA levels in primary cultures of fetal cerebrocortical cells was analyzed. Depolarizing stimuli, such as 56 mM K+ exposure for 30 min, elicited an increase in immunoreactive somatostatin (IR-SS) release to the media and decreased SS mRNA levels. These were increased when exposure to depolarization stimuli was prolonged up to 3 or more hr. At this time, potassium (30 and 56 mM) acted as a secretagogue, stimulating SS secretion, but was also effective in stimulating SS mRNA levels, suggesting that SS secretion can be coupled to SS mRNA accumulation. These changes were inhibited by the Ca2+ channel antagonist verapamil. In contrast, Na+ channel blockade by TTX did not modify the 24 hr potassium-induced increase in SS mRNA, although it partially abolished potassium-induced SS secretion. Examination of the rate of disappearance of SS mRNA levels after inhibition of mRNA transcription by actinomycin-D revealed that K+ stimulation of cerebrocortical cells stabilized the SS mRNA. These results suggest that the induction of SS mRNA expression by K+ is dose dependent, and involves the modulation of ion channels. The time-course study confirmed that the K(+)-induced SS mRNA accumulation is time dependent, chronic activation of the Ca2+ channels being necessary to stimulate SS gene expression. K+ stimulation may also increase the level of SS mRNA in cerebrocortical cells by reducing its rate of degradation.

To determine the possible physiological role of endogenous growth hormone-releasing factor (GRF) in the neuronal content and release of cerebral somatostatin (SS), we studied the effect of endogenous GRF blockade on the immunoreactive SS (IR-SS) content of cells and media in fetal rat cerebral cortical and hypothalamic cells in culture. Cells were cultured in minimum essential medium (MEM) with 10% fetal calf serum and 10% horse serum. After 7–10 days in vitro, media were replaced with MEM without sera containing anti-GRF immunoglobulins G (IgG) for 1, 5 or 24 h. Controls were incubated with equal amounts of IgG from normal rabbit serum (NRS). In another group of experiments, cells were incubated with GRF (10^–11 to 10^–7M) for 1 or 24 h. Long-term exposure (24 h) to anti-GRF IgG resulted in decreased media and intracellular IR-SS content, in both cerebral cortical and hypothalamic cells. 24 h treatment with GRF caused a dose-dependent increase in the IR-SS content of cells and media, the stimulatory action being abolished by the addition of anti-GRF to plates containing 10^–7M GRF. On the contrary, when cells were exposed to anti-GRF IgG for 1 h, IR-SS increased in the media as compared to the control group. Short-term incubation (1 h) with GRF (10^–9 to 10^–7M) resulted in a dose-dependent inhibition of IR-SS content in the cells and media. This inhibitory action was partially prevented by the addition of anti-GRF to plates containing 10^–7M GRF. Patterns of IR-SS cell and media content were qualitatively similar in both cerebral cortical and hypothalamic cells. In plates containing anti-GRF, the accumulation of IR-SS in the cells and media was lower than in the control groups (IgG from NRS) throughout the incubation time.