No abstract

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.

No abstract

Diminished GH secretion is a well known association of obesity. As in obese humans, Zucker fatty rats develop a progressive GH deficiency, present at 6 weeks of age and maximal at 10 to 12 weeks. The aim of this study was to investigate the GH dependence of IGF-I gene expression in liver and extrahepatic tissues of the obese Zucker rat as a model of progressive GH reduction during adult life. Six-and 11-week-old obese Zucker rats and their lean littermates were used to compare body weight, glycemia, insulinemia, serum GH and IGF-I levels and IGF-I mRNA expression in liver, heart, aorta, kidney and skeletal muscle. In comparison with lean controls, obese Zucker rats showed at both ages comparable glycemia, severe hyperinsulinemia (mU/ml, mean ...; 6 weeks 138 10 vs 45 6 P<0·001; 11 weeks 147 14 vs 46 3, P<0·001) and lower GH (ng/ml; 6 weeks 1·7 0·9 vs 2·7 1·1; 11 weeks 1·5 0·9 vs 4·2 1·2) in the presence of similar circulating IGF-I levels (ng/ml; 6 weeks 774 26 vs 694 28; 11 weeks 1439 182 vs 1516 121). Hepatic IGF-I mRNA expression was already reduced at 6 weeks of age due to a significant decrease in the IGF-Ib transcript compared with lean controls (relative units; IGF-Ia: 99 2% vs 100 5%; IGF-Ib: 69 10% vs 100 2%, P<0·05) and this reduction was more marked in 11-week-old animals when both IGF-I transcripts were significantly diminished (relative units; IGF-Ia: 80 6% vs 100 1%, P<0·05; IGF-Ib: 65 5% vs 100 2%, P<0·01). Extrahepatic tissues expressed almost exclusively the IGF-Ia transcript, the amount of which relative to controls was: (1) similar at 6 weeks and decreased at 11 weeks in kidney and skeletal muscle extracts (relative units; kidney: 6 weeks 88 10% vs 100 2%; 11 weeks 76 3% vs 100 4%, P<0·05; vastus lateralis: 6 weeks 95 7% vs 100 10%; 11 weeks 59 4% vs 100 2%, P<0·001); (2) similar at both ages in thoracic aorta (relative units; 6 weeks 121 6% vs 105 5%; 11 weeks: 91 14% vs 100 4%); and (3) increased at both ages in left ventricle extracts (relative units; 6 weeks 114 2% vs 99 9%, P<0·05; 11 weeks 119 7% vs 95 3%, P<0·05).These data support the existence of tissue-specific dependence of IGF-I mRNA on GH levels during adulthood, reflected by the different behavior of IGF-I expression for each tissue in conditions of progressive decrease of GH levels.

Extensive studies exploring the regulation of hypothalamic somatostatin GHRIH release have been reported, but the factors regulating GHRIH release in the cerebral cortex have not been well defined. We have studied the effects of central neurotransmitters on GHRIH secretion by cultured fetal rat cerebral cortical cells and on intracellular GHRIH levels. Cells maintained in vitro for 15-20 days were incubated with dopamine (DA), acetylcholine (ACh), gamma-aminobutyric acid (GABA), norepinephrine (NE), serotonin (SE) or histamine (His) (10(-11) M to 10(3) M) for 30 minutes. Following incubation, immunoreactive GHRIH was measured by RIA in cell extracts and incubation media. DA increased intracellular GHRIH content but have no effect on GHRIH in the media. Both media and intracellular GHRIH content were significantly reduced by GABA and SE. The effect of NE was stimulatory at low (10(-9) M) and inhibitory at high (10(-5) M to 10(-3) M) concentrations. ACh was found to increase media GHRIH and to decrease intracellular GHRIH content; 30 min exposure to His did not significantly modify either media or intracellular GHRH. Our findings with fetal rat cerebral cortical cells in culture demonstrate that endogenous neurotransmitters do have the capacity to directly influence GHRIH regulation.