Resistance of Growth Hormone Secretion to Hypoglycemia in the Mouse

Although previous studies have demonstrated that acute hypoglycemia inhibits growth hormone (GH) secretion due to stimulation (hypothalamic somatostatin (SS) neurones in the rat, the effect of hypoglycemia on GH secretion has not yet been elucidated in the mouse In this study, the effects of insulin-induced hypoglycemia on mouse GH secretion, hypothalamic c-fos expression, GH-releasing hormon (GRH) and SS mRNA levels were investigated in conscious male mice. Seven days after implantation of chronic atrial catheters, bloo samples were taken every 20 min from 1200–1600 h under unrestrained conditions. Insulin was administered iv every 20 min fro1 1200-1240 h to induce moderate hypoglycemia (MH) and severe hypoglycemia (SH), respectively. Expression of hypothalamic c-fos protei was examined 30 min and 60 min after induction of hypoglycemia by immunohistochemistry. Hypothalamic GRH and SS mRNA level were examined 1 h and 3 h after induction of hypoglycemia by Northern blot analysis. The lowest mean plasma glucose levels after insuli injections were 49.1 ± 4.1 mg/dl and 34.2 ± 5.6 mg/dl in conscious mice, respectively. However, pulsatile GH secretion was no significantly altered in either group. Although both MH and SH markedly stimulated c-fos expression in specific hypothalamic nuclc including the paraventricular nucleus, they did not induce c-fos protein in the periventricular nucleus. Neither MH nor SH altere hypothalamic GRH or SS mRNA levels. These results suggest that hypoglycemia does not activate SS neurons which inhibit GH secretio in the mouse.