ACUTE EFFECTS OF HYPOGLYCEMIA AND HYPERGLYCEMIA ON HYPOTHALAMIC GROWTH HORMONE-RELEASING HORMONE AND SOMATOSTATIN GENE-EXPRESSION IN THE RAT

Although previous studies have indicated that changes in the plasma gl ucose concentration alter GH secretion in the rat, the roles of hypoth alamic GH-releasing hormone (GRH) and somatostatin (SS) in the glucose modulation of GH secretion have not yet been elucidated, We investiga ted the acute effects of hypo- and hyperglycemia on hypothalamic GRH a nd SS mRNA in the rat. Long term atrial catheters were implanted in ma le rats, and sequential blood sampling was performed in conscious anim als. Insulin and 50% glucose were administered iv to induce hypo- and hyperglycemia, respectively. At the end of the experiment, the hypotha lamus was removed and total RNA was extracted for Northern blotting. H ypoglycemia completely inhibited pulsatile GH secretion, whereas hyper glycemia partially inhibited GH secretion. Direct effects of insulin i tself on GH secretion in the insulin-induced hypoglycemia were ruled o ut by glucose clamp studies. Hypothalamic SS mRNA levels were increase d dramatically by hypoglycemia (266 +/- 37%) and moderately by hypergl ycemia (183 +/- 45%). Hypothalamic GRH mRNA levels were increased only by hyperglycemia (159 +/- 18%). The GH response to GRH was decreased dramatically by hypoglycemia and moderately by hyperglycemia These res ults indicated that acute changes in the plasma glucose concentration stimulated hypothalamic SS mRNA in the rat.