Glucoreceptors located inside the blood-brain-barrier mediate hypoglycemia-induced LH inhibition in man

OBJECTIVE: To establish the role of hyperinsulinemia and hypoglycemia during the insulin tolerance test (ITT) in the regulation of luteinizing hormone (LH) secretion and the location with respect to the blood-brain barrier (BBB) of the glucosensitive areas controlling LH release. METHODS: The LH-secretory pattern during an ITT (0.15 IU/kg body weight) was evaluated in 8 normal men during infusion with normal saline (control test), glucose or fructose. RESULTS: lnsulin-induced hypoglycemia produced a significant decrement in serum LH levels in the control test, but not when the concomitant infusion of glucose prevented hypoglycemia. Fructose infusion did not change LH decrease during ITT. CONCLUSIONS: These data exclude a direct role of hyperinsulinemia in the mechanism underlying the inhibition of LH secretion during ITT. Furthermore, since glucose but not fructose crosses the BBB, the LH decrease during ITT appears to be generated by hypoglycemia at the level of glucosensitive areas located inside the BBB