GLUCOSE AVAILABILITY MODULATES THE TIMING OF THE LUTEINIZING-HORMONE SURGE IN THE EWE

To determine if glucose availability modulates the timing of the posit ive feedback action of oestrogen on gonadotropin secretion, we monitor ed the estradiol-induced luteinizing hormone (LH) surge in sheep (n = 5/group) made transiently hypoglycemic by insulin. Experiment 1 determ ined an effective insulin treatment, one which would depress tonic LH secretion. Two injections of insulin (5 IU/kg iv) 4 h apart were found to induce extended hypoglycemia (10-13 h) and to decrease the LH puls e frequency for 8 h (5.0 +/- 0.32 pulses/4 h before versus 2.5 +/- 0.3 4 pulses/4 h after insulin; P < 0.05; mean +/- SEM), Using this same p aradigm, experiment 2 determined the influence of the transient hypogl ycemia on the LH surge mechanism, In control sheep, estradiol (subcuta neous implants at hour 0) evoked an LH surge with a latency period of 12.4 +/- 0.5 h, When insulin was administered either before (hours -4 and 0) or after the estradiol stimulus (hours 4 and 8, or 12 and 16), the onset of the LH surge was delayed to 29.0 +/- 2.4 h (average of al l three time groups, P < 0.05). Infusion of glucose from hours 12-30, along with insulin, prevented hypoglycemia and restored the normal tim ing of the oestrogen-induced LH surge to that of controls (15.4 +/- 0. 93 h, P > 0.05), These findings suggest that not only is the tonic mod e of LH secretion sensitive to metabolic fuel availability, but the su rge mode of LH secretion is as well.