title: Glucose availability modulates the timing of the luteinizing hormone surge in the ewe.
abstract: To determine if glucose availability modulates the timing of the positive feedback action of oestrogen on gonadotropin secretion, we monitored the estradiol-induced luteinizing hormone (LH) surge in sheep (n = 5/group) made transiently hypoglycemic by insulin. Experiment 1 determined 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 pulse frequency for 8 h (5.0 +/-0.32 pulses/4 h before versus 2.5+/-0.34 pulses/4 h after insulin; P<0.05; mean +/- SEM). Using this same paradigm, experiment 2 determined the influence of the transient hypoglycemia on the LH surge mechanism. In control sheep, estradiol (subcutaneous 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 all three time groups, P <0.05). Infusion of glucose from hours 12-30, along with insulin, prevented hypoglycemia and restored the normal timing 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 mode of LH secretion sensitive to metabolic fuel availability, but the surge mode of LH secretion is as well.
authors.affiliation: Reproductive Sciences Program, Unit for Laboratory Animal Medicine, University of Michigan, Ann Arbor, USA.
authors.forename: C L
authors.initials: CL
authors.lastname: Medina
authors.forename: S
authors.initials: S
authors.lastname: Nagatani
authors.forename: T A
authors.initials: TA
authors.lastname: Darling
authors.forename: D C
authors.initials: DC
authors.lastname: Bucholtz
authors.forename: H
authors.initials: H
authors.lastname: Tsukamura
authors.forename: K
authors.initials: K
authors.lastname: Maeda
authors.forename: D L
authors.initials: DL
authors.lastname: Foster
mesh_terms: D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects / Q000502:physiology; D004967:Estrogens / Q000494:pharmacology; D005260:Female; D005947:Glucose / Q000502:physiology*; D007986:Luteinizing Hormone / Q000097:blood / Q000378:metabolism*; D012756:Sheep
chemical_list: D004967:Estrogens; D007986:Luteinizing Hormone; D005947:Glucose