Luteinizing hormone secretion as affected by hypophyseal stalk transectionand estradiol-17 beta in ovariectomized gilts

The objectives were to determine hypothalamic regulation of pulsatile lutei nizing hormone (LH) secretion in female pigs and the biphasic feedback acti ons of estradiol-17 beta (E-2-17 beta). In the first study, the minimum eff ective dosage of E-2-17 beta that would induce estrus in ovariectomized gil ts was determined to be 20 mu g/kg body weight. In the second study, ovarie ctomized gilts were assigned randomly on day 0 to treatments: (a) hypophyse al stalk transection (HST), (b) cranial sham-operated control (SOC), and (c ) unoperated control (UOC), On day 3, gilts from each group received a sing le i.m. injection of either E-2-17 beta (20 mu g/kg body weight) or sesame oil. Blood was collected from an indwelling jugular cannula at 15 min inter vals for 3 h before (day -2) and after treatment (day 2) from HST, SOC and UOC gilts. On day 3, blood was collected at 2 h intervals for 12 h after E- 2-17 beta or sesame oil injection and at 4 h intervals thereafter for 108 h . Pulsatile LH secretion in all gilts 2 days after ovariectomy exhibited a frequency of 0.9 +/- 0.06 peaks/h, amplitude of 1.3 +/- 0.13 ng/ml, baselin e of 0.8 +/- 0.07. Serum LH concentrations from SOC and UOC gilts were simi lar on day 2 and profiles did not differ from those on day -2. In HST gilts pulsatile LH release was abolished and mean LH concentration decreased com pared with controls (0 versus 0.9 +/- 0.06 peaks/h and 0.77 +/- 0.03 versus 1.07 +/- 0.07 ng/ml, respectively; P < 0.05). E-2-17 beta or sesame oil di d not affect serum LH concentration in HST gilts, and LH remained constant throughout 120 h (0.7 +/- 0.07 ng/ml). In SOC and UOC control gilts, E-2-17 beta induced a 60% decrease (P < 0.05) in LH concentration within 12 h, an d LH remained low until 48 h, then increased to peak values (P < 0.05) by 7 2 h, followed by a gradual decline to 120 h. Although pituitary weight decr eased 31% in HST gilts compared with controls (228 versus 332 mg, P < 0.05) , an abundance of normal basophils was evident in coronal sections of the a denohypophysis of HST comparable to that seen in control gilts. The third a nd fourth studies determined that hourly i.v. infusions of LHRH (2 mu g) an d a second injection of E-2-17 beta 48h after the first had no effect on th e positive feedback action of estrogen in UOC. However, in HST gilts that r eceived LHRH hourly, the first injection of E-2-17 beta decreased (P < 0.05 ) plasma LH concentrations while the second injection of E-2-17 beta failed to induce a positive response to estrogen. These results indicate that bot h pulsatile LH secretion and the biphasic feedback action of E-2-17 beta on LEI secretion depend on hypothalamic regulatory mechanisms in the gilts. T he isolated pituitary of HST gilts is capable of autonomous secretion of LH ; E-2-17 beta will elicit direct negative feedback action on the isolated p ituitary gland if the gonadotropes are supported by exogenous LHRH, but E-2 -17 beta at high concentrations will not induce positive feedback in isolat ed pituitaries. Thus, the direct effect of E-2-17 beta on the pituitary of monkeys cannot be mimicked in pigs. (C) 2000 Elsevier Science B.V. All righ ts reserved.