PARENT SESSION
PLATFORM SESSION 21: REGULATION OF GONADOTROPINS AND THEIR RECEPTORS: II
Chair: LaBarberra, Andrew¹, ¹
Co-chair: Meehan, Thomas¹, ¹
Harborside D
2:00 PM-4:00 PM

(452) DOSE-RELATED EFFECTS OF RECOMBINANT OVINE LEPTIN ON LUTEINIZING HORMONE SECRETION AFTER SHORT-TERM FASTING IN COWS.

Zieba, Dorota^1,2,3, Amstalden, Marcel^1,2,3, Maciel, Marlon^1,2,3, Keisler, Duane⁴, Raver, Nina⁵, Gertler, Arieh⁵, Williams, Gary^1,2,3, ¹ Animal Reproduction Laboratory, Beeville, TX² Department of Animal Science, College Station, TX³ Center for Animal Biotechnology and Genomics, College Station, TX⁴ Department of Animal Science, Columbia, MO⁵ Institute of Biochemistry, Food Sciences and Nutrition, Rehovot, Israel

ABSTRACT- Leptin, a 16 kDa adipocyte-derived hormone, is a potent regulator of energy metabolism and functions as a metabolic signal to the central reproductive axis. In ruminants, the acute link between the central reproductive axis and changes in nutrient intake is less distinct than in monogastrics, and short-term feed restriction does not measurably reduce pulsatile LH release except in peripubertal heifers. Nevertheless, we have shown recently that fasting permits leptin to modulate LH secretion in cows, and this effect is mediated primarily at the anterior pituitary. In laboratory rodents, the stimulation of LH release by leptin is dose-related. To test the hypothesis that leptin^,s effects on LH secretion in fasted cows are dose-related, we examined the effect of various doses of recombinant ovine leptin (oleptin) on LH secretion. Twenty mature, ovariectomized beef cows, each bearing an estradiol implant to maintain circulating estradiol level at 2-4 pg/ml, were used. All cows were fasted for 60 h with free access to water. Animals were assigned randomly to one of four groups (n=5/group): 1) saline control; 2) oleptin, 0.2 g/kg; 3) oleptin, 2.0 g/kg and 4) oleptin, 20 g/kg. Blood samples were collected at 10-min intervals for 6 h on Days 0 and 2, with saline or oleptin injected i.v. immediately after the first intensive sample on Day 2 (54 h). Leptin caused a dose-related increase (P<0.001) in mean concentrations of circulating LH. The stimulation of LH release by leptin was significant at the lowest (141 % of control) and middle (122% of control) doses employed. These increases appeared to result from an augmentation of basal secretion, as pulse characteristics were not affected. Luteinizing hormone secretion increased slightly (P<0.05) during first h with the highest dose, but did not differ from controls thereafter. Recently, we have shown that leptin modulates at least two aspects of anterior pituitary LH release in ruminants (basal and GnRH-mediated). It is noteworthy that similar to reports in rodents, high doses of leptin in the current experiment did not increase LH release overall. Collectively, our results show that leptin can have a potent effect on LH secretion in fasted cows. However, the effect appears to be dose-related, and this observation should be considered in future studies.

KEY WORDS: Leptin, LH, Cow, Fasting