The Release of LH in Response to Kiss-10 Is Dependent to the Age and Influenced by Serum Concentrations of Estradiol (E2) in Heifers in Tropical Regions.

Abstract

The objective of the study was to evaluate the influence of the age of heifers on the release of LH in response to Kiss-10. Heifers (4, 7, and 11 months old) received an i.v. injection of kiss-10 (5 micrograms/kg). Serum concentrations of LH, FSH and estradiol (E2) were quantified by radioimmunoassay and data were subjected to analysis of variance. Serum concentrations of LH and FSH were analyzed in a repeated measurements design in blood samples collected every 15 minutes from 4 hours before to 4 hours after administration of Kiss-10. Serum concentrations of E2 were analyzed in a completely randomized design from a blood sample collected just before treatment with Kiss-10. Serum concentrations of FSH were similar (P > 0.3) in heifers of 4, 7, and 11 months old (21 ± 2.2, 18 ± 2.2, and 15 ± 2.4 ng/ml, respectively) and administration of Kiss-10 increased (P < 0.01) secretion of FSH compared to the concentrations during the pre-treatment period (33 ± 1.9 vs. 18 ± 1.3 ng/ml), and the magnitude of the stimulatory effect of Kiss-10 was similar among age groups. Similarly, serum concentrations of LH did not differ (P > 0.1) in heifers of 4, 7, and 11 months old (1.4 ± 0.26, 2.3 ± 0.41, and 2.3 ± 0.4 ng/ml, respectively). In all three age groups LH increased (P < 0.01) in response to Kiss-10 compared to the pre-treatment period (3.7 ± 0.60 vs. 1.1 ± 0.21 ng/ml). However, the stimulatory effect of Kiss-10 on LH secretion was higher (P < 0.01) in the 11 versus the 4 month old heifers when evaluated as either mean LH (4.8 ± 0.88 vs. 2.6 ± 0.41 ng/ml) or area under the curve of LH (36.5 ± 7 vs. 18.9 ± 3 arbitrary units). The greater LH secretion induced by Kiss-10 detected in 11 month old heifers was associated with higher (P < 0.06) serum concentrations of E2 compared to 4 month old heifers (1.68 ± 0.51 vs. 0.43 ± 0.11 pg/ml). We speculate that increased ovarian activity that occurs as the heifers approach puberty increases sensitivity of the hypothalamic-pituitary axis to Kiss-10. (poster)