18 EFFECT OF GnRH TREATMENT IN LH SECRETION IN INTACT AND OVARIECTOMIZED NELLORE HEIFERSD. Cardoso A , F. F. Guerra B and G. P. Nogueira C
A Regional Pole of Development of the Paulista Extreme West - APTA, Araçatuba, SP, Brazil;
B FUNEP, Penápolis, SP, Brazil;
C DAPSA, University of Sao Paulo State-UNESP, Araçatuba, SP, Brazil
Reproduction, Fertility and Development 22(1) 167-167 http://dx.doi.org/10.1071/RDv22n1Ab18
Published: 8 December 2009
The gonadotropin-releasing hormone (GnRH) and its analogues become control reproductive function in farm animals. The GnRH agonist (GnRHa), which is used in veterinary practice, has a longer half-life and higher affinity to GnRH receptors than the native decapeptide. GnRH or GnRHa effects depend on the species, dose, route, and the moment of administration. As Nellore is the main bovine breed in Brazil, corresponding to 70% of Brazilian herds, more studies on its reproductive physiology are needed. The objective was to study luteinizing hormone (LH) secretion in intact (I, n = 4) and ovariectomized (Ov, n = 4) prepubertal Nellore heifers (18 months) after gonadorelin acetate administration (25 μg, iv; Lecirelina, Gestran Plus, ARSA S.R.L., Buenos Aires, Argentina) from samples collected every 15 min for 10 h. LH was quantified by RIA, with sensitivity of 0.03 ng mL-1 and CV of 11%. Maximum LH amplitude was considered as the peak for each period. Peaks were identified as an increase 2 times higher than intraassay CV and involving at least 3 samples. The time for the highest peak occurrence was determined by GraphPad Prism program (Prism 3.00 for Windows, GraphPad Software, San Diego CA, USA). Results were compared by the ANOVA followed by t-test either paired or not using GraphPad InStat (3.06 version). After GnRH treatment, maximum LH peak amplitude was higher in Ov (12.48 ± 1.41 ng mL-1) (P ≤ 0.05) than in intact heifers (6.42 ± 1.30 ng mL-1), but there was no significant difference (P ≥ 0.05) on LH pulse frequency, with only one peak in both groups during the 10-h period. Although there was a small delay in peak occurrence in I (63.75 ± 21.48 min) compared with Ov heifers (45 ± 16.43 min), there was no difference (P ≥ 0.05) between groups. The results suggested a higher hypothalamus sensitivity to GnRH in absence of gonadal steroids or a higher LH storage during the hypergonatropism period.
Support by FAPESP (São Paulo, Brazil); fellowship from CAPES.