Differential pituitary and gonadal function of Chinese Meishan and European white composite boars: effects of gonadotropin-releasing hormone stimulation, castration, and steroidal feedback.

Abstract

Considerable breed differences exist in concentrations of gonadotropins between Chinese Meishan boars and meat-type crossbred boars of European origin. To define mechanisms that might be responsible, sensitivity to GnRH stimulation, hormonal clearance, and gonadal pituitary feedback effects were monitored in two experiments. In the first trial, jugular catheters were placed into mature Meishan (n = 7) and white composite boars (n = 8). Boars were repetitively sampled after administration of two doses of GnRH (150 and 1500 ng/kg BW) a week apart. Before administration of GnRH, FSH was greater in Meishan boars (554 +/- 50 vs. 102 +/- 8 ng/ml, p < 0.01). Both breeds responded with increases in FSH, LH, and testosterone; but concentrations of gonadotropins in Meishan boars were increased for a greater duration than in white composite boars. By 160 min after GnRH administration, gonadotropins were declining in white composite boars. Maximal plasma concentrations of FSH after GnRH stimulation were greater with the higher GnRH dosage than with the lower (1058 +/- 113 vs. 730 +/- 70 in Meishan and 184 +/- 40 vs. 141 +/- 14 ng/ml in white composite boars). In the second trial, boars were assigned to three groups related to their circulating FSH concentrations: European white composite boars (FSH = 100 ng/ml), Meishan boars with low FSH (FSH = < 500 ng/ml), and Meishan boars with high FSH (FSH > 750 ng/ml; n = 5 per breed group). Boars were cannulated and repetitively sampled during three protocols: 1) low GnRH stimulation (15 ng/kg BW), 2) pre- and postcastration, and 3) steroidal treatment (0.5 mg testosterone propionate and 20 micrograms estradiol cypionate/kg BW; then 24 days later, a 20-strength low-dosage steroid treatment). At the low GnRH stimulation, LH and testosterone concentrations were increased but FSH concentrations were not altered. Rate of increase of FSH and decline of testosterone concentrations were not different between the three treatment groups after castration. Castration levels of FSH were maintained and further increased with time. Rates of reduction in FSH concentration after gonadal steroidal stimulation were not different between breed groups, but Meishan boars escaped the exogenous steroid suppression earlier than white composite boars (p < 0.01). Differences in concentrations of gonadotropins in Meishan and white composite boars of European origin seem to be at the pituitary level of control and not related to gonadal factors.