Phenotypic variation in testosterone and luteinizing hormone production among boars: differential response to gonadotropin releasing hormone and adrenocorticotropic hormone.

Variation in ability of boars to produce testosterone and luteinizing hormone (LH) in response to both gonadotropin releasing hormone (GnRH) and adrenocorticotropic hormone (ACTH) stimulation, as well as quantitative relationships between pretreatment and posttreatment responses, were assessed in a population of 38 boars of similar age and breeding. Peripheral testosterone concentrations following either GnRH or ACTH increased (P less than 0.01) to peak circulating levels of 7.16 +/- 0.62 and 8.42 +/- 0.81 ng/ml by 120 and 45 min, respectively. Post-GnRH testosterone area varied from 7.44 to 50.84 ng/ml X h (CV = 47.44%) and post-ACTH testosterone area ranged from 3.05 to 28.78 ng/ml X h (CV = 46.09%). GnRH-induced increases in testosterone were preceded by elevations (P less than 0.01) in peripheral LH concentrations but ACTH had no effect upon LH levels. Post-GnRH area varied from 7.07 to 125.45 ng/ml X h (CV = 76.61%). Significant (P less than 0.01) correlations were obtained between pre-GnRH and post-GnRH testosterone areas (r = 0.58) and between pre-ACTH and post-ACTH testosterone areas (r = 0.67). Nonsignificant (P greater than 0.10) correlations were obtained between post-GnRH and post-ACTH testosterone areas (r = 0.006) and between post-GnRH testosterone and LH areas (r = 0.09). The testosterone producing ability of boars was highly variable and their innate ability to produce testosterone influenced their response to GnRH and ACTH. Additionally, the mechanisms by which GnRH and ACTH influence testosterone production in boars appear to differ. Variation in the ability of boars to produce testosterone could not be explained on the basis of differences in circulating levels of LH.