Serum luteinizing hormone and testosterone in bulls during exposure to elevated ambient temperature

Eight mature Angus bulls were used to evaluate the effects of exposure to 34 C on concentrations of serum luteinizing hormone (LH) and testosterone before and after treatment with gonadotropin releasing hormone (GnRH). After a 3-week period of adjustment at 22 C, the bulls were randomly assigned to either a control (22 +/- 1 C) or a heat stress ( 34 +/- 1C) treatment for 15 days. Blood was sampled via jugular cannula at 30-min intervals for 12 hr on days -2, 6 and 15 of treatment. After the 12 hr sampling period, 200 micrograms of GnRH were rapidly infused and blood samples were collected at 15-min intervals for 1 hr and then at 30-min intervals for 5 hours. Average serum LH concentrations, the areas under 12-hr LH profiles and the magnitude of episodic releases of LH were reduced (P greater than .05) in heat stressed bulls on day 6 of treatment, but only average LH concentration was reduced (P greater than .10) on day 15. Average serum testosterone concentrations, the areas under 12-hr testosterone profiles and the number and magnitude of testosterone secretory spikes were similar for heat stressed and control bulls on day 6 of treatment. However, serum testosterone concentrations were slightly reduced (P greater than .13) after 15 days of heat stress. Testosterone and LH responses to GnRH infusion were not influenced by heat stress. These results indicate that exposure of bulls to increased ambient temperatures causes a transitory reduction in serum LH concentrations after 6 days of exposure but that serum testosterone is not significantly altered.