Changes in testicular steroidogenic acute regulatory (STAR) protein, steroidogenic enzymes and testicular morphology associated with increased testosterone secretion in bulls receiving the luteinizing hormone releasing hormone agonist deslorelin.

Testosterone secretion and the expression and relative contents of steroidogenic acute regulatory (StAR) protein and steroidogenic enzymes cholesterol side-chain cleavage cytochrome P450 (P450SCC), 3beta-hydroxysteroid dehydrogenase/delta(5)-->delta(4)-isomerase (3 beta-HSD), and (17)alpha-hydroxylase cytochrome P450/C17-20 lyase (P450(17)alpha) were determined in testicular tissues of bulls treated with a LHRH agonist. Testis morphology and spermatogenesis were also examined. In Experiment 1, bulls (30-mo-old) received no treatment (control, n = 7) or were implanted for 10 days with the LHRH agonist deslorelin (n = 7). Bulls were castrated on Day 10 and testis tissues prepared for Western and Northern blotting. At castration, bulls implanted with deslorelin had greater plasma testosterone (5-fold) and testis content of testosterone (10-fold) compared with control bulls. Relative content (per micrograms total testis protein or RNA) of StAR protein, 3beta-HSD, P450SCC, and mRNA for P450(17)alpha in bulls treated with deslorelin ranged from 3- to 6-fold that of control bulls. In Experiment 2, bulls (20-mo-old) were left untreated (control, n = 6) or implanted with deslorelin (n = 12) for 120 days. On Day 120, bulls were castrated and right testis tissues prepared for morphology. Testis volume and weight were increased (P < 0.01) in bulls treated with deslorelin compared with control bulls. Stereological analysis revealed that this increase occurred in all compartments (seminiferous epithelium, lumen and interstitium) studied, but was significant (P < 0.01) only for the seminiferous epithelium. Absolute numbers of round spermatids per testis were increased (P < 0.05) in bulls treated with deslorelin compared with control bulls. Increased testosterone secretion in bulls treated with deslorelin was associated with increased testicular StAR protein and steroidogenic enzymes. Bulls treated long-term with deslorelin had a faster rate of testis growth and increased daily sperm production at the end of the experiment.