Down-regulation of testicular aromatization in the horse.

This study aims to explore the process of aromatization in horses, specifically the conversion of androgens into estrogens in the testicles. The researchers discovered a potent aromatase and observed changes in hormone levels after administering testosterone or human chorionic gonadotrophin (hCG) to both castrated and non-castrated male horses. The results suggest that a decrease in estrogen levels following injections may be attributed to a reduction in aromatase synthesis, not a lack of substrate.

Methods and Procedures

• Experimental horses (geldings and stallions) were given a single intramuscular injection of testosterone (750 mg of testosterone bexahydrobenzoate) or an intravenous injection of human chorionic gonadotrophin (hCG) (10,000 IU).
• Blood and urine samples were collected daily from the day of injection (D0) to the tenth day post-injection (D10) for analysis.
• Levels of unconjugated and conjugated (after solvolysis) androgens and estrogens were measured in these samples.

Results and Findings

• The experiment revealed a sharp increase in levels of plasma estrogens in stallions, which peaked one day prior to androgen levels.
• Both testosterone and hCG treatments confirmed the existence of a potent testicular aromatase capable of converting endogenous and exogenous androgens into conjugated estrogens.
• The patterns of estrogen increase following testosterone or hCG administration were found to be very similar, suggesting the estrogen rise induced by hCG might be due partly to increased availability of testosterone.

Conclusion and Implications

• The sudden drop in plasma estrogen levels was not due to a lack of substrate, as two successive androgen injections did not succeed in maintaining high estrogen levels.
• Since estrogens were unable to inhibit the testicular aromatase activity in vitro, the researchers concluded that the drop in estrogen levels could be due to a decrease – or ‘down-regulation’ – of aromatase synthesis.
• Such down-regulation of testicular aromatization could have significant implications for understanding hormone regulation in horses and potentially other species.