We have shown previously that administration of an orally active competitive aromatase inhibitor 4-(5,6,7,8- tetrahydroimidazo [1,5a] pyridin-5-yl) benzonitrile monohydrochloride to adult male beagles increases peripheral blood LH and testosterone concentrations, and that testes from treated dogs produce more testosterone when perfused in vitro than age-matched controls. In the present study we posed the question of whether the increased testosterone secretion by testes from these same aromatase-treated dogs was due to Leydig cell hypertrophy or hyperplasia, and if the latter, whether cytoplasmic organelles are increased. Beagles were treated with the inhibitor at a dosage of 2.5 mg/ kg · day for 25 weeks and were euthanized by an overdose of iv sodium pentobarbital; testes were perfusion-fixed, embedded, and sectioned for stereological analysis. There were no significant differences in testis volume and absolute volumes of seminiferous tubules, blood vessels, lymphatic space, macrophage cells, and mesenchymal cells between the control and treated dogs. In contrast absolute interstitium volume and the absolute volume of Leydig cells per testis were significantly increased (P < 0.05) in treated dogs. This increased Leydig cell volume per testis was due to increased volume of individual Leydig cells rather than to increases in Leydig cell number per testis. Additional studies showed that the surface area per Leydig cell of smooth endoplasmic reticulum, outer and inner mitochondrial membranes, and membranes of lipid droplets per testis were significantly higher (p < 0.05) in the treated dogs as compared to the controls. In summary, the results of this study lead us to conclude that aromatase inhibition in the mature dog causes Leydig cell hypertrophy rather than hyperplasia and increased surface area per Leydig cell of subcellular organelles that contain enzymes involved in steroid biosynthesis.
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