Testicular endocrine function in GH receptor gene disrupted mice

The consequences of disruption of GH receptor gene in GH receptor knockout mice on testicular function were evaluated. Adult male GH receptor knockout mice and their normal siblings were divided in to two subgroups and treated with either saline or ovine LH (0.3 microg/g BW) in saline. One hour after saline or LH administration, blood was obtained via heart puncture. Plasma IGF-I, LH, FSH, PRL, androstenedione, and testosterone levels were measured by RIAs. Testicular LH and PRL receptor numbers as well as pituitary LHbeta-subunit and testicular sulfated glycoprotein-2 mRNA levels were measured. Also, testicular morphometric analysis was performed. Unlike in normal, wild-type mice, the circulating IGF-I was undetectable in GH receptor knockout mice. The plasma PRL levels were (P<0.01) higher in GH receptor knockout mice than in their normal siblings. The basal LH secretion was similar in normal and GH receptor knockout mice. However, the circulating FSH levels were lower (P<0.001) in GH receptor gene disrupted mice. Administration of LH resulted in a significant (P<0.001) increase in plasma testosterone levels in both GH receptor knockout and normal mice. However, this testosterone response was attenuated (P < 0.01) in GH receptor knockout mice. Plasma androstenedione responses were similar in both GH receptor knockout and normal mice. Testicular LH and PRL receptor numbers were significantly decreased in GH receptor knockout mice. The results of the morphometric analysis of the testis revealed that the Leydig cell volume per testis was reduced in mice with GH receptor gene disruption. The steady-state of LHbeta-subunit and testicular sulfated glycoprotein-2 mRNA levels were not different in GH receptor knockout mice relative to their normal siblings. The present in vivo study demonstrates that in GH receptor knockout mice, LH action on the testis in terms of testosterone secretion is significantly attenuated and suggests that this is due to a decrease in the number of testicular LH receptors. The reduced number of PRL receptors may contribute to the diminished responsiveness of testicular steroidogenesis to LH by decreased ability to convert androstenedione to testosterone. These changes are most likely due to the absence of circulating IGF-I. These findings provide evidence that systemic IGF-I plays a major modulatory role in testicular endocrine function.