Reduced urea flux across the blood-testis barrier and early maturation in the male reproductive system in UT-B-null mice

A urea-selective urine-concentrating defect was found in transgenic mice deficient in urea transporter (UT)-B. To determine the role of facilitated urea transport in extrarenal organs expressing UT-B, we studied the kinetics of [C-14] urea distribution in UT- B- null mice versus wild- type mice. After renal blood flow was disrupted, [C-14] urea distribution was selectively reduced in testis in UT-B-null mice. Under basal conditions, total testis urea content was 335.4 +/- 43.8 mu g in UT-B-null mice versus 196.3 +/- 18.2 mu g in wild- type mice (P < 0.01). Testis weight in UT-B-null mice (6.6 +/- 0.8 mg/ g body wt) was significantly greater than in wild- type mice (4.2 +/- 0.8 mg/ g body wt). Elongated spermatids were observed earlier in UT- B- null mice compared with wild type mice on day 24 versus day 32, respectively. First breeding ages in UT- B knockout males (48 +/- 3 days) were also significantly earlier than that in wild- type males (56 +/- 2 days). In competing mating tests with wild- type males and UT-B-null males, all pups carried UT-B-targeted genes, which indicates that all pups were produced from breeding of UT-B-null males. Experiments of the expression of follicle-stimulating hormone receptor (FSHR) and androgen binding protein (ABP) indicated that the development of Sertoli cells was also earlier in UT-B-null mice than that in wild- type mice. These results suggest that UT-B plays an important role in eliminating urea produced by Sertoli cells and that UT-B deletion causes both urea accumulation in the testis and early maturation of the male reproductive system. The UT-B knockout mouse may be a useful experimental model to define the molecular mechanisms of early puberty.