Angiotensin-converting enzyme in the male rat reproductive system: autoradiographic visualization with [3H]captopril.

[3H]Captopril autoradiography visualizes angiotensin-converting enzyme (ACE; EC 3.14.5.1) in the reproductive tract of male rats. [3H]Captopril binds to testicular slices with high affinity (Kd = 4.4 nM) and displays a pharmacological profile similar to that of ACE activity. High densities of [3H] captopril autoradiographic silver grains are found over spermatid heads and in the lumen of seminiferous tubules in stages I-VIII and XII-XIV. Tubules in stages IX-XI exhibit only one fifth the level of binding. The basal epithelium and interstitial tissue are not labeled. The initial segment of the epididymis contains very low levels of grains. The head of the epididymis demonstrates intense grain density at the luminal surface of the epithelium, with little luminal labeling. In a progression to the tail of the epididymis, epithelium labeling declines, and luminal grains increase. The lumen of the vas deferens is also labeled. ACE from testis and several regions of the epididymis has been categorized with respect to its particulate vs. soluble nature and its ability to be precipitated by an antirat lung ACE monoclonal antibody. Testicular ACE is particulate and not immunoprecipitable. The distribution of immunoprecipitable particulate ACE in the epididymis is similar to that of autoradiographic silver grains over the epithelium. The concentration of particulate but nonprecipitable ACE gradually rises from the initial segment to the tail of the epididymis, similar to the distribution of luminal [3H]captopril-associated grains. Soluble ACE activity, present in equal concentration throughout the epididymis and not immunoprecipitable, may not be detected by autoradiography of [3H]captopril.