Protein Synthesis and Secretion by the Rat Caput Epididymidis in Vivo: Influence of the Luminal Microenvironment1

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Abstract
The role of intraluminal factors in regulating the functions of the epididymal epithelium is unknown and virtually unexplored. Simultaneous in vivo microperfusion and microperifusion procedures were carried out on rat caput epididymal tubules to examine the effects of the intraluminal microenvironment on protein synthesis and secretion. Caput tubules were perifused with 35S-methionine for 3 h while the tubule lumen was subjected to a stop-flow perfusion of either native caput content from another epididymis (NCC), artificial caput fluid containing no testicular factors (ACF), or rete testis fluid (RTF). Protein synthesis and secretion were measured quantitatively as trichloroacetic acid- (TCA) precipitable 35S-methionine-labeled proteins (cpm/μI) and qualitatively as autoradiograms of electrophoresed 35S-methionine-labeled proteins. Electrophoresed proteins, i.e., those in fluids collected by micropuncture, were: perifused interstitial fluid (IF), lumen fluid from the perifused but not perfused length of tubule (LF), and the perfusion fluid (PF) re-collected from the tubule lumen. Undiluted tubule extract (TE) of the perfused length of tubule was also analyzed. ACF in the lumen caused a significant reduction in proteins synthesized and secreted by the epithelium, and RTF in the lumen returned protein synthesis and secretion to control values. For example, TCA-precipitable 35S-methionine-labeled proteins in TE of tubules perfused with NCC, ACF, or RTF were 107.6 ± 13.1, 58.9 ± 6.9, and 121.9 ± 19.7 × 103 cpm/μl, respectively. Autoradiograms reinforced this data. Subsequently, tubules were perfused with ACF or ACF containing physiological concentrations of sex hormone-binding globulin (SHBG; an androgen-binding protein [ABP] homolog), 5α-dihydrotestosterone (DHT), or SHBG + DHT. DHT alone and DHT + SHBG stimulated protein synthesis and secretion, but SHBG alone stimulated protein synthesis almost twice as much as DHT alone. For example, TCA-precipitable 35S-methionine-labeled proteins in TE of tubules perfused with ACF, ACF + DHT, ACF + SHBG, and ACF + DHT + SHBG, respectively, were 58.9 ± 6.9, 108.0 ± 28.4, 200.7 ± 30.9, and 148.1 ± 19.5 × 103 cpm/μl. In all of the experiments mentioned, secretion changed in proportion to synthesis, but secretion was never stimulated independently of synthesis. It is speculated that ABP is a Sertoli cell protein with a role in the short-loop regulation of caput epididymal function.

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