The secretory pathway in the mouse epididymis as shown by electron microscope radioautography of principal cells exposed to monensin

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Abstract
The secretory pathway in principal cells of the mouse epididymis was studied using in vitro labeling and electron microscope radioautography of tissue exposed to the ionophore monensin. After a 5‐minute pulse of 3H‐leucine, control samples of caput epididymidis were incubated in a modified Krebs‐Ringer solution (MKRH medium), while experimental specimens were placed in the same medium, to which 1 μM monensin had been added. At intervals between 5 minutes and 4 hours, samples were fixed and prepared for electron microscope radioautography. Analysis of control specimens revealed heaviest labeling of the rough and the sparsely granulated endoplasmic reticulum early in the experiment followed by a fall in radioactivity, maximal labeling of the Golgi apparatus at 30 minutes, and a pronounced rise in the percentage of grains associated with the apical cell surface and the epididymal lumen beginning 1 hour after administration of precursor. In monensin‐treated epididymides, radioactive material accumulated in the Golgi region while the normal increase in labeling of the apical surface and the lumen was completely inhibited for at least 2 hours. The percentage of grains attributed to coated vesicles was also reduced in samples exposed to monensin. In contrast, labeling patterns of the abundant, sparsely granulated, endoplasmic reticulum and the rough endoplasmic reticulum were very similar in monensin‐treated and control specimens. The concomitant alterations in labeling of the Golgi apparatus and the lumen demonstrate that the Golgi apparatus participates in intracellular transport of secretory proteins in epididymal principal cells, and is not bypassed as previously suggested. The percentage of grains associated with the sparsely granulated endoplasmic reticulum suggests that much of the synthesis of secretory protein in the principal cells occurs in this organelle, and the lack of alteration of its labeling in the presence of a monensin‐induced block at the level of the Golgi apparatus indicates that the sparsely granulated endoplasmic reticulum lies before the Golgi apparatus in the secretory pathway. It is speculated that vesicles play a role in transport of secretory pathway. It is speculated that vesicles play a role in transport of secretory protein from the Golgi apparatus to the lumen.