Electron microscopic immunolocalization of caltrin proteins in guinea pig seminal vesicles

Caltrins, the small, basic proteins of the seminal vesicle secretion that inhibit calcium transport into epididymal spermatozoa, and consequently the onset of the acrosome reaction and the hyperactivated motility, were localized in the epithelial cells and the lumen of the seminal vesicles of the guinea pig by an immunocytochemical procedure and electron microscopy. Rabbit antisera against each protein (caltrin I or II), and goat anti-rabbit IgG antiserum labeled with colloidal gold were used to detect the caltrin immunoreaction. The subcellular distribution of the gold labeling was occasionally localized in the rough endoplasmic reticulum but mainly within big secretory vacuoles containing low electron-dense material, which are components of the Golgi complex known as condensing vacuoles. These are involved in the intracellular transport, storage, and discharge of secretory proteins. Gold-labeled material released to the lumen was also detected. There was no clear evidence that the discharge was mediated by an exocytotic process. Immunoreaction was observed neither in the electron-dense core nor in the electron-lucent halo of the typical secretory granules of the epithelial cells of the seminal vesicles. Using light microscope immunocytochemistry, intense positive immunoreactivity was detected in the material secreted to the lumen but not on the epithelial cell layer. Only those cells undergoing a degenerative process and showing a picnotic nucleus and condensed cytoplasmic matrix exhibited detectable immunoreaction when gold label and silver intensification were applied. The same distribution of the immunoprobes was obtained by electron or light microscopy when antiserum to either I or II was used. It would appear that the two caltrin proteins of the guinea pig are synthesized in the rough endoplasmic reticulum of the epithelial cells and transported quickly to the Golgi complex where the secretory vacuoles (condensing vacuoles) are formed. The proteins are transported by the secretory vacuoles to the apical ends of the cells to be discharged into the lumen.