G-protein activation enhances Ca+2-dependent lipid secretion of the rat Harderian gland

We studied the secretory mechanism of the Harderian gland of rats. After perfusion with HEPES-buffered Ringer's solution containing NaF (10 mM) with AlCl₃ (10 M), a G-protein activator, the glandular cells of the Harderian gland showed massive exocytosis and apocrine-like protrusions on the luminal surface. Some of the secretory vacuoles aggregated within the cytoplasm, and large vacuoles were formed. Contraction of the myoepithelial cells covering the glandular endpieces caused a narrowing of the glandular lumina, which contained cytoplasmic fragments, and deformation of the basal contour of the glandular end-pieces. The basal regions of the glandular cells also bulged between the myoepithelial cells. Secretory vacuoles were also discharged to the lateral cell surface, and the intercellular spaces were dilated. The enhanced secretory activities of the glandular cells and the contraction of the myoepithelial cells were similar to those in rats stimulated with 10 M carbachol (CCh). However, dilatation of the endoplasmic reticulum in glandular cells (type A cells), which leads to the formation of small vesicles, was observed in those glands stimulated by NaF+AlCl₃, but not in those stimulated by CCh. Removal of Ca⁺² from the perfusing HR or addition of EDTA (0.5 mM) diminished and inhibited NaF+AlCl₃- or CCh-enhanced secretory activity of the glandular cells and also allayed the deformation of glandular cells caused by myoepithelial cell contraction. The present results demonstrate the involvement of G-proteins and Ca²⁺-influx in the lipid secretion of glandular cells and in the contraction of myoepithelial cells of the Harderian gland in rats.