Immunohistochemical localization of inositol 1,4,5-trisphosphate receptors in non-neural tissues, with special reference to epithelia, the reproductive system, and muscular tissues

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Abstract
Although the pharmacological properties and distribution of inositol 1,4,5-trisphosphate (IP3)-sensitive calcium stores vary considerably among tissues, studies of its localization have been devoted mostly to the central nervous system. In this report we have analyzed the localization of IP3 receptors in diverse non-neural tissues of the mouse, using polyclonal antibodies raised against purified IP3 receptors through immunoblotting and immunohistochemistry. These antibodies mainly recognized type 1 IP3 receptors, although they also reacted with other types of IP3 receptors. The receptors were localized in the apical surface areas of highly polarized epithelia, such as the choroid plexus, retinal pigment epithelium, and columnar epithelium lining the interlobular ducts in the sublingual and submaxillary salivary glands. Immunoelectron microscopy of choroidal cells revealed that IP3 receptors are localized on the surfaces of several structures, including clear vesicles, tubules and vesicular profiles of smooth endoplasmic reticulum, rough endoplasmic reticulum and a part of the nuclear envelope, as well as clusters of ribosomes in the cytoplasmic matrix. The surface areas of ciliated epithelia, such as those of the trachea and oviduct, also stained positive. The cytoplasmic distribution was homogeneous in mesangial cells, myoepithelial cells, and endothelial cells accompanying muscle, as well as in a population of endocrine cells. Intense immunostaining was found in the surface areas as well as in the cytoplasm of diverse smooth muscle cells. Staining intensity of smooth muscle varied among cells in the same tissue. Staining was intense in the cytoplasm of premature oocytes in the primary follicle, but then attenuated as these cells matured. The Sertoli cells with clusters of elongating maturing sperm were stained, but mature sperm were unstained. These results indicate a heterogeneous cellular distribution, and possibly a heterogeneous subcellular distribution, of a population of IP3 receptors in a variety of non-neural tissues.

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