Northern blot analysis demonstrated that the concentration of inositol 1,4,5-trisphosphate (IP3) receptor mRNA was greatest in cerebellar tissue. Moderate amounts of IP3 mRNA were present in brain tissue without cerebellum and tissue of the thymus, heart, lung, liver, spleen, kidney, and uterus. Small amounts of IP3 receptor mRNA were observed in skeletal muscle and testicular tissue. Regional distribution of IP3 mRNA in various tissues was also examined by in situ hybridization. A considerable amount of IP3 receptor mRNA was located in smooth muscle cells, such as those of the arteries, bronchioles, oviduct and uterus. In addition, secondary oocytes surrounded by Graafian follicles in the ovary were found to have large amounts of IP3 receptor mRNA. The present studies suggest a functional importance of the IP3 second-messenger system in these cell types.
