Novel mechanism of intracellular calcium release in pituitary cells

Abstract

In sea urchin eggs an enzymatic metabolite of β-NAD +, called cyclic ADP-ribose (cADPR), is as potent and powerful a releaser of sequestered intracellular Ca 2+ as is inositol 1,4,5-trisphosphate (IP 3). The enzyme that synthesizes cADPR is present in several vertebrate animal tissues, but the Ca 2+-releasing activity of cADPR has not been described in mammalian cells. We report here that incubation of β-NAD + with cell-free extracts of several rat tissues (including pituitary gland) generates a product which releases intracellular Ca 2+ stores in permeabilized rat pituitary GH 4C 1 cells. This product has the biological characteristics of cADPR (it acts after depletion of the IP 3 stores and after blockade of the IP 3 receptor by heparin). The response is mimicked, in a concentration-dependent manner, by authentic cADPR and is desensitized by prior incubation with cADPR. We conclude that cADPR is not only synthesized by certain mammalian cells but also acts in such cells to release compartmentalized intracellular Ca 2+ by a mechanism that differs from that used by IP 3. Therefore, cADPR may serve, in addition to IP 3, as a second messenger for intracellular Ca 2+ mobilization in mammalian cells.