Muscarinic receptor-induced phosphoinositide hydrolysis at resting cytosolic Ca2+ concentration in PC12 cells

Abstract

In PC12 cells, cultured in the presence of nerve growth factor to increase their complement of muscarinic receptors, treatment with carbachol induces muscarinic receptor-dependent rises in free cytosolic Ca 2+ as well as hydrolysis of membrane phosphoinositides. Experiments were carried out to clarify the relationship between these two receptor-triggered events. In particular, since inositol-l,4,5-trisphosphate (the hydrophilic metabolite produced by the hydrolysis of phosphatidylinositol- 4,5-bisphosphate) is believed to mediate intracellularly the release of Ca 2+ from nonmitochondrial store(s), it was important to establish whether it can be generated at resting cytoplasmic concentration of Ca 2+ (≈0.1 µM). Cells incubated in Ca2+-free medium were depleted of their cytoplasmic Ca 2+ stores by pretreatment with ionomycin. When these cells were then treated with carbachol, their cytosolic concentration of Ca 2+ remained at the resting level, whereas inositol- 1,4,5-trisphosphate generation was still markedly stimulated. Our results demonstrate that an increase in the concentration of cytosolic Ca 2+ is not a necessary intermediate between receptor activation and phosphoinositide hydrolysis, and therefore support the second-messenger role of inositol-l,4,5- trisphosphate. © 1985, Rockefeller University Press., All rights reserved.