Measurement of calcium fluxes in permeabilized cells using a ⁴⁵Ca²+ uptake and release assay.

Many cell surface receptors activate phosphoinositidase(s) C, via G proteins that catalyze the hydrolysis of phosphatidylinositol 4,5-biphosphate to produce the second messengers, inositol(1,4,5)trisphosphate [Ins(1,4,5)P(3)] and diacylglycerol. Ins(1,4,5)P(3) interacts with specific receptor populations of ligand-gated channels to mobilize nonmitochondrial intracellular calcium (Ca(2+)) stores. Because Ins(1,4,5)P(3) is very hydrophilic, it cannot readily cross the intact plasma membrane. Consequently, Ins(1,4,5)P(3)-induced Ca(2+) release was initially demonstrated in permeabilized pancreatic acinar cells, and all subsequent studies in cells have involved the introduction of Ins(1,4,5)P(3) by rendering a cell population permeable, using microinjection techniques or by the presentation of chemically modified membrane-permeable Ins(1,4,5)P(3) analogs, such as photolabile "caged Ins(1,4,5)P(3)" (5). An alternative approach involves disruption of the plasma membrane and preparation of microsomes from the intracellular vesicular Ca(2+) stores, however, these preparations exhibit a loss of Ins(1,4,5)P(3) responsiveness compared to cells. The author will describe a (45)Ca(2+)-release assay used to monitor Ins(1,4,5)P(3)-induced Ca(2+) mobilization from nonmitochondrial intracellular Ca(2+) stores using "cytosol-like" buffer (CLB) and permeabilized SH-SY5Y neuroblastoma cell populations.