The relationship between cellular ion deregulation and acute and chronic toxicity

Abstract

Cell injury proceeds through a predictable series of stages as it progresses from reversible to irreversible injury (or “point of no return”) and ends eventually in cell death. Ion deregulation is strongly implicated in this process and, in particular, the deregulation of cytosolic Ca²⁺ ([Ca²⁺]_i) which is thought by most to be a critical step in the transition from reversible to irreversible injury. [Ca²⁺]_i is normally maintained at approximately 100 μm, a level 10,000 times lower than for extracellular Ca²⁺ ([Ca²⁺]_e). Deregulation may affect any of three Ca²⁺ buffering systems: the plasma membrane, the mitochondria, and the endoplasmic reticulum. Perturbation of [Ca²⁺]_i is intimately related to perturbation of other ions, including, H⁺, Na⁺, and K⁺. In normal cells, [Ca²⁺]_i elevation is also linked to activation of oncogenes as well as cell division, initiation, wound repair, differentiation, and possibly tumor promotion. In all models of acute injury for which we have measured [Ca²⁺]_i, including ischemia, HgCl₂ and calcium inophores, [Ca²⁺]_i always became elevated. This elevation results from influx of [Ca²⁺]_e (ionomycin), redistribution from intracellular stores (NEM, KCN), or from both sources (HgCl₂). The degree of [Ca²⁺]_i elevation is correlated with the degree of injury (as determined by blebbing and morphological changes) and cell killing. More recently, much work has been focused on the role of [Ca²⁺]_i in neoplasia. Many stimuli, including the promoter TPA and transforming growth factor β have been shown to affect normal and transformed cells differently. Both cause differentiation in normal human bronchial epithelial cells but stimulate growth in transformed cells. We propose that deregulation of ions, especially [Ca²⁺]_i, plays an important role, if not a key role, in the initiation of acute and chronic cell injury, including neoplasia. Increases in [Ca²⁺]_i appear to accelerate degradative processes and, unless regulated, lead to cell death.