Extracellular Ca2+-dependent elevation in cytosolic Ca2+ potentiates HgCl2-induced renal proximal tubular cell damage

While normal fluctuations of cytosolic Ca2+ ([Ca2+]i) occur physiologically, the deregulation of cellular Ca2+ homeostasis leads to cellular injury. The contribution of [Ca2+]i to the process of cellular damage was assessed in a model system where HgCl2 was used to induce plasma membrane damage in renal tubular cells. In the presence of 1.37 mM extracellular Ca2+, HgCl2 (10-50 microM) induced a slow, dose-dependent, 4-6 fold increase in [Ca2+]i (as measured by Quin 2) by 10 min of exposure, which could be abolished by prior incubation of the cells with dithiothreitol. Correlates of cellular injury, i.e., decrease in cell viability, change in cellular morphology, such as bleb formation, membrane distortion and mitochondrial swelling, were induced after HgCl2 addition. The rate and dose-responses of these changes were similar to that of [Ca]i elevation. When cells were exposed to HgCl2 in the absence of added extracellular Ca2+, there was no increase in [Ca2+]i and both the rate and extent of cell damage were reduced. When Ca2+ was readded to the extracellular medium after HgCl2, there was a rapid elevation of [Ca2+]i, increased cell killing and bleb formation. The observed correlation between [Ca2+]i elevation, decreased cell viability and morphological aberrations in terms of (i) dose-dependency for HgCl2, (ii) requirement for high extracellular Ca2+, and (iii) rate of change, suggests that HgCl2-induced renal cell damage involves the entry of Ca2+ from the extracellular milieu which potentiates the progression of cellular injury.