Extracellular Ca2+-Dependent Elevation in Cytosolic Ca2+ Potentiates HgCl2-Induced Renal Proximal Tubular Cell Damage

Abstract

While normal fluctuations of cytosolic Ca ^{2 +} ([Ca ²⁺] i) occur physiologically, the deregulation of cellular Ca ^{2 +} homeostasis leads to cellular injury. The contribution of [ Ca ²⁺]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 Ca ^{2 +}, HgCl₂ (10-50 μM) 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 Ca ²⁺ , there was no increase in [Ca²⁺ ]_i and both the rate and extent of cell damage were reduced. When Ca ^{2 +} was readded to the extracellular medium after HgCl₂, there was a rapid elevation of [Ca ^{2 +} ]i, increased cell killing and bleb formation. The observed correlation between [Ca ^{2 +} ]i elevation, decreased cell viability and morphological aberrations in terms of (i) dose-dependency for HgCl₂, (ii) requirement for high extracellular Ca ^{2 +}, and (iii) rate of change, suggests that HgCl₂-induced renal cell damage involves the entry of Ca ^{2 +} from the extracellular milieu which potentiates the progression of cellular injury.