Rise in cytosolic Ca2+ and collapse of mitochondrial potential in anoxic, but not hypoxic, rat proximal tubules.

It has been suggested that ischemic renal proximal tubular cell injury is mediated by an increase in cytosolic calcium concentrations ((Ca2+)i). However, measurements of (Ca2+)i in rat or rabbit proximal tubules exposed to hypoxia or anoxia have yielded ambiguous results. This study explored the possibility that the severity of oxygen deprivation and the energy state of the mitochondria are important determinants of (Ca2+)i. To this end, (Ca2+)i (measured with fura-2) and the mitochondrial membrane potential (measured with rhodamine 123) were studied simultaneously in individual rat proximal tubules in hypoxic and anoxic conditions. (Ca2+)i did not change during hypoxia, but increased rapidly during anoxia. Increases in (Ca2+)i were only observed in parallel with a decrease of rhodamine 123 fluorescence, which indicates a collapse of the mitochondrial membrane potential. The increase in (Ca2+)i during anoxia was prevented by incubating the tubules in a low Ca2+ medium, which did not interfere with the collapse of the mitochondrial membrane potential. Both hypoxic and anoxic incubation led to cell death, as assessed by the fluorescent dye propidium iodide. These results clearly demonstrate that the level of oxygen deprivation is critical in determining changes in (Ca2+)i. Because cell damage occurred in both hypoxic and anoxic conditions. It was concluded that an increase in (Ca2+)i is not a necessary prerequisite for the development of ischemic cell injury.