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 tu bules exposed to hypoxia or anoxia have yielded ambiguous results, Thi s study explored the possibility that the severity of oxygen deprivati on and the energy state of the mitochondria are important determinants of (Ca2+)(i). To this end, (Ca2+)(i) (measured with fura-2) and the m itochondrial membrane potential (measured with rhodamine 123) were stu died simultaneously in individual rat proximal tubules in hypoxic and anoxic conditions. (Ca2+)(i) did not change during hypoxia, but increa sed rapidly during anoxia, Increases in (Ca2+)(i) were only observed i n parallel with a decrease of rhodamine 123 fluorescence, which indica tes a collapse of the mitochondrial membrane potential, The increase i n (Ca2+)(i) during anoxia was prevented by incubating the tubules in a low Ca2+ medium, which did not interfere with the collapse of the mit ochondrial membrane potential. Both hypoxic and anoxic incubation led to cell death, as assessed by the fluorescent dye propidium iodide. Th ese results clearly demonstrate that the level of oxygen deprivation i s critical in determining changes in (Ca2+)(i). Because cell damage oc curred in both hypoxic and anoxic conditions, if was concluded that an increase in (Ca2+)(i) is not a necessary prerequisite for the develop ment of ischemic cell injury.