Evidence for role of cytosolic free calcium in hypoxia-induced proximal tubule injury

The role of cytosolic free Ca²⁺ ([Ca²⁺](i)) in hypoxic injury was investigated in rat proximal tubules. [Ca²⁺](i) was measured using fura-2 and cell injury was estimated with propidium iodide (PI) in individual tubules using video imaging fluorescence microscopy. [Ca²⁺](i) increased from ~ 170 to ~ 390 nM during 5 min of hypoxia. This increase preceded detectable cell injury as assessed by PI and was reversible with reoxygenation. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 100 μM) reduced [Ca²⁺](i) under basal conditions (~ 80 nM) and during hypoxia (~ 120 nM) and significantly attenuated hypoxic injury. When [Ca²⁺](i) and hypoxic cell injury were studied concurrently in the same individual tubules, the 10 min [Ca²⁺](i) rise correlated significantly with subsequent cell damage observed at 20 min. 2 mM glycine did not block the rise in [Ca²⁺](i), yet protected the tubules from hypoxic injury. These results indicate that in rat proximal tubules, hypoxia induces an increase of [Ca²⁺](i) which occurs before cell damage. The protective effect of BAPTA supports a role for [Ca²⁺](i) in the initiation of hypoxic proximal tubule injury. The glycine results, however, implicate calcium-independent mechanisms of injury and/or blockade of calcium-mediated processes of injury such as activation of phospholipases or proteases.