IDENTIFICATION OF THE NOVEL CALCIUM-MEDIATED CELLULAR EVENTS IN THE PATHOGENESIS OF HYPOXIA-INDUCED PROXIMAL TUBULAR INJURY

Acute renal failure (ARF) is a clinical syndrome that is frequently pr esent in hospitalized patients and is associated with high mortality. The duration of the clinical course of ARF correlates with a mortality that remains high despite the availability of dialysis treatment. The virtual complete recovery of renal function in those patients who sur vive ARF, as well as the minimal renal histological abnormalities, sug gest that there are reversible components in the pathophysiology of AR F. A better understanding of the pathogenesis of ARF is therefore need ed to allow interventions that would prevent the need for haemodialysi s and improve survival. A role of cytoplasmic free calcium [Ca2+](i) i n hypoxia-induced proximal tubule damage has been proposed. To further investigate the role of [Ca2+](i) in mediating hypoxic proximal tubul ar injury, a video imaging technique has been developed in which [Ca2](i) can be measured simultaneously with propidium iodide (PI) stainin g of nuclei as an index of hypoxia-induced membrane damage. Hypoxia in rat proximal tubules is associated with a significant rise in [Ca2+]( i) which precedes evidence of membrane damage as assessed by PI staini ng. This rise in [Ca2+](i) activates calpain, a Ca2+-dependent cystein e protease and constitutive nitric oxide synthase (NOS), the Ca2+ depe ndent form of NOS. Inhibition of calpain with mechanistically and chem ically dissimilar inhibitors provides marked cytoprotection against hy poxic and ionomycin induced proximal tubular injury. Similarly, inhibi tion of NOS with L-NAME, acidosis, tetrahydrobiopterin depletion and l ow extracellular calcium is associated with marked cytoprotection agai nst hypoxic cellular injury. These observations are consistent with th e early rise in [Ca2+](i), initiating hypoxic injury by activating NOS and calpain.