THE ROLE OF THE CALCIUM-DEPENDENT ENZYMES NITRIC-OXIDE SYNTHASE AND CALPAIN IN HYPOXIA-INDUCED PROXIMAL TUBULE INJURY

A role of cytoplasmic free calcium [Ca2+](i) in hypoxia-induced proxim al tubule damage has been proposed. To further investigate the role of [Ca2+](i) in mediating hypoxic proximal tubular injury, a video imagi ng technique has been developed in which [Ca2+](i) can be measured sim ultaneously, with propidium iodide (PI) staining of nuclei as an index of hypoxia-induced membrane damage. Hypoxia in rat proximal tubules i s associated with a significant rise in [Ca2+](i) which precedes evide nce of membrane damage as assessed by PI staining. This rise in [Ca2+] (i) activates calpain, a Ca2+-dependent cysteine protease, and constit utive nitric oxide synthase (NOS), the Ca2+-dependent form of NOS. Inh ibition of either calpain or nitric oxide synthase provides marked cyt oprotection against hypoxic proximal tubular injury. These observation s are consistent with the early rise in [Ca2+](i) initiating hypoxic i njury by activating NOS and calpain.