Calcium overload is suggested to play a fundamental role in the process of rod apoptosis in chemical-induced and inherited retinal degenerations. However, this hypothesis has not been tested directly. We developed an in vitro model utilizing isolated rat retinas to determine the mechanisms underlying Ca Apoptosis is an active mode of cell death that is induced by a variety of physiological and pathological stimuli. Convergent evidence suggests that mitochondria and caspases play a central and fundamental role in the effector or executioner phase of apoptosis (1). Early during the effector phase, the mitochondrial permeability transition pore (PTP), 1 a megachannel in the inner mitochondrial membrane, is opened by a variety of apoptotic inducers such as elevated matrix Ca 2ϩ , pro-oxidants, and thiol-reactive agents (1-3). This leads to mitochondrial depolarization (decrease in ⌬⌿ m ) and subsequently to the release of cytochrome c and/or apoptosis-inducing factor from mitochondria to cytosol or nuclei (1, 4, 5). Caspases are activated, which cleave downstream death substrates and activate endonucleases that cleave genomic DNA into fragments resulting in the apoptotic nuclear morphology (6, 7). The opening of the mitochondrial PTP and the apoptotic process can be inhibited by a diverse group of agents such as Bcl-2, Bcl-x L , bongkrekic acid, and CsA (1).Sustained increases in intracellular [Ca 2ϩ ] trigger apoptosis in a diverse array of in vivo and in vitro systems (1, 8). Results from several studies suggest that elevated rod photoreceptor [Ca 2ϩ ] plays a key role in the process of apoptotic rod cell death in humans and animals during inherited retinal degenerations, retinal diseases and injuries, and chemical exposure. These include patients with retinitis pigmentosa and cancer-associated retinopathy (9, 10), lead-exposed rats (11-13), retinal degeneration mice (13,14), rats injected with anti-recoverin monoclonal antibodies (15), rats with hypoxic-ischemic injury (16), and rats with light induced damage (17).Several neurotoxic heavy metals, transition metals, and organometals produce neuronal apoptosis in whole animals and cultured cells (11, 12, 18 -25). For example, low to moderate level Pb 2ϩ exposure produces apoptotic rod and bipolar cell death in developing and adult rats (11,12) and apoptotic neuronal cell death in primary cultured cells (23,24). Similarly, Cu 2ϩ and methyl mercury produce apoptosis in the developing and mature olfactory epithelium and cerebellum, respectively (18,22). Although the molecular mechanisms underlying the apoptosis induced by these different metals are unknown, there are two possible, though not mutually exclusive, triggering mechanisms: one is Ca 2ϩ overload and the other is the generation of ROS. Both in vivo and in vitro lead exposure increase rod and retinal intracellular [Ca 2ϩ ] during cell death (26, 27). Hg 2ϩ , tributyltin, and methyl mercury also cause Ca 2ϩ overload in cultured neuronal cells prior to cell death (19,20,28). Moreover, Hg 2ϩ , Mn 2ϩ , Cu 2ϩ...
