title: Mitochondria buffer non-toxic calcium loads and release calcium through the mitochondrial permeability transition pore and sodium/calcium exchanger in rat basal forebrain neurons.
abstract: Mitochondria participate in intracellular Ca2+ buffering and signalling. They are also major mediators of cell death. Toxic Ca2+ accumulation in mitochondria is widely believed to initiate cell death in many cell types by opening the permeability transition pore (PTP). In non-neuronal cells, the PTP has been implicated as a Ca2+ release mechanism in physiological Ca2+ signalling. In neurons, Ca2+ release from mitochondria has been attributed primarily to mitochondrial Na+/Ca2+ exchange. Using fura-2 ratiometric microfluorimetry in acutely dissociated rat basal forebrain neurons, we show that mitochondria are able to buffer non-toxic Ca2+ loads arising from caffeine-sensitive internal stores or from extracellular influx through voltage gated channels. We also show that these non-toxic Ca2+ loads are reversibly released from mitochondria through the PTP and the Na+/Ca2+ exchanger. Evoked Ca2+ transients have characteristic peak and shoulder features mediated by mitochondrial buffering and release. Depolarizing mitochondria with carbonyl cyanide m-chlorophenylhydrazone (CCCP, 5 microM) causes release of mitochondrial Ca2+ and prevents Ca2+ uptake. In CCCP, the magnitudes of evoked Ca2+ transients are increased, and the peak and shoulder features are eliminated. The PTP antagonist, cyclosporin A, (CSA, 2 microM) and the Na+/Ca2+ exchange blocker, clonazepam, (CLO, 20 microM) reversibly inhibited both the shoulder features of evoked Ca2+ transients and Ca2+ transients associated with CCCP application. We suggest that central neuronal mitochondria buffer and release Ca2+ through the PTP and Na+/Ca2+ exchanger during physiological Ca2+ signalling. We also suggest that CLO blocks both the PTP and the mitochondrial Na+/Ca2+ exchanger.
authors.affiliation: Department of Medical Pharmacology and Toxicology, College of Medicine, Texas A&M University System Health Science Center, College Station 77843-1114, USA.
authors.forename: D
authors.initials: D
authors.lastname: Murchison
authors.forename: W H
authors.initials: WH
authors.lastname: Griffith
mesh_terms: D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002998:Clonazepam / Q000494:pharmacology; D016572:Cyclosporine / Q000494:pharmacology; D008297:Male; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D009474:Neurons / Q000378:metabolism*; D010539:Permeability; D016548:Prosencephalon / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D011916:Rats, Inbred F344; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D002021:Buffers; D019831:Sodium-Calcium Exchanger; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D002998:Clonazepam; D016572:Cyclosporine; D002118:Calcium