Mitochondrial clearance of cytosolic Ca(2+) in stimulated lizard motor nerve terminals proceeds without progressive elevation of mitochondrial matrix [Ca(2+)].
This study used fluorescent indicator dyes to measure changes in cytosolic and mitochondrial [Ca(2+)] produced by physiological stimulation of lizard motor nerve terminals. During repetitive action potential discharge at 10-50 Hz, the increase in average cytosolic [Ca(2+)] reached plateau at levels that increased with increasing stimulus frequency. This stabilization of cytosolic [Ca(2+)] was caused mainly by mitochondrial Ca(2+) uptake, because drugs that depolarize mitochondria greatly increased the stimulation-induced elevation of cytosolic [Ca(2+)], whereas blockers of other Ca(2+) clearance routes had little effect. Surprisingly, during this sustained Ca(2+) uptake the free [Ca(2+)] in the mitochondrial matrix never exceeded a plateau level of approximately 1 microM, regardless of stimulation frequency or pattern. When stimulation ceased, matrix [Ca(2+)] decreased over a slow ( approximately 10 min) time course consisting of an initial plateau followed by a return to baseline. These measurements demonstrate that sustained mitochondrial Ca(2+) uptake is not invariably accompanied by progressive elevation of matrix free [Ca(2+)]. Both the plateau of matrix free [Ca(2+)] during stimulation and its complex decay after stimulation could be accounted for by a model incorporating reversible formation of an insoluble Ca salt. This mechanism allows mitochondria to sequester large amounts of Ca(2+) while maintaining matrix free [Ca(2+)] at levels sufficient to activate Ca(2+)-dependent mitochondrial dehydrogenases, but below levels that activate the permeability transition pore.
David
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Department of Physiology and Biophysics, University of Miami School of Medicine, Miami Florida 33101, USA.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D000968:Antimycin A / Q000031:analogs & derivatives / Q000494:pharmacology; D001369:Axons / Q000502:physiology*; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002998:Clonazepam / Q000031:analogs & derivatives / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D007425:Intracellular Membranes / Q000502:physiology; D008116:Lizards; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008931:Mitochondria, Muscle / Q000187:drug effects / Q000502:physiology*; D008954:Models, Biological; D009045:Motor Endplate / Q000502:physiology; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000187:drug effects; D018482:Muscle, Skeletal / Q000187:drug effects / Q000294:innervation / Q000502:physiology*; D009411:Nerve Endings / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology*; D009840:Oligomycins / Q000494:pharmacology; D013841:Thiazepines / Q000494:pharmacology; D014403:Tubocurarine / Q000494:pharmacology
D007211:Indoles; D009840:Oligomycins; D013841:Thiazepines; C032456:antimycin; D002110:Caffeine; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D002998:Clonazepam; D000968:Antimycin A; C079032:CGP 37157; D002217:Carbachol; D002118:Calcium; D014403:Tubocurarine; C000543:cyclopiazonic acid