Inhibition of mitochondrial Ca2+ uptake affects phasic release from motor terminals differently depending on external [Ca2+].
We investigated how inhibition of mitochondrial Ca2+ uptake affects stimulation-induced increases in cytosolic [Ca2+] and phasic and asynchronous transmitter release in lizard motor terminals in 2 and 0.5 mM bath [Ca2+]. Lowering bath [Ca2+] reduced the rate of rise, but not the final amplitude, of the increase in mitochondrial [Ca2+] during 50-Hz stimulation. The amplitude of the stimulation-induced increase in cytosolic [Ca2+] was reduced in low-bath [Ca2+] and increased when mitochondrial Ca2+ uptake was inhibited by depolarizing mitochondria. In 2 mM Ca2+, end-plate potentials (epps) depressed by 53% after 10 s of 50-Hz stimulation, and this depression increased to 80% after mitochondrial depolarization. In contrast, in 0.5 mM Ca2+ the same stimulation pattern increased epps by approximately 3.4-fold, and this increase was even greater (transiently) after mitochondrial depolarization. In both 2 and 0.5 mM [Ca2+], mitochondrial depolarization increased asynchronous release during the 50-Hz train and increased the total vesicular release (phasic and asynchronous) measured by destaining of the styryl dye FM2-10. These results suggest that by limiting the stimulation-induced increase in cytosolic [Ca2+], mitochondrial Ca2+ uptake maintains a high ratio of phasic to asynchronous release, thus helping to sustain neuromuscular transmission during repetitive stimulation. Interestingly, the quantal content of the epp reached during 50-Hz stimulation stabilized at a similar level ( approximately 20 quanta) in both 2 and 0.5 mM Ca2+. A similar convergence was measured in oligomycin, which inhibits mitochondrial ATP synthesis without depolarizing mitochondria, but quantal contents fell to <20 when mitochondria were depolarized in 2 mM Ca2+.
Talbot
Janet D
JD
Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33136, USA.
David
Gavriel
G
Barrett
Ellen F
EF
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects*; D003600:Cytosol / Q000378:metabolism; D004558:Electric Stimulation; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008116:Lizards; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D009046:Motor Neurons / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000378:metabolism; D009840:Oligomycins / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects; D013572:Synaptic Vesicles / Q000378:metabolism; D013997:Time Factors
D004791:Enzyme Inhibitors; D009840:Oligomycins; D002118:Calcium