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Presynaptic function is altered in snake K⁺-depolarized motor nerve terminals containing compromised mitochondria

Michelle A. Calupca, Chris Prior *, Laura A. Merriam, Gregory M. Hendricks and Rodney L. Parsons

1. Presynaptic function was investigated at K⁺-stimulated motor nerve terminals in snake costocutaneous nerve muscle preparations exposed to carbonyl cyanide m-chlorophenylhydrazone (CCCP, 2 M), oligomycin (8 g ml^-1) or CCCP and oligomycin together.

2. Miniature endplate currents (MEPCs) were recorded at -150 mV with two-electrode voltage clamp. With all three drug treatments, during stimulation by elevated K⁺ (35 mM), MEPC frequencies initially increased to values > 350 s^-1, but then declined. The decline occurred more rapidly in preparations treated with CCCP or CCCP and oligomycin together than in those treated with oligomycin alone.

3. Staining with FM1-43 indicated that synaptic vesicle membrane endocytosis occurred at some CCCP- or oligomycin-treated nerve terminals after 120 or 180 min of K⁺ stimulation, respectively.

4. The addition of glucose to stimulate production of ATP by glycolysis during sustained K⁺ stimulation attenuated the decline in MEPC frequency and increased the percentage of terminals stained by FM1-43 in preparations exposed to either CCCP or oligomycin.

5. We propose that the decline in K⁺-stimulated quantal release in preparations treated with CCCP, oligomycin or CCCP and oligomycin together could result from a progressive elevation of intracellular calcium concentration ([Ca²⁺]_i). For oligomycin-treated nerve terminals, a progressive elevation of [Ca²⁺]_i could occur as the cytoplasmic ATP/ADP ratio decreases, causing energy-dependent Ca²⁺ buffering mechanisms to fail. The decline in MEPC frequency could occur more rapidly in preparations treated with CCCP or CCCP and oligomycin together because mitochondrial Ca²⁺ buffering and ATP production were both inhibited. Therefore, the proposed sustained elevation of [Ca²⁺]_i could occur more rapidly.

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