Calcium buffering in presynaptic nerve terminals. Free calcium levels measured with arsenazo III.
The particulate fraction from osmotically shocked synaptosomes ('synaptosomal membrances') sequesters Ca when incubated with ATP]containing solutions. This net accumulation of Ca can reduce the free [Ca2+] of the bathing medium to sub-micromolar levels (measured with arsenazo III). Two distinct types of Ca sequestration site are responsible for the Ca2+ buffering. One site, presumed to be smooth endoplasmic reticulum, operates at low [Ca2+] (less than 1 microM), and has a relatively small capacity. Ca sequestration at this site is prevented by the Ca2+ ionophore, A-23187, but not by mitochondrial poisons. The secone (mitochondrial) site, in contrast, is blocked by the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone, and oligomycin. Since the intraterminal organelles can buffer [Ca2+] to about 0.3-0.5 microM, this may be an upper limit to the normal resting level of [Ca2+]i in nerve terminals. In the steady state, total cell Ca and [Ca2+]i will be governed principally be Ca transport mechanisms in the plasmalemma; the intracellular organelle transport systems then operate in equilibrium with this [Ca2+]. During activity, however, Ca rapidly enters the terminals and [Ca2+]i rises. The intracellular buffering mechanisms then come into play and help to return [Ca2+]i toward the resting level; the non-mitochondrial Ca sequestration mechanism probably plays the major role in this Ca buffering.
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001150:Arsenazo III; D001391:Azo Compounds; D001692:Biological Transport / Q000187:drug effects; D001921:Brain / Q000378:metabolism*; D002021:Buffers; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D007700:Kinetics; D008433:Mathematics; D051381:Rats; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*
D001391:Azo Compounds; D002021:Buffers; D001150:Arsenazo III; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D000001:Calcimycin; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D002118:Calcium