Simultaneous measurement of evoked release and [Ca2+]i in a crayfish release bouton reveals high affinity of release to Ca2+.
The opener neuromuscular junction of crayfish was used to determine the affinity of the putative Ca2+ receptor(s) responsible for evoked release. Evoked, asynchronous release, and steady-state intracellular Ca2+ concentration, [Ca2+]ss, were measured concomitantly in single release boutons. It was found that, as expected, asynchronous release is highly correlated with [Ca2+]ss. Surprisingly, evoked release was also found to be highly correlated with [Ca2+]ss. The quantal content (m) and the rate of asynchronous release (S) showed sigmoidal dependence on [Ca2+]ss. The slope log m/log [Ca2+]ss varied between 1.6 and 3.3; the higher slope observed at the lower [Ca2+]o. The slope log S/log [Ca2+]ss varied between 3 and 4 and was independent of [Ca2+]o. These results are consistent with the assumption that evoked release is controlled by the sum of [Ca2+]ss and the local elevation of Ca2+ concentration near the release sites resulting from Ca2+ influx through voltage-gated Ca2+ channels (Y). On the basis of the above, we were able to estimate Y. We found Y to be significantly <10 microM even for [Ca2+]o = 13.5 mM. The dissociation constant (Kd) of the Ca2+ receptor(s) associated with evoked release was calculated to be in the range of 4-5 microM. This value of Kd is similar to that found previously for asynchronous release.
The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem 91904, Israel.
R
R
Ravin
H
H
Parnas
M E
ME
Spira
N
N
Volfovsky
I
I
Parnas
D000818:Animals; D003400:Astacoidea; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D004558:Electric Stimulation; D005110:Extracellular Space / Q000378:metabolism; D005456:Fluorescent Dyes; D016257:Fura-2; D007424:Intracellular Fluid / Q000378:metabolism*; D008959:Models, Neurological; D008962:Models, Theoretical; D009469:Neuromuscular Junction / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism; D017729:Presynaptic Terminals / Q000378:metabolism*
D015220:Calcium Channels; D005456:Fluorescent Dyes; D018377:Neurotransmitter Agents; D002118:Calcium; D016257:Fura-2