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J Neurophysiol 81: 634-642, 1999;
0022-3077/99 $5.00
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The Journal of Neurophysiology Vol. 81 No. 2 February 1999, pp. 634-642
Copyright ©1999 by the American Physiological Society

Simultaneous Measurement of Evoked Release and [Ca2+]i in a Crayfish Release Bouton Reveals High Affinity of Release to Ca2+

R. Ravin, H. Parnas, M. E. Spira, N. Volfovsky, and I. Parnas

The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem 91904, Israel

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 µM 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 µM. This value of Kd is similar to that found previously for asynchronous release.

0022-3077/99 $5.00 Copyright © 1999 The American Physiological Society



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