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Depolarization, intracellular calcium and exocytosis in single vertebrate nerve endings


Lindau,  Manfred
Department of Molecular Cell Research, Max Planck Institute for Medical Research, Max Planck Society;

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Lindau, M., Stuenkel, E. L., & Nordmann, J. J. (1992). Depolarization, intracellular calcium and exocytosis in single vertebrate nerve endings. Biophysical Journal, 61(1), 19-30. doi:10.1016/S0006-3495(92)81812-X.

We have investigated the temporal relationship between depolarization, elevation of [Ca2+]i and exocytosis in single vertebrate neuroendocrine nerve terminals. The change of [Ca2+]i and vasopressin release were measured with a time resolution of less than 1 s in response to K(+)−induced depolarization. Exocytosis was also monitored in the whole−terminal patch−clamp configuration by time resolved capacitance measurements while [Ca2+]i was simultaneously followed by fura−2 fluorescence measurements. In intact as well as patch−clamped nerve terminals sustained depolarization leads to a sustained rise of [Ca2+]i. The rate of vasopressin release from intact nerve terminals rises in parallel with [Ca2+]i but then declines rapidly towards basal (t1/2 approximately 15 s) despite the maintained high [Ca2+]i indicating that only a limited number of exocytotic vesicles can be released. We demonstrate that in nerve terminals exocytosis can be followed during step depolarization by capacitance measurements. The capacitance increase starts instantaneously whereas [Ca2+]i rises with a half time of several hundred milliseconds. An instantaneous steep capacitance increase is followed by a slow increase with a slope of 25−50 fF/s indicating the sequential fusion of predocked and cytoplasmic vesicles. During depolarization the capacitance slope declines to zero with a similar time course as the vasopressin release indicating a decrease in exocytotic activity. Depolarization per se in the absence of a sufficient rise of [Ca2+]i does not induce exocytosis but elevation of [Ca2+]i in the absence of depolarization is as effective as in its presence. The experiments suggest that a rapid rise of [Ca2+]i in a narrow region beneath the plasma membrane induces a burst of exocytotic activity preceding the elevation of bulk [Ca2+]i in the whole nerve terminal