Depolarization, intracellular calcium and exocytosis in single vertebrate nerve endings.
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.
Lindau
M
M
Biophysics Group, Freie Universität Berlin, Germany.
Stuenkel
E L
EL
Nordmann
J J
JJ
D000818:Animals; D001127:Arginine Vasopressin / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D005089:Exocytosis; D007700:Kinetics; D008433:Mathematics; D008564:Membrane Potentials; D008954:Models, Biological; D009411:Nerve Endings / Q000502:physiology*; D010904:Pituitary Gland, Posterior / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains
D001127:Arginine Vasopressin; D002118:Calcium