Ba2+ ions evoke two kinetically distinct patterns of exocytosis in chromaffin cells, but not in neurohypophysial nerve terminals.
The coupling between divalent cations and exocytosis of large dense-cored vesicles (LDCV) was studied with capacitance-detection techniques in nerve terminals of the rat neurohypophysis (NHP) and bovine chromaffin cells. Ba2+ substitution for Ca2+ produced kinetically distinct responses in the two preparations. In NHP terminals, Ba2+ ions behave as weak substitutes for Ca2+. Exocytotic events occur principally during depolarizing pulses, i.e., events are "stimulus-coupled" to Ba2+ entry through voltage-gated Ca2+ channels. Stimulus-coupled exocytosis apparently requires elevated submembrane cation concentrations that dissipate rapidly on hyperpolarization-induced Ca(2+)-channel closure. Intracellular dialysis of NHP terminals with Ba2+ does not evoke exocytosis, nor does it interfere with depolarization-evoked Ca2+ influx and exocytosis. In chromaffin cells, Ba2+ ions evoke a small quantity of stimulus-coupled secretion, but the dominant response is an additional pronounced poststimulus capacitance increase that outlasts channel closures by 20-50 sec. "Stimulus-decoupled" exocytosis is slow (approximately 25-40 fF/sec) compared with Ca(2+)-evoked stimulus-coupled exocytosis (approximately 1000 fF/sec). Decoupled secretion is not attributable to Ba2+ displacement of intracellular Ca2+ ions, because it is insensitive to 10 mM EGTA or thapsigargin. Slow exocytosis is initiated by inclusion of Ba2+ ions in the recording pipette and continues steadily for 5-12 min, producing a total increase of several thousand fF, which ultimately doubles or triples the original cell-surface area. We propose that two pathways of regulated exocytosis with distinct kinetics and divalent cation sensitivity exist in chromaffin cells. Only a single kinetic pattern is detected in NHP terminals, suggesting that mechanisms for secretion are not universally distributed in excitable cells.
Seward
E P
EP
Department of Neurobiology and Anatomy, Medical College of Pennsylvania, Philadelphia 19129, USA.
Chernevskaya
N I
NI
Nowycky
M C
MC
D000818:Animals; D001464:Barium / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology; D002412:Cations / Q000494:pharmacology; D002417:Cattle; D002838:Chromaffin System / Q000187:drug effects*; D005089:Exocytosis / Q000502:physiology*; D007700:Kinetics; D010904:Pituitary Gland, Posterior / Q000187:drug effects*; D013997:Time Factors
D002412:Cations; D001464:Barium; D002118:Calcium