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 Ca²⁺-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 (∼25-40 fF/sec) compared with Ca²⁺-evoked stimulus-coupled exocytosis (∼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.