Non-SG fusion in whole-cell recordings is strongly inhibited by cell swelling. (A and B) Bar graphs give exchange current densities (top) and capacitance responses (bottom) for two batches of BHK cells in which membrane fusion was activated by outward Na/Ca exchange current. (A) Effects of cytoplasmic osmolarity on fusion responses. The left bar graphs give results for isoosmotic solution, the middle bars for cytoplasmic solution with 200 mM sucrose, and the right bars for cytoplasmic solution diluted 30% with distilled water. (B) Effects of extracellular osmolarity on fusion responses. From left to right the bar graphs are with (1) standard extracellular solution for 2 min after cell opening, (2) standard extracellular solution with the NMG-aspartate concentration reduced by 80 mM for 2 min after cell opening, (3) standard solution reapplied for 2 min after applying hypoosmotic solution for 2 min, and (4), as in (3) with 5 μM wortmannin in all cytoplasmic solutions. (C) Capacitance responses of RBL cells for pipette perfusion of cytoplasmic solution with 200 μM free Ca. The left bar graph indicates the response magnitude for control cells, the middle graph for cells swollen with hyperosmotic cytoplasmic solution (200 mM sucrose) for 2 min, and the right bar for cells swollen with extracellular solution in which the NMG concentration was reduced by 80 mM.