FIGURE 8.

Capacitance and amperometric recordings show that granule exocytosis is followed by compensatory endocytosis. A possible explanation may be provided by compound cavicapture, a mechanism involving the transient fusion of a large piece of membrane formed by the cumulative fusion of several secretory granules with the cell membrane. Secretory granules can form aggregates of granules through cavicapture. The capacitance trace (blue) shows a small-capacitance flicker (kiss-and-run endocytosis) at 1, followed by four on-steps (steps 2–5), reflecting four secretory granules fusing sequentially with the plasma membrane and releasing their serotonin contents. A first resealing of the entire cavity is observed at step 6 as a large off-step. Later, a larger on-step (step 7) may be interpreted as a new fusion of the granule chain including an additional vesicle (perhaps the first vesicle that fused transiently by the kiss-and-run mechanism). The granule aggregate fused transiently by cavicapture up to three times (steps 7–12) until its final disconnection from the plasma membrane. The cavity tends to increase (steps 11 and 12) or decrease (steps 7 and 8) by membrane flow to or from the cell surface (53). In addition, the second granule of the chain may close instead the first one, leaving the first granule bound to the plasma membrane (step 8).