Fusion Gains Independence

Elevated intracellular Ca 2+ concentrations [Ca 2+ ] appear to be a rather universal trigger of massive membrane ca-pacitance increases, presumably refl ecting exocytosis of tiny vesicles (Borgonovo et al., 2002). The [Ca 2+ ] required to stimulate this response is very high. In this issue Yaradanakul et al. (see p. 29) report experiments with baby hamster kidney (BHK) cells expressing the Na + /Ca 2+ exchanger NCX1. In whole-cell patch clamp experiments, using an intracellular solution with high (40 mM) Na + concentration and a Na +-free extracellular solution, switching extracellular [Ca 2+ ] from very low (in the presence of 0.5 mM EGTA) to 2 mM is a new trick that makes it work. The massive Ca 2+ infl ux that is produced (mediated bv the Na + /Ca 2+ exchanger running in reverse) raises intracellular free [Ca 2+ ] to ‫ف‬ 200 μ M, which triggers a pronounced capacitance increase. The response is only partially inhibited when ATP is replaced by the nonhydrolyzable analogue AMPPNP. A Ca 2+-dependent capacitance increase, distinct from secretory granule exocytosis, was fi rst discovered in rat peritoneal mast cells (Almers and Neher, 1987) and was subsequently reported to occur in many other cell types 2+ ] increase required to induce the response in BHK cells agrees with the previously reported requirement for intracellular [Ca 2+ ] exceeding 100 μ M to induce corresponding capacitance increases in many other cells — though the response in mast cells was apparent already at ‫ف‬ 3 μ M free intracellular [Ca 2+ ] (Almers and Neher, 1987). Phosphoinositides have for many years been implicated to play a signifi cant role in regulated exocytosis present detailed studies on the role of phosphoinositides in the response that is triggered by high [Ca 2+ ] in BHK cells. The Ca 2+ infl ux activates PI(4,5)P 2 breakdown but phosphoinositide metabolism turns out to be neither suffi cient nor necessary for the membrane-fusion response. Activation of PI(4,5)P 2 breakdown in the absence of a suffi ciently high [Ca 2+ ] increase does not stimulate fusion and PLC inhibitors as well as peptides binding PI(4,5)P 2 do not interfere with the activation of the Ca 2+ infl ux – induced fusion response. These results Abbreviations used in this paper: BHK, baby hamster kidney; RBL, rat basophilic leukemia; TeTx, tetanus toxin. indicate that the regulation of this fusion response is quite different …