Exocytosis begins with formation of a small fusion pore which then expands allowing rapid release of granular contents. We studied the influence of cytoplasmic free Ca2+ ([Ca2+]i) on the conductance of the initial pore and on the dynamics of subsequent expansion in horse eosinophils using the patch clamp technique. The mean initial conductance is ∼200 pS independent of [Ca2+]i. This value is close to that previously found in beige mouse mast cells. The pore subsequently expands by 18 nS/s at [Ca2+]i < 10 nM, by 40 nS/s at [Ca2+]i = 1.5 μM and by 90 nS/s at [Ca2+]i = 10 μM. These results show that the structure of the initial fusion pore is independent of cytoplasmic Ca2+. However, the pore expansion is a Ca2+-dependent process modulating secretion at a step later than vesicle-plasma membrane fusion.
