ATP inhibits onset of exocytosis in permeabilised mast cells

ATP is not required for exocytosis from permeabilised mast cells, and therefore there is no direct role for protein phosphorylation in the late stages of the activation pathway. We have measured the timecourse of exocytosis from permeabilised cells triggered to release hexosaminidase following addition of Ca²⁺ to cells equilibrated for 2 min with GTP--S. If ATP is included at the time of permeabilisation, then exocytosis commences after a delay, the duration of which depends on the square root of the product [Ca²⁺][GTP--S], and which may extend to beyond 3 min. When ATP is excluded then the maximal rate of exocytosis is established within 3 secs of completing the effector combination. These results suggest that the achievement of a new steady-state, induced by Ca²⁺ and GTP--S, and required for exocytosis is inhibited by ATP. From this we conclude that dephosphorylation of an unknown regulator protein may comprise a step in the exocytotic pathway.