Exocytotic and cytolytic release of histamine from mast cells treated with Portuguese man-of-war (Physalia physalis) venom.

Electron microscopic observations suggest that venom from isolated nematocysts of the stinging tentacles of the Portuguese man-of-war, Physalia physalis, causes histamine release via a rapid, short-duration exocytosis of granules and a slower, long-duration lysis of mast cells. Fine structural changes in mast cells are concurrent with histamine release and are independent of the presence of leukocytes. Vesiculation of the plasma membrane and release of granules nearest the cell surface occur within 10 sec after exposure to 100 micrograms venom/10(5) cells. Released granules and granules retained in plasma membrane invaginations are fibrous and less electron opaque than more centrally located granules. Complex channels to the external medium continue to form, and within 1 min, characteristics of both degranulation and cytolysis are well advanced. Mitochondria are swollen or disrupted. Microridges are absent. Intracellular granules are significantly fewer in venom-treated mast cells, but are more widely separated than in controls. This suggests that degranulation occurs at early stages but is halted as cytolysis proceeds.