Immediate sustained permeability in cells is increased by:
Direct endothelial injury, resulting in endothelial cell necrosis and detachment:

This effect is usually encountered in necrotizing injury, and is due to direct damage to the endothelium by the injurious stimulus e.g. In severe burns or lytic lesions.
Neutrophils adhere to the endothelium, and may also injure the endothelium.
In most instances, leakage starts immediately after the injury and is sustained at a high level for several hours, until the damaged vessels are thrombosed or repaired.
This reaction is immediate sustained response. All levels of microcirculation are affected, including venules, capillaries, and arterioles.
Endothelial cell detachment is often associated with platelet adhesion and thrombosis.
Direct injury and necrosis
Endothelial gaps by histamine
Endothelial gaps by serotonin
Leucocyte mediated endothelial injury