The ultrastructure of frog microvessels following perfusion with the ionophore A23187

The ultrastructure of frog mesenteric capillaries and venules has been examined after the permeability of these vessels to fluid and macromolecules has been increased to a measured extent by perfusion with the ionophore A23187. An average 4.5-fold increase in hydraulic permeability in thirteen vessels was associated with the presence of gaps between the endothelial cells and marked attenuation of endothelial cytoplasm with the appearance of many fenestrations. The changes in ultrastructure suggested much larger increments in permeability than had been measured in these same vessels in vivo.