Effect of histamine on the ultrastructure of mucosal circulatory vessels in rat large intestine.

Histamine, applied intravenously at various concentrations (3 micrograms, 125 micrograms, 250 micrograms/100 g body weight) caused 90 and 210 s after the injection ultrastructural alterations in the endothelial cells of rat colonic mucosal microvessels. The most prominent changes occurred with the 250 micrograms/100 g/90 s combination, including an increase of vesicular population, formation of vacuole-like spaces, increase in the luminal evaginations of the fenestrated capillaries, as well as an increase in size of the Golgi complex. Only few gaps were distinguished in some venules after the administration of 250 micrograms/100 g histamine and a circulation time of 210 s. Intact vessels were detected at all concentrations. Results indicate, that histamine affects structures involved in transvascular transport, mainly by increasing the number of pinocytotic vesicles. They also suggest a possible differential response within the endothelial cell population towards histamine action.