How do kinins affect vascular tone?

Because kinins affect vascular tone, it is assumed that kinins act directly on smooth muscle. However, a direct interaction is difficult to conceive. Vessels containing smooth muscle are lined by a continuous endothelium with tight junctions. In addition, kinins act on endothelial cells to cause the release of prostaglandin-related substances; possibly through receptors. Furthermore, endothelial cells have a great capacity for hydrolyzing kinins to inactive products. Hence, even invoking active transport, less than 1% of kinins might be expected to reach the first layer of smooth muscle cells. However, kinins may not act directly on smooth muscle as endothelial cells and smooth muscle cells form specialized cell contacts. Myoendothelial junctions occur, and we have shown, in pulmonary arterioles, that smooth muscle cells send large numbers of projections into the cytoplasm of the endothelial cells. In addition, smooth muscle cells attach directly to the abluminal surface of endothelial cells, as do pericytes. Thus, there is a morphologic basis by which kinins can affect tone of smooth muscle without acting directly on smooth muscle cells.