The influence of tone upon responses of small and large vessels to serotonin.

In the dog foreleg, serotonin antagonizes extremes of vascular tone induced by neurogenic means. It produces net dilatation when the bed is constricted and net constriction when the bed is dilated. This bidirectional response derives ultimately from the facts that changes in nervous activity change calibers of small vessels without greatly altering calibers of large vessels, and that serotonin produces small vessel dilatation at the same time that it constricts large vessels. When small vessels are already neurogenically dilated, serotonin cannot dilate them further, but continues to constrict large vessels. Thus, the net effect is constriction. When small vessels are highly constricted, serotonin dilates small vessels more than it constricts large vessels. The net effect is dilatation. The study also indicates why it is more difficult to demonstrate a similar antagonism when tone is varied by humoral means.