Intestinal Vascular Responses to 5‐Hydroxytryptamine

The in vivo and in vitro effects or 5‐hydroxytryptamine (5‐HT) were investigated on the mesenteric vasculature of the cat. Close i.a. injections (10 μg) or infusions (10–15 μg/min) into the superior mesenteric artery of chloralose anesthetized cats generally induced an intestinal vasodilatation of 60–150 % above control which was unaltered by α‐or β‐adrenergic or cholinergic receptor blockade, and which after its development was unaffected by intestinal motility responses. Following nerve blockade by tetrodotoxin, the vasodilator response to 5‐HT was changed to vasoconstriction. During 5‐HT infusions of 20–50 μg/min intestinal blood flow and intestinal capillary filtration coefficient (CFG), measured plethysmographical‐ly, increased significantly above control, while at infusions of 10–15 μg/min 5‐HT increased CFC without increasing intestinal blood flow. In in vitro studies on strips or segments of mesenteric arteries and veins from the cat and rat, 5‐HT constantly increased isometric tension in all vessels studied. The response was unaltered by adrenergic or cholinergic receptor blockade and was not related to passive tension applied to the vessels nor to the presence or absence of noradrenaline in the bath. The divergent in vivo and in vitro response are discussed in terms of the following possible mechanisms: 1. A differential effect of 5‐HT with constriction of large vessels and dilatations of smaller ones; 2. A direct constrictor effect on the vascular smooth muscle via specific serotonic receptors, as well as an indirect dilator effect, perhaps mediated through local nervous pathways.