Effects of 5-Hydroxytryptamine on Capillary and Arteriovenous Anastomotic Blood Flow in the Human Hand and Forearm and in the Pig Hind Leg

The effects of intraarterially infused serotonin (5-HT) on capillary and arteriovenous anastomotic (AVA) blood flow were investigated in the hand and forearm of 19 healthy volunteers, and in the hind leg of six anesthetized pigs using radioactive microspheres with a diameter of 15 μm. The 5-HT2-receptor antagonist ketanserin was used in an attempt to identify the receptors involved. None of the drugs in the doses used induced systemic hemodynamic effects. Low doses of 5-HT significantly increased forearm blood flow with a maximum response at the dose of 1 ng/kg/min (68 ± 14%, p < 0.05), whereas only at the highest dose of 80ng/kg/min was a net decrease in forearm blood flow measured ( - 28 ± 6%. p < 0.05). Conversely, finger blood flow was not influenced by the lower doses of 5-HT, whereas a major reduction was observed at the highest dose ( - 90 ± 3%). Ketanserin increased both total forearm blood flow and AVA blood flow. The drug blunted the constrictor response to 5-HT in the forearm but only slightly attenuated this response in the finger. The percentage AVA blood flow in the human hand and forearm was not influenced by an infusion of 5-HT at 80 ng/kg/min alone. However, after pretreatment with ketanserin, which itself increased the AVA component, this dose of 5-HT significantly reduced AVA flow. In the pig, total femoral blood flow was not influenced by 5-HT, but AVA blood flow was significantly reduced and capillary skin blood flow increased. It is concluded that in both humans and pigs intraarterially infused 5-HT decreases AVA blood flow but that this effect is seemingly not mediated by 5-HT2 receptors but, as previously reported in the porcine carotid circulation, may involve 5-HT1-like receptors. The fact that low doses of 5-HT did not induce a net vasodilation in the finger is attributed to the relative absence of skeletal muscles and abundance of AVAs in the finger.