CONTROL OF GILL FILAMENT BLOOD-FLOW BY SEROTONIN IN THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

The effects of exogenously applied serotonin [5-hydroxytryptamine (5-HT)] on the distal arterial vasculature of gill filaments were observed using an epi-illumination microscope equipped with a water-immersion objective and connected to a video camera. In addition, ventral aortic flow (Q) and celiac artery pressure (P-CA) were measured. Intra-arterial injection of serotonin (100 nmol/kg) completely stopped the blood flow in the distal part of the filaments and caused a rapid decrease of P-CA. Repeatedly, the flow reduction was found to coincide with a constriction of the distal portion of the efferent filamental vasculature. Because there was no concomitant reduction in Q, it is concluded that a redistribution of blood to more proximal parts of the filaments occurred. After treatment with the serotonergic receptor antagonist methysergide, the vasoconstrictor effect of serotonin on the filamental vasculature was eliminated, while a decrease in P-CA was still observed. The results demonstrate a specific site(s) for the serotonergic vasoconstriction in the distal portion of the filament.