The actions of neuropeptide Y and peptide YY on the hepatic arterial and portal vascular beds of the anaesthetized dog.

1. The vascular actions of the two peptides, neuropeptide Y (NPY) and peptide YY (PYY) were compared with the transmitter noradrenaline (NA) on the arterial and portal vascular beds of the in situ liver of the anaesthetized dog. 2. The sole vascular response of the hepatic arterial vasculature to intra-arterial administration of either NPY or PYY was vasoconstriction; the duration of these responses was longer than that to NA. 3. The maximum hepatic arterial vasoconstrictor responses to PYY and to NPY were significantly different and they were both significantly less than the maximum to NA (P less than 0.001). 4. In contrast to its activity on the splenic arterial vasculature PYY was not more potent, on a molar basis, than NPY as an hepatic arterial vasoconstrictor agent. However, both peptides were significantly more potent than NA (P less than 0.005). 5. Neither peptide, when injected intraportally, caused any change in intrahepatic portal inflow resistance. 6. Both peptides when administered intraportally in doses which were free of systemic effects caused hepatic arterial vasoconstriction.