The actions of the peptides, neuropeptide Y and peptide YY, on the vascular and capsular smooth muscle of the isolated, blood-perfused spleen of the dog.

The two peptides, neuropeptide Y (NPY) and peptide YY (PYY) were compared for potency on the vascular and extravascular smooth muscle of the isolated, blood-perfused spleen of the dog. The only vascular response to both NPY and PYY was vasoconstriction; the maximum effect was to arrest splenic arterial blood flow completely. On a molar basis both NPY and PYY were significantly more potent (P less than 0.01) as splenic arterial vasoconstrictors than the transmitter noradrenaline (NA). PYY was approximately 7 times more potent as a vasoconstrictor than NPY. In contrast to their potency on the vascular smooth muscle, NPY and PYY were significantly (P less than 0.01) less potent than NA in causing contraction of the splenic capsule. The two peptides were equipotent in eliciting this contraction.