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

1 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. 2 The only vascular response to both NPY and PYY was vasoconstriction; the maximum effect was to arrest splenic arterial blood flow completely. 3 On a molar basis both NPY and PYY were significantly more potent (P < 0.01) as splenic arterial vasoconstrictors than the transmitter noradrenaline (NA). 4 PYY was approximately 7 times more potent as a vasoconstrictor than NPY. 5 In contrast to their potency on the vascular smooth muscle, NPY and PYY were significantly (P < 0.01) less potent than NA in causing contraction of the splenic capsule. The two peptides were equipotent in eliciting this contraction.