Neuropeptide Y inhibits relaxations of the guinea pig uterine artery produced by VIP

Interactions between neuropeptides contained in autonomic vasodilator neurons supplying the guinea pig uterine artery were investigated in isolated segments of the artery precontracted with prostaglandin F2 alpha. Neither somatostatin-14 (10(-6) mol.l-1) nor dynorphin A(1-17) (10(-6) mol.l-1) had direct effects on vascular tone, and did not affect relaxations produced by guinea pig vasoactive intestinal peptide (gpVIP). Both the porcine and the guinea pig forms of neuropeptide Y (NPY; 10(-7)-10(-5) mol.l-1) caused transient contraction of the precontracted arteries. NPY also inhibited relaxations of the artery produced by gpVIP, an action which was not directly related to the NPY contractions. NPY caused both a concentration-dependent rightward shift in the gpVIP concentration-response curve, and a reduction in size of the maximum relaxation to gpVIP. NPY (10(-6) mol.l-1) also produced a rightward shift in the concentration-response curves for the vasodilators forskolin and glyceryl trinitrate, but did not reduce the maximum relaxations to these compounds. Thus NPY, which is colocalized with VIP in vasodilator neurons supplying the uterine artery, can greatly reduce the vasodilator potency of VIP.