Neuropeptide Y Y1 receptor mechanisms in sympathetic vascular control

The Y1 receptor is the predominant vascular NPY receptor subtype in pig hind limb and kidney. Thus, vascular responses to exogenous and endogenous NPY were almost or totally abolished in the presence of the Y1 receptor antagonist BIBP 3226. Furthermore, dose-dependent renal vasoconstriction was evoked by a Y1, but not a Y2, receptor agonist, and this could be strongly reduced by the Y1 receptor antagonist SR 120107A. Moreover, the expression of Y1 receptors in pig kidney and renal artery was indicated by RT-PCR and mRNA for Y1 receptors was detected in small intrarenal arteries using in situ hybridization. In contrast, the pig spleen contains both Y1 and Y2 receptors. In vivo, both Y1 and Y2 receptor agonists evoked dose-dependent splenic vasoconstriction, which was strongly reduced and not influenced, respectively, by SR 120107A. Accordingly, RT-PCR indicated expression of both Y1 and Y2 receptors in pig spleen. Presence of pig splenic Y2, but not Y1, receptors was also demonstrated in autoradiographic and membrane receptor binding studies. The presence of Y1 receptors in dog spleen was demonstrated in vivo, by RT-PCR, autoradiographic and membrane receptor binding, the latter also indicating existence of Y2 receptors. In addition, the Y1 receptor was also demonstrated in dog kidney in vivo and by RT-PCR. 2. Selectivity of SR 120107A for Y1 receptors was demonstrated, as Y1 agonist binding in dog spleen was displaced with great affinity, in contrast to Y2 agonist binding in dog and pig spleen. Furthermore, both SR 120107A and BIBP 3226 potently displaced tritiated BIBP 3226 binding from Y1 receptors in dog splenic membranes. BIBP 3226 exerted potent and dose-dependent antagonistic effects on contractions evoked by NPY in guinea-pig caval vein in vitro. The inhibition was competitive as the slope of the Schild plot was not significantly different from unity. SR 120107A appeared as effective as BIBP 3226 to antagonize NPY-evoked contractions in this vessel and neither antagonist affected contractions evoked by NA. SR 120107A potently antagonized Y1 receptor mediated vasoconstriction evoked in pig kidney and spleen in vivo. In contrast, vasoconstrictor responses in vivo mediated via other receptors, including Y2, were not affected. SR 120107A was also shown to have a long duration of action in vivo. BIBP 3226 exerted dose-dependent and equally potent antagonism on vascular responses to exogenous and endogenous, neurogenically released, NPY in vivo. The elimination of BIBP 3226 from plasma fit a two-compartment model, resulting in a half-life of 2 and 20 min of the alpha- and beta-phase, respectively. It is concluded that continuous infusions of this latter antagonist are preferable to infections during in vivo experiments, since non-specific effects can be avoided, and the duration of antagonistic action is under better control. 3. This study presents the final pharmacological evidence for the involvement of endogenous NPY in sympathetic vasoconstriction. It was demonstrated that neurogenically released NPY acting on Y1 receptors mediates long-lasting contractions of the guinea-pig vena cava in vitro. Thus, both SR 120107A and BIBP 3226 almost abolished the long-lasting phase of contraction evoked by high frequency stimulation of perivascular sympathetic nerves in this vessel, leaving merely an initial adrenergic peak contraction. Enantioselective inhibition of the neurogenically-evoked contractions in the caval vein was demonstrated, as BIBP 3435, the S-enantiomer of BIBP 3226 and virtually devoid of Y1 receptor affinity, was largely without effect. Evidence was also presented for the involvement of endogenous NPY in nonadrenergic vasoconstriction in vivo. Thus, SR 120107A abolished the long-lasting phase of nonadrenergic vasoconstriction evoked in hind limb and nasal mucosa upon high frequency stimulation of sympathetic nerves in the reserpine-treated pig in vivo. (ABSTRACT TRUNCATED)