Neuropeptide Y Y1 receptor mechanisms in sympathetic vascular control

Sammanfattning: Neuropeptide Y Yl receptor mechanisms in sympathetic vascular control. By Rickard E. Malmström, 1997. Department of Physiology and Pharmacology, Karolinska Institute, S- 17177 Stockholm, Sweden. It was demonstrated that the Y1 receptor was the predominant vascular neuropeptide Y (NPY) receptor in pig kidney and hind limb as exogenous and endogenous NPY evoked vasoconstrictor responses that were almost or totally abolished by the selective non-peptide Y1 receptor antagonist, BIBP 3226. Furthermore, renal vasoeonstriction was evoked by NPY and a peptide Y1 agonist, but not by a Y 2 agonist, and these responses were strongly reduced by another non-peptide Y, receptor antagonist, SR 120107A. Moreover, expression of Y1 receptors in pig kidney and renal arteries was indicated by reverse transcriptase-polymerase chain reaction (RT PCR) and mRNA for Y1 receptors was detected in small intrarenal arteries with in situ hybridization. The NPY receptor population in the pig spleen seems to consist of both Y1 and Y2 receptors, since peptide agonists with preference for either subtype evoked splenic vasoeonstriction. Splenic vasoconstriction evoked by the Y1 agonist was markedly reduced by SR 120107A. RT-PCR indicated expression of both Y1 and Y2 receptors in pig spleen and the existence of splenic Y2 receptors was also demonstrated with membrane and autoradiograpbic receptor binding. The predominant NPY receptor in both dog spleen and kidney is the Y1 receptor as demonstrated by in vivo studies, RT-PCR and reeeptor binding. The Y1-selectivity of SR 120107A was demonstrated by the fact that the compound displaced binding of an iodinated Y1, but not Y2, reeeptor ligand from membranes and sections of pig and dog spleen. Moreover, both SR 120107A and BIBP 3226 potently displaced tritiated BIBP 3226 binding from Y1 receptors in dog spleen. Increasing concentrations of BIBP 3226 caused a rightward shift in the concentration-response curves to NPY without influencing the maximal NPY-evoked contraction in guinea-pig vena cava. The antagonism appeared competitive as the slope (0.84) of the Schild plot was not significantly different from unity, with a pA2 value of 8.0. SR 120107A appeared as effective as BIBP 3226 in antagonizing NPY-evoked contractions in this vessel. SR 120107A potently inhibited Y1 receptor mediated vasoconstriction in the pig in vivo, without influencing vascular responses exerted via Y2, a, P2X1 and angiotensin II reeeptors. In addition, the Y, receptor antagonism of SR 120107A was of long (>3h) duration in vivo. BIBP 3226 exerted dose-dependent and equal antagonism on vascular responses to both endogenous and exogenous NPY in the pig in vivo. The elimination of BIBP 3226 from plasma fit a two-compartment model with half-lives of 2 and 20 min for the A- and B- phase, respectively. The final pharmacological evidenee for NPY as a mediator of sympathetic vasoeonstrietion was presented. Thus, neurogenically released NPY mediates long-lasting contraction of the guinea-pig caval vein in vitro, as shown by the inhibitory effects of both BIBP 3226 and SR 120107A. In the presence of either antagonist, only an initial rapid adrenergic phase of contraction remained upon high frequency transmural electrical field stimulation in this vessel. The neurogenic contractions were largely unaffected by the S-enantiomer to BIBP 3226, BIBP 3435, which is virtually inactive on Y1 receptors. Evidence was also presented for the involvement of NPY in nonadrenergic sympathetic vasoconstriction evoked in the reserpine-treated pig in vivo. Thus, SR 120107A strongly reduced the long-lasting phase of vasoconstriction evoked in nasal mucosa and hind limb by high frequency sympathetic nerve stimulation, leaving merely an initial rapid phase of constnction. Furthermore, the reserpine-resistant sympathetic vasoconstrietion in pig kidney was almost abolished by SR 120107A, whereas both the peak and duration of this response were redueed in the spleen. In eontrast, the role of NPY in sympathetie vaseular control is less obvious in the control pig, in which noradrenaline (NA) levels are intact and the NPY release is smaller due to prejunctional a2-reeeptor regulation. Reperfusion after two h of renal isehaemia was associated with venous overflow of NA, but not of NPY-like immunoreaetivhy (Ll). In addition, the renal sympathetic nerve-evoked overflow of NA, but not of NPY-LI, was reduced in parallel with reduced renal vasoconstnctor responses to nerve activation and exogenous agonists. The vascular responses as well as the nerve-evoked overflow of NA were partially restored a further two h after reperfusion. No overflow of either NA or NPY-LI was seen upon reperfusion after 15 min of renal ischaemia but an enhanced overflow of NPY-LI, but not NA, was observed upon sympathetic nerve stimulation and this was paralleled by an augmented vasoconstnctor response that in turn was significantly inhibited by BIBP 3226. Furthermore, the renal vasoconstrictor response to Y, receptor activation by exogenous agonists was markedly prolonged after 15 min ischaemia and this prolonged response was nearly abolished by BIBP 3226. These results suggest that, presumably due to an impaired local degradation, the role of neurogenically released NPY in renal sympathetic vasoconstriction is enhanced after short-term ischaemia compared to control conditions. It is concluded that NPY may serve as a sympathetic mediator of vasoconstriction, preferentially acting on the Y1 receptor. The role of NPY in sympathetic vascular control is enhanced after reserpine treatment and short-term ischaemia, but is less obvious in the control situation. Furthermore, BIBP 3226 and SR 120107A are selective Y, receptor antagonists both in vitro and in vivo. Key words: Sympathetic vasoconstriction, neuropeptide Y1 Y, receptor antagonists, BIBP 3226, SR 120107A, ischaemia, pig. ISBN 91-628-2527-5