Hemodynamic characterization of a novel neuropeptide Y receptor antagonist

Enhanced neuropeptide Y immunoreactivity and vasoconstriction in mesenteric small arteries from spontaneously hypertensive rats

Enhanced sympathetic nerve activity is thought to play a role in the pathogenesis of hypertension. The purpose of the present study was to investigate the mechanisms underlying the enhanced vasocontractile response to perivascular stimulation of mesenteric arteries isolated from female spontaneously hypertensive rats (SHR). Innervation of mesenteric small arteries was evaluated by immunohistochemistry and confocal microscopy while functional studies were conducted in a microvascular myograph. The distribution of nerve terminals immunoreactive for tyrosine hydroxylase (TH) and neuropeptide Y (NPY) was similar in mesenteric small arteries from Wistar-Kyoto (WKY) and SHR rats. However, immunointensity of TH or NPY immunoreactivities were much higher in small arteries from SHR compared to WKY. Expressed as percentage of contractions elicited by 124 mM K(+), concentration-response curves for noradrenaline (NA) and NPY were shifted leftward in SHR compared with WKY rats. The combination of noradrenaline (1 microM) and NPY (10 nM) contracted mesenteric arteries from WKY and SHR to higher levels than compared to either contractile agent added alone. The NPY Y(1) receptor antagonist, BIBP 3226, inhibited these contractions with 87 +/- 0.7 and 80 +/- 1.3% (p < 0.05, n = 6) in arteries from WKY and SHR rats, respectively. In arteries incubated with the alpha(1)-adrenoceptor antagonist, prazosin, and preactivated with vasopressin, electrical field stimulation evoked contractions which were more pronounced in mesenteric arteries from SHR compared to WKY rats. BIBP 3226 partially inhibited these contractions. In vasopressin-activated arteries BIBP 3226 caused rightward shifts of the concentration-response curves for NPY in mesenteric arteries from SHR rats, but in addition it also abolished the maximal NPY contraction in arteries from WKY rats. In the presence of BIBP 3226, low concentrations (1 pM to 10 nM) of NPY caused relaxations in arteries from WKY, but not in segments from SHR rats. Mechanical removal of the endothelium abolished NPY relaxation in arteries from WKY. In arteries activated with vasopressin and exposed to either forskolin or sodium nitroprusside, the addition of NPY evoked contractions which were more pronounced in arteries from SHR compared to WKY arteries. The present study suggests that enhanced NPY content and vasoconstriction to NPY in arteries from hypertensive rats can contribute to the enhanced sympathetic nerve activity and vascular resistance in female hypertensive rats. Endothelial cell dysfunction as well as alterations in smooth muscle response to NPY seem to contribute to the enhanced vasoconstriction in arteries from hypertensive animals.

Role of NPY Y1 receptors in cardiovascular control in the conscious rabbit

Prejunctional neuropeptide Y (NPY) Y1 receptors on cardiac sympathetic neurons mediate transient inhibition of chronotropic responses in rabbit isolated right atria. The function of these receptors remains speculative. We investigated a possible functional role for these receptors in modulation of the baroreceptor-heart rate (HR) reflex in the conscious rabbit. Mean arterial pressure (MAP) responses to a range of doses of the Y1 receptor agonist [Leu31,Pro34]NPY (1-8 microg/kg, i.v.) were constructed in ganglion-blocked rabbits. After administration of the selective Y1 receptor antagonist GR231118(150 microg/kg, i.v.), two-point [Leu31,Pro34]NPY dose-pressor responses were assessed. Linear regression analysis of the relation between the shift in the [Leu31,Pro34]NPY dose-pressor response lines against time was used as an estimate of the functional half-life of GR231118. GR231118 shifted the two-point [Leu31,Pro34]NPY dose-pressor response relation by 10- to 30-fold. A single estimate of the functional half-life of a bolus dose of GR231118 was 25 +/- 2 min. This determination allowed a steady-state Y1-receptor blockade to be established by a bolus and infusion. In a separate group of rabbits, the baroreceptor-HR reflex was assessed before and 30 min after administration of GR231118 (150 microg/kg bolus, then 150 microg/ kg/h, i.v.). GR231118 caused an initial transient pressor response and bradycardia, followed by a depressor response and a more sustained tachycardia. Infusion of GR231118 had no effect on the baroreceptor-HR reflex. Prejunctional Y1 receptors appear not to mediate a tonic inhibition of cardiac sympathetic neurotransmission in the conscious rabbit during physiological manipulations in MAP. However, activation of postjunctional Y1 receptors by neuronal or circulating NPY may be important in maintenance of vascular tone in the conscious rabbit.

Neuropeptide Y Y1 receptors in vascular pharmacology

The existence of neurogenic mediator candidates apart from noradrenaline and acetylcholine involved in the control of vascular tone has attracted enormous attention during the past few decades. One such mediator is neuropeptide Y (NPY), which is co-localized with noradrenaline in sympathetic perivascular nerves. Stimulation of sympathetic nerves in vitro and in vivo causes non-adrenergic vasoconstriction which can be blocked by experimental manipulations that inhibit NPY mechanisms. Thus, the vasopressor response to stimulation of sympathetic nerves can be attenuated by chemical or surgical sympathectomy, treatment with reserpine or other pharmacological agents, and tachyphylaxis to NPY or by NPY antagonists. The NPY field was long plagued by a lack of specific antagonists, but with the recently developed, selective, non-peptide and stable NPY antagonists it has now become possible to study subtypes of this receptor family. For instance, it has become clear that the NPY Y1 receptor mediates most of the direct peripheral effects of NPY on vascular tone. These antagonists promise to stimulate NPY research and will likely unravel the true significance of NPY in cardiovascular control under physiological conditions as well as in pathophysiological states.

A comparison of actions of neuropeptide Y (NPY) agonists and antagonists at NPY Y1 and Y2 receptors in anaesthetized rats

The pancreatic polypeptide family includes three members, neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP), with sequence homology between members and species varying from approximately 50 to 80%. Some of these peptides were compared in the mammalian cardiovascular system for activity mediated by actions on pre- (Y2) and post-junctional (Y1) NPY receptors. NPY and PYY, with sequence homology of 67% have similar actions on Y1 and Y2 receptors. Rat pancreatic polypeptide (rPP) with sequence homology of approximately 50% is inactive at both. This study reports that the chimeric peptide, hPP1-11/NPY12-36 and the truncated peptide NPY2-36 show similar activity to NPY mediated through both receptor types in vivo, while salmon PYY (sPYY), with 81% homology to NPY, has improved potency at both receptor subtypes. NPY3-36 has equal activity with NPY on actions mediated through Y2 receptors, but significantly reduced activity mediated through Y1 receptors. Two NPY antagonists were also examined: PYX2 was inactive in vivo and 1229U91 showed potent, long-lasting activity on Y1 receptor-mediated effects.

Role of neuropeptide Y in cold stress-induced hypertension

Chronic cold stress (4 degrees C) produced a sustained increase in mean arterial pressure in both normotensive and borderline hypertensive rats (BHR). The high blood pressure in BHRs was significantly reversed by a neuropeptide Y (NPY) Y1 receptor antagonist suggesting that NPY is involved in mediating stress-induced hypertension. Corresponding increases in adrenal NPY messenger RNA and NPY immunoreactivity were found during the stress; furthermore, chronic cold stress also potentiated the pressor response of rats to a subsequent acute stress test in which NPY has been shown to play a role. These results suggest that chronic cold stress-induced hypertension is mediated by elevated NPY release and vascular tone as a result of increased NPY gene expression and storage.

Hemodynamic profile of neuropeptide Y in dogs: effect of ganglionic blockade

The hemodynamic effects of neuropeptide Y (NPY) were examined over a dose range of 0.03-30 nmol/kg, i.v., in anesthetized open-chest, ventilated dogs with and without ganglionic blockade. In normal (non-ganglion-blocked) animals, NPY produced significant, dose-dependent, and sustained (lasting 15-45 min) increases in mean arterial blood pressure and systemic vascular resistance (SVR) with a threshold dose of 0.3 nmol/kg and a maximum effective dose of 10 nmol/kg. Cardiac index (CI) decreased at doses > 1 nmol/kg, but stroke volume was not altered; heart rate (HR) decreased significantly at and above the 3 nmol/kg dose. No significant changes were observed in the left ventricular dP/dt (LVdP/dt) or the contractility index (LVdP/dt divided by systolic pressure). In ganglion-blocked animals, pressor and SVR responses to NPY were similar to those seen in normal animals but HR was not affected and a small but significant decrease in CI was seen only at the 30 nmol/kg. Furthermore, whereas LVdP/dt of ganglion-blocked dogs increased significantly at and above the 1 nmol/kg dose, the contractility index increased slightly only with the 10 and 30 nmol/kg doses. These data indicate that NPY produces sustained hypertension in dogs secondary to peripheral vasoconstriction, has a weak, direct positive inotropic action on the heart, and lacks chronotropic effects.