Neuropeptide Y in cerebrovascular function: comparison of membrane potential changes and vasomotor responses evoked by NPY and other vasoconstrictors in the guinea pig basilar artery

Effects of prostaglandin F2alpha on membrane currents in rabbit middle cerebral arterial smooth muscle cells

Although PGF(2alpha) affects contractility of vascular smooth muscles, no studies to date have addressed the electrophysiological mechanism of this effect. The purpose of our investigation was to examine the direct effects of PGF(2alpha) on membrane potentials, Ca(2+)-activated K(+) (K(Ca)) channels, delayed rectifier K(+) (K(V)) channels, and L-type Ca(2+) channels with the patch-clamp technique in single rabbit middle cerebral arterial smooth muscle cells (SMCs). PGF(2alpha) significantly hyperpolarized membrane potentials and increased the amplitudes of total K(+) currents. PGF(2alpha) increased open-state probability but had little effect on the open and closed kinetics of K(Ca) channels. PGF(2alpha) increased the amplitudes of K(V) currents with a leftward shift of the activation and inactivation curves and a decrease in the activation time constant. PGF(2alpha) decreased the amplitudes of L-type Ca(2+) currents without any significant change in threshold or apparent reversal potentials. This study provides the first finding that the direct effects of PGF(2alpha) on middle cerebral arterial SMCs, at least in part, could attenuate vasoconstriction.

Histamine-induced depolarization: ionic mechanisms and role in sustained contraction of rabbit cerebral arteries

The role of membrane depolarization in the histamine-induced contraction of the rabbit middle cerebral artery was examined by simultaneous measurements of membrane potential and isometric force. Histamine (1-100 microM) induced a concentration-dependent sustained contraction associated with sustained depolarization. Action potentials were observed during depolarization caused by histamine but not by high-K(+) solution. K(+)-induced contraction was much smaller than sustained contraction associated with the same depolarization caused by histamine. Nifedipine attenuates histamine-induced sustained contraction by 80%, with no effect on depolarization. Inhibition of nonselective cation channels with Co(2+) (100-200 microM) reversed the histamine-induced depolarization and relaxed the arteries but induced only a minor change in K(+)-induced contraction. In the presence of Co(2+) and in low-Na(+) solution, histamine-evoked depolarization and contraction were transient. We conclude that nonselective cation channels contribute to histamine-induced sustained depolarization, which stimulates Ca(2+) influx through voltage-dependent Ca(2+) channels participating in contraction. The histamine-induced depolarization, although an important and necessary mechanism, cannot fully account for sustained contraction, which may be due in part to augmentation of currents through voltage-dependent Ca(2+) channels and Ca(2+) sensitization of the contractile process.

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.

Characterization of neuropeptide Y receptors mediating contraction, potentiation and inhibition of relaxation

In addition to its direct vasoconstrictive effect, neuropeptide Y (NPY) potentiates noradrenaline-(NA) induced contraction and inhibits acetylcholine-(ACh) induced relaxation: The aim of the present study was to elucidate the NPY receptor subtypes responsible for mediating these three responses. NPY, peptide YY (PYY) and pro34NPY (a NPY Y1 receptor agonist) induced equipotent and equally strong concentration-dependent contractions of guinea pig basilar arteries. NPY13-36 (a NPY Y2 receptor agonist), however, caused only weak contraction with significantly lower potency. The NPY-induced contraction was significantly inhibited by the selective NPY Y1 receptor antagonist BIBP3226 (1 microM). NPY, PYY and pro34NPY but not NPY13-36 significantly potentiated the NA-induced contraction in guinea pig mesenteric arteries. The potentiation was significantly inhibited by BIBP3226 (1 microM). In precontracted guinea pig basilar arteries, ACh induced a concentration-dependent relaxation which was significantly inhibited by NPY, PYY and NPY13-36 but not by pro34NPY. BIBP3226 had no significant effect on the NPY-induced inhibition of the relaxation. These results suggests that the NPY Y1 receptors mediate the direct contraction and the potentiation of the NA-induced contraction but not the inhibition of the ACh-induced relaxation. This effect seems to be mediated by another NPY receptor subtype, presumably by the Y2 receptor, as judged from the agonist potency order.

Ca2+ handling mechanisms underlying neuropeptide Y-induced contraction in canine basilar artery

The effects of neuropeptide Y on isometric tension simultaneously measured with cytosolic Ca2+ concentration ([Ca2+]cyt) and Ca2+ sensitivity of contractile elements were studied in isolated canine basilar arteries. Neuropeptide Y (1-100 nM) increased [Ca2+]cyt and tension in a concentration-dependent and parallel manner, whereas 9,11-dideoxy-11 alpha,9 alpha-epoxymethano prostaglandin F2 alpha (U46619) (10-100 nM), a thromboxane A2 mimetic, produced a large contraction with a small increase in [Ca2+]cyt. Ca2+ channel antagonists such as d-cis-diltiazem (10 mM) abolished both [Ca2+]cyt and tension augmented by neuropeptide Y. In Ca(2+)-free solution containing 0.2 mM EGTA, neuropeptide Y did not change [Ca2+]cyt and tension, whereas U46619 transiently increased both of them. Furthermore, neuropeptide Y apparently did not affect the Ca2+ sensitivity when assessed in the artery permeabilized with Staphylococcus aureus alpha-toxin, whereas U46619 augmented it. These findings suggest that neuropeptide Y-induced contraction in the canine basilar artery is produced mainly by Ca2+ influx through L-type Ca2+ channels.

alpha-Trinositol blocks neuropeptide Y-induced inositolphosphate formation in cerebral vessels

Neuropeptide Y (NPY) induces contraction of guinea-pig basilar arteries via activation of Y1 receptors. This contraction is blocked by D-myo-inositol-1,2,6-triphosphate (alpha-trinositol). Previous binding studies have shown that alpha-trinositol has no effect at Y1 or Y2 binding sites thus the antagonistic effect should occur at the level of a second messenger. We have examined the effects of NPY on the formation of inositol phosphates (IP) and have looked for an antagonistic effect of alpha-trinositol. NPY (10(-9)-3 x 10-(-7) M) induced strong concentration-dependent contraction of basilar arteries from young guinea-pigs (weight 200-250 g) (Emax: 76.4 +/- 11.1%) but not of arteries from old guinea-pigs (weight > 500 g) (Emax: 2.8 +/- 1.5%). [Pro34]NPY and PYY induced contraction of similar magnitude and potency, whereas NPY13-36 had only a weak effect. This demonstrates an effect via the Y1 type of NPY receptor. The contraction induced by NPY was blocked by alpha-trinositol (p < 0.05). LiCI (2 x 10-4) M), used to inhibit IP breakdown, had no effect on the contraction induced by NPY. NPY (10(-10)-10(-8) M) increased the formation of IP in cerebral vessels from young guinea-pigs from 357 +/- 48 cpm/mg w.w. to 900 +/- 233 cpm/mg w.w. However, there was no alteration in IP formation in cerebral vessels from old guinea-pigs (NPY 10(-9)-10(-7) M). In the presence of alpha-trinositol (10(-8)-10(-6) M) the NPY induced stimulation of IP formation was totally abolished.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of K+ channels in neuropeptide Y-induced vasoconstriction in rabbit cerebral basilar artery

The role of K+ channels in the direct vasoconstrictive response induced by neuropeptide Y was investigated in isolated basilar arteries of rabbits and in vivo in rats. K+ channel openers, either BRL38227 or diazoxide, caused dose-dependent and complete relaxation of isolated arteries precontracted by neuropeptide Y. Exposure to both BRL38227 and diazoxide shifted the concentration-response curves for neuropeptide Y to the right without changing the maximal response. However, BRL38227 antagonized the angiotensin II-induced vasoconstriction noncompetitively. In vivo, the pressor responses produced by neuropeptide Y were significantly inhibited by pretreatment with BRL38227 in anesthetized rats. These results show that K+ channel openers antagonize neuropeptide Y-induced vasoconstriction in a competitive manner and suggest that blockade of K+ channels contributes, at least in part, to the direct vasoconstrictive effect of neuropeptide Y.

Selective constriction of small cutaneous arteries by NPY matches distribution of NPY in sympathetic axons

This study has begun to investigate some functional implications of the differential localization of neuropeptide Y (NPY) in sympathetic neurons supplying different arterial segments in the cutaneous circulation of the guinea-pig ear. Responses of the main ear artery to exogenous NPY and norepinephrine (NE) were examined in vitro by measuring isometric tension. Responses of smaller arterial vessels to application of exogenous NPY or NE to the adventitial surface were examined in anaesthetized, ventilated guinea-pigs, by measuring changes in internal vessel diameter using video microscopy. Some arterial segments subsequently were examined for the presence of immunoreactivity (IR) to tyrosine hydroxylase (TH) and NPY. NPY (1 nM-10 microM) contracted the main ear artery (EC50 = 10 nM; max. contraction = 30% KCl), and 1 nM NPY produced slight potentiation of contractions produced by NE. In vivo, local applications of NPY (1-10 microM) constricted only a subpopulation of arterial vessels (23 of 41). All vessels constricted by NPY were innervated by axons containing IR to both TH and NPY, and as a population, were more proximal in the arterial tree (branch orders 3 to 6) than were vessels insensitive to NPY (branch orders 4 to 8). Most vessels insensitive to NPY were arterioles and arterio-venous anastomoses < 40 microns in diameter, which were innervated by axons containing TH-IR but not NPY-IR. In contrast, local application of NE (1-30 microM) constricted all vessels examined in vivo. When present, NPY constrictions had a longer latency (15-45 s) and duration (3-4 min) than NE constrictions of the same vessel segments. In vivo, NPY sometimes potentiated the peak amplitude of NE constrictions (2 of 7 vessels), but only in vessels where NPY also produced direct constriction. These results reveal an excellent correlation between the localization of NPY in sympathetic axons, and the location of postsynaptic NPY receptors throughout the cutaneous arterial system. Any NPY released in response to strong activation of cutaneous sympathetic neurons is likely to act preferentially on the proximal cutaneous arteries, and to lead to a more prolonged constriction of these arteries than of more distal arterioles and arterio-venous anastomoses.

Neuropeptide Y potentiates noradrenaline-evoked vasoconstriction by an intracellular calcium-dependent mechanism

The potentiating effect of neuropeptide Y (NPY) was examined by testing the influence of putative inhibitors of calcium entry on the NPY-enhanced contractile response to noradrenaline in the guinea pig uterine artery. In order to examine the involvement of voltage sensitive calcium entry mechanisms we recorded the effect of noradrenaline and NPY on the membrane potential. NPY (100-300 nM) enhanced noradrenaline-evoked vasoconstriction. The potentiation by NPY was most prominent in low noradrenaline concentrations (30-300 nM) and the pD10 (-log molar concentration of agonist eliciting 10% of maximum contraction) value was increased from 6.43 +/- 0.07 to 6.97 +/- 0.11 (P < 0.001, n = 6). Inhibition of extracellular calcium influx shifted concentration-dependently to the right the concentration-response curve for noradrenaline but potentiation by NPY still remained. The intracellular calcium chelator quin-2 AM selectively abolished the NPY-induced enhancement of the contractile response to noradrenaline. In contrast, quin-2 AM (10-30 microM) had no inhibitory effect on the contractile response to noradrenaline per se. It is suggested that NPY initiates an intracellular calcium-sensitive mechanism which increase alpha-adrenoceptor sensitivity. This results in a significant increase of sarcoplasmic calcium and stronger contractile responses to noradrenaline.

Cerebral vasoconstrictor mediators

Endogenous cerebral vasoconstrictor mediators regulate vascular resistance and blood flow in the brain as a whole and in various regions and participate in the pathogenesis of cerebral circulatory disturbances. Vasoconstrictors are effective in the treatment of diseases associated with cerebral vasodilatation. There are variations in the response of cerebral arteries from primate and subprimate mammals; therefore, information as to similarities and differences in their response is quite important in evaluating the physiological role, involvement in pathogenesis and therapeutic usefulness of the mediators in healthy men and patients. In this review we described characteristics of the action of vasoconstrictors (amines, peptides, prostanoids, and others) on isolated cerebral arteries from mammals, including humans and monkeys.

Electrophysiology of cerebral blood vessels

In spite of the relatively large amount of in vitro and in vivo data indicating that, in a number of ways, cerebral arteries are pharmacologically different from peripheral arteries, the mechanisms responsible for these differences are far from clear. An understanding of these mechanisms is particularly important for a rational approach to the treatment of disorders of the cerebral circulation including migraine, hypertension and the responses of cerebral vessels to subarachnoid haemorrhage. This review outlines electrophysiological data which are available from cerebrovascular smooth muscle cells, including the possibility that inwardly-rectifying potassium channels, active at potentials close to the resting membrane potential, are intimately involved in the changes in smooth muscle tone which couple blood flow to regional changes in nerve cell activity. The membrane potential changes in response to perivascular nerve stimulation, noradrenaline, 5-hydroxytryptamine and endothelium-derived hyperpolarizing factor are also described, together with the underlying membrane mechanisms and their relationship to smooth muscle contraction and relaxation.

Unmasking the vasoconstrictor response to neuropeptide Y and its interaction with vasodilating agents in vitro

Neuropeptide Y (NPY) is a powerful vasoconstrictor in vivo but is usually much less active on isolated blood vessels. The contractile effect of NPY was examined in the isolated rat femoral artery exposed to various degrees of vasoconstriction. The effects of NPY on the relaxation induced by vasodilator agents was also studied. NPY (< or = 1 microM) had no contractile effect. In vessels pretreated with a low concentration of phenylephrine (0.3-1.0 microM), NPY evoked a concentration-dependent contraction, which was similar in intact and in endothelium-deprived vessels. Other vessels were contracted with phenylephrine (3-10 microM) and relaxed with histamine (0.1 mM). Subsequent addition of NPY elicited a contraction which was much greater than that observed in vessels pretreated with phenylephrine only. The Y1 receptor agonist, [Pro34]NPY, but not the Y2 receptor agonist, NPY 13-36, evoked a concentration-dependent contraction in phenylephrine-pretreated vessels. Acetylcholine (ACh) induced endothelium-dependent relaxation in vessels contracted with phenylephrine. NPY (0.1 microM) induced a rightward shift of the concentration-response curve and a lower maximum relaxation in response to ACh. NPY was without effect on the dilatation evoked by nitroprusside, histamine or forskolin. In conclusion, under appropriate vasoconstrictor and vasodilator influence, NPY can act at Y1 receptors to evoke vasoconstriction in the femoral artery via endothelium-independent mechanisms. In addition, NPY seems to attenuate the endothelium-dependent relaxation induced by ACh. These actions of NPY may contribute to explain the strong vascular effects of the peptide in vivo.

Endothelium-independent potentiating effects of neuropeptide Y in the rat tail artery

The role of the endothelium in the potentiating action of neuropeptide Y (NPY) to contraction induced by KCl, alpha, beta-methylene ATP (mATP), and noradrenaline (NA) was tested on rat tail arteries. Endothelium-intact and denuded ring segments and freshly isolated single smooth muscle cells were used in the study. Contraction responses to KCl and mATP were potentiated by NPY (50 nM) in both intact and denuded arteries. Contraction to NA was potentiated by NPY at 500 nM but not at 50 nM. The potentiation effect of NPY was antagonized by nifedipine. Similarly, the shortening of single smooth muscle cells in response to KCl and mATP was potentiated by NPY (50 nM). The noradrenaline response was potentiated by NPY at 500 nM but not at 50 nM. Our results suggest that the potentiating effect of NPY is more specific to contraction mediated by nifedipine-sensitive calcium channels and is not dependent on the presence of an intact endothelium.

The role of neuropeptide Y in cardiovascular regulation

Neuropeptide Y (NPY) is a 36 amino acid peptide present in the brain, the adrenal medulla and peripheral sympathetic nerves. The localization and mode of release of NPY led to the proposal that this peptide plays an important role in modulating the contribution of the sympathetic nervous system to blood pressure control. In this paper Bernard Waeber and colleagues review the current knowledge about the mechanisms involved in NPY signal transduction and the different mechanisms whereby NPY, released by the peripheral nervous system, may influence vascular tone and cardiac function.

Neuropeptide Y antagonistic properties of D-myo-inositol-1.2.6-trisphosphate in guinea pig basilar arteries

The antagonistic properties on neuropeptide Y (NPY)-induced contraction of the guinea pig basilar artery of D-myo-inositol-1.2.6-triphosphate (PP56) has been examined using a sensitive in vitro system. It was observed that PP56 did not per se cause contraction or relaxation of precontracted vessel segments. However, it was found to be a non-competitive antagonist of NPY-induced contraction. This effect was observed in the concentration range 10(-8)-10(-6) M PP56. A slight potentiation of endothelin I-induced contraction was seen at high concentrations (10(-3) M). In contrast there was no modulation of the contractile effects elicited by bradykinin, noradrenaline, 5-hydroxytryptamine (5-HT) or prostaglandin F2 alpha (PGF2 alpha) apart from a slight reduction in maximum effect at 10(-4) M and 10(-3) M PP56. PP56 was observed to possess antihistaminic and anticholinergic properties in the concentration range 10(-5) M-10(-3) M. The relaxant effects of vasoactive intestinal peptide, calcitonin gene-related peptide, neurokinin A and substance P were only modified to a minor extent by PP56 in concentrations of 10(-4) M and 10(-3) M. In conclusion, PP56 appears to be the first non-peptide which potently and rather selectively antagonizes NPY-induced contractions of the guinea pig basilar artery. In high concentrations, PP56 may modify the responses of other agents tested, including histamine and acetylcholine.