Vasodilatation and modulation of vasoconstriction in canine subcutaneous adipose tissue caused by activation of β-adrenoceptors

The present experiments were undertaken to study the balance between vascular α- and β-adrenoceptors in canine subcutaneous adipose tissue during sympathetic nerve stimulation and noradrenaline injections. Propranolol potentiated and prolonged the vasoconstrictor response to close i.a. injections of noradrenaline. The vasoconstriction induced by brief nerve stimulation (0.5 to 8 Hz) was, however, unaltered by the β-adrenoceptor blockade. During prolonged nerve stimulation the vasoconstrictor response was well maintained at 1.5 Hz but at 4 Hz there was a gradual escape. The escape phenomenon at 4 Hz was diminished by propranolol. The β₁-selective antagonist practolol, like propranolol, potentiated and prolonged the vasoconstriction induced by noradrenaline injections and reduced the vasoconstrictor escape during prolonged nerve stimulation at 4 Hz. Furthermore, the vasodilatation induced by noradrenaline injection or nerve stimulation during α-adrenoceptor blockade was diminished by practolol. Practolol also blocked the lipolytic response to noradrenaiine and nerve stimulation. The β₂-selective antagonist H35/25 blocked the effects of the β₂-selective agonist salbutamol but failed to alter noradrenaline as well as nerve stimulation induced vascular and lipolytic β-adrenoceptor responses. The present results provide further support for the hypothesis that vascular β-adrenoceptors in adipose tissue are humoral (noninnervated), preferentially activated by circulating noradrenaline. Moreover, both vascular and lipolytic β-adrenoceptors activated by noradrenaline in adipose tissue are best classified as β₁-adrenoceptors.     

Time-dependent effects of ischaemia on neuropeptide Y mechanisms in pig renal vascular control in vivo

We have investigated the effects of ischaemia on neuropeptide Y (NPY) mechanisms involved in sympathetic vascular control of the pig kidney in vivo. Reperfusion after 2 h of renal ischaemia was associated with local overflow of noradrenaline (NA) but not of NPY-like immunoreactivity (-LI). Renal sympathetic nerve stimulation 10 min into reperfusion evoked markedly reduced vasoconstrictor effects and significantly less overflow of NA (reduced by 70% from the pre-ischaemic conditions), whereas NPY-LI overflow was unaltered. Renal vasoconstrictor responses to exogenous peptide YY (PYY), phenylephrine and angiotensin II were strongly attenuated after this ischaemic period, while vasoconstriction to α,β-methylene ATP was maintained to a larger extent. The renal vascular responses and NA overflow had become partially normalized within a 2 h recovery period. In contrast, the renal vasoconstrictor response and the overflow of NPY-LI upon sympathetic nerve stimulation were enhanced after 15 min of renal ischaemia. In parallel, the PYY-evoked renal vasoconstriction was selectively and markedly prolonged after the 15 min of ischaemia. In the presence of the NPY Y₁ receptor antagonist BIBP 3226, the augmented vascular response to nerve stimulation was significantly attenuated. We conclude that reperfusion after 2 h of renal ischaemia is associated with local overflow of NA, whereas the sympathetic nerve-evoked release of NA and the reactivity of the renal vasculature to vasoconstrictor stimuli are reversibly reduced. Furthermore, possibly due to an impaired local degradation, the role of neurogenically released NPY in renal sympathetic vasoconstriction is enhanced after short-term (15 min) ischaemia compared with control conditions.     

Vasodilatation and modulation of vasoconstriction in canine subcutaneous adipose tissue caused by activation of beta-adrenoceptors.

The present experiments were undertaken to study the balance between vascular alpha- and beta-adrenoceptors in canine subcutaneous adipose tissue during sympathetic nerve stimulation and noradrenaline injections. Propranolol potentiated and prolonged the vasoconstrictor response to close i.a. injections of noradrenaline. The vasoconstriction induced by brief nerve stimulation (0.5 to 8 Hz) was, however, unaltered by the beta-adrenoceptor blockade. During prolonged nerve stimulation the vasoconstrictor response was well maintained at 1.5 Hz but at 4 Hz there was a gradual escape. The escape phenomenon at 4 Hz was diminished by propranolol. The beta1-selective antagonist practolol, like propranolol, potentiated and prolonged the vasoconstriction induced by noradrenaline injections and reduced the vasoconstrictor escape during prolonged nerve stimulation at 4 Hz. Furthermore, the vasodilatation induced by noradrenaline injection or nerve stimulation during alpha-adrenoceptor blockade was diminished by practolol. Practolol also blocked the lipolytic response to noradrenaline and nerve stimulation. The beta2-selective antagonist H35/25 blocked the effects of the beta2-selective agonist salbutamol but failed to alter noradrenaline as well as nerve stimulation induced vascular and lipolytic beta-adrenoceptor responses. The present results provide further support for the hypothesis that vascular beta-adrenoceptors in adipose tissue are humoral (noninnervated), preferentially activated by circulating noradrenaline. Moreover, both vascular and lipolytic beta-adrenoceptors activated by noradrenaline in adipose tissue are best classified as beta1-adrenoceptors.     

Frequency-dependent differences in the responses of the capsular and vascular smooth muscle of the spleen of the dog to sympathetic nerve stimulation

1. The responses of the capsular and vascular smooth muscle to splenic nerve stimulation have been studied simultaneously in the isolated blood-perfused dog's spleen.2. Low frequencies of splenic nerve stimulation (below 1.0 Hz) caused pronounced contraction of the splenic capsule but little or no constriction of the splenic vascular bed.3. Splenic contraction reached a maximum at stimulation frequencies of 1-2 Hz; maximum vasoconstriction occurred at frequencies of 7-10 Hz.4. The separation of responses of the capsular and vascular smooth muscle was mimicked by close arterial infusions of either adrenaline or noradrenaline.5. The maximum responses of the splenic vascular smooth muscle to nerve stimulation, adrenaline and noradrenaline were not significantly different.6. The maximum reduction in spleen volume to sympathetic nerve stimulation was significantly greater than the maximum response to close arterial noradrenaline.7. The maximum contractions of the spleen to adrenaline and noradrenaline were not significantly different. At concentrations producing submaximal responses adrenaline was more potent than noradrenaline.8. It is suggested that the frequency dependent separation of smooth muscle responses to sympathetic nerve stimulation is due to a different sensitivity of the capsular and vascular smooth muscle to the chemical transmitter noradrenaline.9. The results are discussed in the context of the function of the dog's spleen.     

Inhibition of neuropeptide Y-induced augmentation of noradrenaline-induced vasoconstriction by D-myo-inositol 1,2,6-trisphosphate in the rat mesenteric arterial bed

The effect of the neuropeptide Y antagonist D-myo-inositol-l,2,6-trisphosphate (α-trinositol) was tested against modulatory actions mediated by neuropeptide Y in the isolated rat mesenteric arterial bed. Neuropeptide Y (1 and 10 nM) had no direct postjunctional effects, but augmented vasoconstrictor responses to noradrenaline and to sympathetic nerve stimulation to an extent which was greater with the higher concentration of neuropeptide Y. The augmenting effect of neuropeptide Y at 1 nM on vasoconstriction induced by lower doses of noradrenaline was antagonized by α-trinositol (1 μM), producing a shift to the right of the dose-response curve. A lower concentration of α-trinositol (0.1 μM) had no inhibitory effect on responses to noradrenaline. Augmentation by the higher concentration of neuropeptide Y (10 nM) of noradrenaline-induced vasoconstriction was not affected by α-trinositol at concentrations of up to 10μM. α-Trinositol did not significantly antagonize neuropeptide Y-induced augmentation of vasoconstrictor responses to sympathetic nerve stimulation, α-Trinositol alone did not affect vasoconstrictor responses to noradrenaline, potassium, or to sympathetic nerve stimulation. In the raised-tone preparation (tone raised with methoxamine) in the presence of guanethidine (5 μM) to block sympathetic neurotransmission, perivascular nerve stimulation caused vasodilatation due to activation of sensory-motor nerves. Neuropeptide Y inhibited sensory-motor nerve induced vasodilatation in a concentration-dependent manner but this was not affected by α-trinositol (1 μM). These results suggest that α-trinositol can be a useful functional antagonist of neuropeptide Y-induced augmentation of vasoconstrictor responses to noradrenaline in the rat mesenteric arterial bed. Antagonistic effects of α-trinositol on neuropeptide Y-mediated pre-junctional inhibition of sensory-motor neurotransmission were not evident.     

Neuropeptide Y may mediate effects of sympathetic nerve stimulations on colonic motility and blood flow in the cat

The present study investigated sympathetic mechanisms involved in the regulation of colonic motility and blood flow in the cat. Infusion of neuropeptide Y (NPY) close i.a. produced an inhibition of colonic motility and a vasoconstriction of long duration but no post-infusion vasodilatation. In contrast to NPY, porcine pancreatic polypeptide did not evoke any vascular or motility response. On a molar basis, NPY was 25 times more potent than noradrenaline in producing 50% reduction of the colonic blood flow. These vascular and motility effects of NPY were resistant to guanethidine, phentolamine, phenoxybenzamine and propranolol. Thus, the action of NPY on vascular and colonic smooth muscle did not seem to be mediated via adrenergic receptors. Noradrenaline administered close i.a. produced inhibition of colonic motility, and vasoconstriction followed by a rapid vasodilatation. These effects were completely blocked by combined alpha- and beta-adrenoceptor blockade. Electrical stimulation of the splanchnic and lumbar colonic nerves produced an overall inhibition of colonic motility, and vasoconstriction of the proximal and distal colon, respectively, with a rapid post-stimulatory vasodilatation. After combined alpha- and beta-adrenoceptor blockade the inhibitory effect of the nerve stimulations on colonic motility partly remained together with a marked vasoconstriction, which was most pronounced upon lumbar colonic nerve stimulation. All vascular effects of sympathetic nerve stimulation were eradicated by guanethidine, which also abolished the inhibitory motility response to splanchnic nerve stimulation. However, lumbar colonic nerve stimulation elicited a colonic contraction, possibly due to stimulation of afferent C-fibres. The present findings indicate the existence of a sympathetic nonadrenergic neuronal mechanism mediating vasoconstriction and inhibition of colonic motility in the cat. Thus, NPY may be released from noradrenergic neurons to act on colonic smooth muscle and vessels.     

Neuropeptide Y and differential sympathetic control of splenic blood flow and capacitance function in the pig and dog

The roles of different mediators in the sympathetic regulation of the pig and dog spleens were investigated using a preparation with intact vascular perfusion in vivo. Sympathetic nerve stimulation caused overflow of neuropeptide Y-like immunoreactivity (NPY-LI) and noradrenaline (NA), arterial vasoconstriction, increase in venous blood flow and haematocrit. The dog spleen responded to single impulse stimulation, whereas more prolonged stimulation was required to elicit vascular responses in the pig spleen. Furthermore, the maximal splenic capacitance response was about 10 times larger in the dog than in the pig. After depletion of neuronal NA content by reserpine combined with preganglionic denervation, about 70% of the splenic arterial vasoconstrictor responses in the dog and pig still remained at 5 Hz stimulation. Fifty per cent of the capacitance response evoked by nerve stimulation still remained in the pig while in the dog spleen the capacitance response was virtually abolished after reserpine. The stimulation-evoked overflow of NPY-LI in pig spleen was increased several fold after reserpine treatment as compared to controls reaching levels in the venous effluent where exogenous NPY evokes vasoconstriction. In the dog spleen, overflow of NPY-LI was only observed after reserpine. Administration of NA caused arterial vasoconstriction with an initial increase in venous blood flow while NPY mainly reduced arterial blood flow. It is concluded that NA is involved in both the splenic arterial vasoconstriction and the capacitance responses while a non-adrenergic splenic vasoconstriction at least in the pig may be mediated by NPY.     

Comparison of the effects of stimulation of the sympathetic vasomotor nerves and the adrenal medullary catecholamines on the hind limb blood vessels of the rabbit

1. The responses to sympathetic nerve stimulation and to the adrenal medullary hormones have been studied in the hind limb vascular beds of the anaesthetized rabbit.2. Simultaneous measurements of femoral arterial blood pressure and of femoral venous blood flow indicate that stimulation of the sympathetic nerves decreases the calculated vascular conductance in both the intact and skinned hind limbs. Evidence is presented to show that these changes are due to vasoconstriction.3. The vasoconstriction in both skin and muscle vascular beds reaches a maximum at frequencies of stimulation around 15 Hz. No vasodilatation is obtained at any frequency of stimulation.4. The rabbit adrenal gland secretes only adrenaline during splanchnic nerve stimulation at frequencies between 3 and 60 Hz. The amounts liberated from both glands over this frequency range are 25-500 ng.kg body wt.(-1) min(-1).5. Intravenous infusions of adrenaline in concentrations similar to those liberated by the adrenal glands during splanchnic nerve stimulation, and of noradrenaline, cause only vasoconstrictor responses in skin and muscle.6. Simultaneous stimulation of the sympathetic nerves to the hind limb and infusion of adrenaline in quantities that could be liberated by splanchnic nerve stimulation at equivalent frequencies shows that the vasoconstrictor effects exerted by the individual components are additive, though the effects produced by the direct sympathetic nerve supply overshadow those produced by the catecholamine.7. The results are discussed in the context of the possible vascular role of the adrenal medullary hormones in the rabbit.     

Possible involvement of neuropeptide Y in sympathetic vascular control of canine skeletal muscle

Sympathetic nerve stimulation (2 min, 2 and 10 Hz) increased perfusion pressure in the blood perfused canine gracilis muscle in situ after pretreatment with atropine, desipramine and beta-adrenoceptor antagonists. This vasoconstriction was accompanied by clear-cut increases in the overflow of endogenous noradrenaline (NA) at both frequencies and, at 10 Hz but not at 2 Hz, also of neuropeptide Y-like immunoreactivity (NPY-LI). The irreversible alpha-adrenoceptor antagonist phenoxybenzamine enhanced the nerve stimulation induced overflows of NA and NPY-LI five- to eightfold and threefold, respectively. The fractional overflows of NA and NPY-LI per nerve impulse were similar in response to the high-frequency stimulation, indicating equimolar release in relation to the tissue contents of the respective neurotransmitter. The maximal vasoconstrictor response elicited by 10 Hz was reduced by about 50% following a dose of phenoxybenzamine which abolished the effect of exogenous NA and the remaining response was more long-lasting. Local i.a. infusion of NPY evoked long-lasting vasoconstriction in the presence of phenoxybenzamine, while the stable adenosine 5(1)-triphosphate (ATP) analogue alpha-beta-methylene ATP was without vascular effects. Locally infused NPY reduced the nerve stimulation evoked NA overflow by 31% (P less than 0.01) at 1 microM in arterial plasma, suggesting prejunctional inhibition of NA release. In conclusion, NPY-LI is released from the canine gracilis muscle upon sympathetic nerve stimulation at high frequencies. There is nerve stimulation evoked vasoconstriction, which is resistant to alpha-adrenoceptor blockade. This may in part be mediated by NPY released together with NA from the sympathetic vascular nerves.     

Influence of adenosine on the vascular responses to sympathetic nerve stimulation in the canine subcutaneous adipose tissue

Adenosine appears to regulate resting blood flow in canine subcutaneous adipose tissue. Sympathetic nerve stimulation has been shown to enhance the adenosine production in this tissue. This study therefore tested the possibility that adenosine may influence the vascular responses to sympathetic nerve stimulation. Intraarterial infusion of adenosine (5–20 μM in arterial blood) increased the resting vascular conductance (from 0.048 ± 0.007 to 0.095 ± 0.013 ml ± min^-1100 g^-1± mmHg^-1) and the percental reduction in vascular conductance due to sympathetic nerve stimulation (4 Hz) by 34 per cent (p<0.05) and to i. a.noradrenaline by 27 per cent (p<0.05). The vasodilator response due to nerve stimulation after α-blockade was reduced by adenosine. Dipyridamole (0.5–1.5 μM) + EHNA (3–10 μM), which increases plasma adenosine levels, had similar effects to adenosine, while theophylline (30–80 μM) decreased the vasoconstrictor response. The vasoconstrictor escape was enhanced by EHNA alone and in combination with dipyridamole, but was reduced by theophylline. On the other hand, the poststimulatory hyperemia was unaffected by adenosine, dipyridamole and EHNA, and theophylline. The results show that adenosine does not reduce the magnitude of the initial vasoconstrictor response in proportion to the increase in resting blood flow. The autoregulatory escape in adipose tissue during nerve stimulation appears to be mediated both by adenosine and by noradrenaline acting on β-adrenoceptors. Poststimulatory hyperemia does not seem to be greatly influenced by exogenous or endogenous adenosine     

Neuropeptide Y (NPY) and sympathetic control of blood flow in oral mucosa and dental pulp in the cat

Neuropeptide Y (NPY) immunoreactivity (-IR) was found to be present in perivascular nerves in the cat dental pulp and oral mucosa. Many ganglion cells in the superior cervical ganglion also contained NPY-IR. Ligation of the inferior alveolar or lingual nerves produced an accumulation of NPY-IR in axons proximal to the site of ligation, suggesting an anterograde axonal transport of the peptide. After unilateral sympathectomy the NPY-IR disappeared in the dental pulp and oral mucosa on the ipsilateral side. Reversed phase high performance liquid chromatography showed that the main peak of NPY-like immunoreactivity found in the superior cervical ganglion co-chromatographed with synthetic porcine NPY. Changes in blood flow in dental pulp or oral mucosa were measured indirectly by recording local clearance of ¹²⁵I during electrical stimulation of the sympathetic nerve or during close intra-arterial infusion of noradrenaline or NPY. All three procedures resulted in a pronounced decrease in local blood flow of a similar magnitude in both tissues. After a-adrenoceptor blockade with phentolamine, the vasoconstrictor effect of noradrenaline was abolished. However, the effect of sympathetic stimulation after phentolamine was only partially reduced (23–54%) and the response to NPY was almost unaffected by the a-receptor blockade. The remaining effect of sympathetic stimulation after phentolamine was abolished by guanethidine. However, the response to NPY was not changed by the latter drug. In conclusion, the vasoconstrictor response in the dental pulp and oral mucosa caused by activation of sympathetic nerves is more resistant to phentolamine than the response induced by infusion of exogenous noradrenaline. Since NPY is probably co-localized with noradrenaline in the sympathetic perivascular nerves and NPY reduces local blood flow,it is proposed that this peptide is involved in sympathetic vascular control in oral tissues.     

Evaluation of presynaptic alpha-receptor function in the canine renal vascular bed

The functional significance of presynaptic alpha-receptor modulation of sympathetic nerves was examined in vivo in the canine renal vascular bed. In pentobarbital-anesthetized dogs, the vasoconstrictor response to renal nerve stimulation and exogenous norepinephrine was compared before and during intra-arterial infusions of epinephrine, oxymetazoline, clonidine, and norepinephrine. Only epinephrine produced a modest decrease in stimulation-induced vasoconstriction at 1 Hz. After pretreatment with desipramine, intra-arterial infusions of epinephrine or norepinephrine did not alter stimulation-induced vasoconstrictor responses relative to exogenous norepinephrine. Further, neither yohimbine nor phentolamine (10(-9) to 10(-3) g, intra-arterial) produced a distinctly increased vasoconstrictor response to nerve stimulation relative to exogenous norepinephrine. Thus, studies using alpha-receptor agonists, antagonists, and inhibition of neuronal uptake failed to reveal a physiologically significant alpha-receptor-mediated negative feedback mechanism for stimulation-induced vasoconstriction in the canine renal vascular bed.     

Sympathetic vascular control of the pig nasal mucosa: (I) increased resistance and capacitance vessel responses upon stimulation with irregular bursts compared to continuous impulses

An in vivo model is described in which pentobarbital anaesthetized pigs were used to study the sympathetic nervous control of the nasal mucosal vascular bed. Changes in blood flow in the sphenopalatine artery (representing nasal blood flow) and in the volume of the nasal cavity (mainly reflecting blood content in venous sinusoids), upon electrical stimulation of the cervical sympathetic trunk, were recorded simultaneously. Single impulses (15V, 5 ms) reduced both the arterial flow and the volume of the nasal mucosa. The effects of nerve stimulation with a continuous train of impulses at 0.59, 2 and 6.9 Hz were compared with those caused by stimulation with the irregular bursting pattern, triggered by recorded human sympathetic vasoconstrictor nerve activity, with the same average frequencies. Both types of stimulation reduced nasal blood flow and volume, but the responses were significantly larger with burst stimulation at 0.59 Hz. The volume reduction was already maximal at 0.59 Hz while the blood flow response increased further higher frequencies. Local intra-arterial pretreatment with the α-adrenoceptor antagonist phenoxybenzamine significantly attenuated the flow and volume responses to single impulses, while clear-cut reductions in blood flow (by 40%) and volume (by 80%) remained, upon stimulation, at 6.9 Hz. Noradrenaline given intra-arterially caused a dose-dependent reduction in nasal blood flow and volume. The noradrenaline effects were blocked by phenoxybenzamine treatment. The results show that the pig nasal mucosa represents a model where both blood flow and volume changes can be studied in parallel in vivo. Furthermore, stimulation with the firing pattern of human vasoconstrictor nerves, i.e. irregular bursts, causes larger vascular responses in the pig nasal mucosa compared to a continuous stimulation. The large residual vascular responses to sympathetic nerve stimulation at high frequency after a-adrenoceptor blockage may be mediated by some other non-adrenergic transmitter substance(s).     

Sympathetic vascular control of the pig nasal mucosa (2): Reserpine-resistant, non-adrenergic nervous responses in relation to neuropeptide Y and ATP

The possible occurrence of non-adrenergic mechanisms in the sympathetic vascular control of the nasal mucosa was studied in vivo using reserpine-treated pigs (1 mg kg-1, i.v., 24 h earlier) in combination with pharmacological blockade of alpha-adrenoceptors by local phenoxybenzamine (1 mg kg-1, i.a.) infusion. The nasal mucosal depletion (99%) of the content of noradrenaline (NA) in reserpinized animals was not influenced by preganglionic denervation while the depletion (44%) of neuropeptide Y (NPY) was prevented. Upon stimulation with single shocks, 25% of the arterial blood flow reduction and 47% of the nasal mucosal volume reduction (reflecting contraction of venous sinusoids) were still present after reserpine as compared with controls. In reserpinized animals, the vascular responses were slow developing and long-lasting, and about 60% remained at 0.59 Hz and more than 80% at 6.9 Hz. The vascular effects after reserpine were, however, subjected to fatigue, which may explain why phenoxybenzamine treatment still reduced the functional effects in the absence of NA. Local intra-arterial injections of NA, NPY and the metabolically stable adenosine-5'-triphosphate analogue alpha, beta-methylene ATP (mATP) caused reduction in both arterial blood flow and nasal mucosal volume. The C-terminal fragment of NPY (NPY 13-36) also induced nasal vasoconstriction although with a fivefold lower potency than NPY 1-36. Adenosine-5'-triphosphate caused a biphasic vascular effect with vasodilatatory actions at low doses and a short-lasting vasoconstriction followed by vasodilatation at very high doses (100-fold higher than the threshold response to mATP). In contrast to the response to NA, the long-lasting vascular effects of NPY and mATP were resistant to phenoxybenzamine treatment. In conclusion, although NA is likely to mediate most of the sympathetic vascular responses to low-frequency stimulation in the pig nasal mucosa, a large resistance and capacitance vessel component upon high-frequency stimulation seems to be non-adrenergic and mimicked by NPY rather than ATP.     

Mechanisms underlying pre- and postjunctional effects of neuropeptide Y in sympathetic vascular control

The effects of porcine neuropeptide Y (NPY) regarding sympathetic vascular control were studied in vitro on isolated rat blood vessels. The 10(-9)M NPY enhanced (about two-fold) the contractile responses to transmural nerve stimulation (TNS), noradrenaline (NA) and adrenaline (about two-fold) in the femoral artery. Higher concentrations of NPY (greater than 10(-8)M) caused an adrenoceptor-resistant contraction per se. The TNS-evoked [3H]NA efflux was significantly reduced by NPY in a concentration-dependent manner (threshold 10(-9)M). The calcium antagonist, nifedipine, abolished the contractile effects of NPY and the NPY-induced enhancement of NA contractions but did not influence the prejunctional inhibition of [3H]NA release. Receptor-binding studies showed that the ratio of alpha 1-to alpha 2-adrenoceptors in the femoral artery was 30:1. The NPY did not cause any detectable change in the number of alpha 1-or alpha 2-adrenoceptor binding sites or in the affinity of alpha 2-binding sites, as revealed by prazosin- and clonidine-binding, respectively. The NPY also inhibited the TNS-evoked [3H]NA release (by 42-86%) in the superior mesenteric and basilar arteries and in femoral and portal veins. The NPY still depressed TNS-evoked [3H]NA secretion from the portal vein in the presence of phentolamine. The NPY caused a clear-cut contraction in the basilar artery, increased the contractile force of spontaneous contractions in the portal vein, while only weak responses were observed in the superior mesenteric artery and femoral vein. The NA-induced contraction was markedly enhanced by NPY in the superior mesenteric artery, only slightly enhanced in the portal vein and uninfluenced in the femoral vein. In conclusion, in all blood vessels tested, NPY depresses the TNS-evoked [3H]NA secretion via a nifedipine-resistant action. Furthermore, NPY exerts a variable, Ca2+-dependent vasoconstrictor effect and enhancement of NA and TNS contractions.     

Sympathetic vascular control of the pig nasal mucosa (III): co-release of noradrenaline and neuropeptide Y

The overflows of noradrenaline (NA) and neuropeptide Y like immunoreactivity (NPYLI) and vascular responses upon sympathetic nerve stimulation were analysed in the nasal mucosa of pentobarbital anaesthetized pigs. In controls, a frequency-dependent increase in NA overflow was observed whereas detectable release of NPY-LI occurred only at 6.9 Hz. Parallel decreases in blood flow in the sphenopalatine artery and vein and in nasal mucosa volume (reflecting blood volume in the venous sinusoids) were observed. The laser Doppler flowmeter signal (reflecting superficial blood flow) increased upon low and decreased upon high frequency stimulation. Twenty-four hours after reserpine pretreatment and preganglionic decentralization, the NA overflow was abolished while a frequency-dependent release of NPY-LI occurred. Forty, 60 and 80% of the vasoconstrictor responses then remained upon stimulation with a single impulse, 0.59 and 6.9 Hz, respectively. Both the vasoconstriction and NPY-LI overflow, however, were subjected to fatigue upon repeated stimulation. In reserpinized animals release of NPY-LI and vasoconstrictor responses were larger upon stimulation with irregular bursts at 0.59 Hz compared to effects seen at stimulation with continuous impulses. Pre-treatment with the a-adrenoceptor antagonist phenoxybenzamine or the monoamine reuptake inhibitor, desipramine, enhanced NA overflow by 2–3 and 1.5 times at 0.59 and 6.9 Hz, respectively. Phenoxybenzamine significantly reduced the nerve-evoked vascular responses while the release of NPY-LI at 6.9 Hz was increased. Desipramine increased the functional responses but reduced the NPY-LI overflow. During tachyphylaxis to the vasoconstrictor effects of the stable adenosine 5′-triphosphate (ATP) analogue α-β-methylene ATP (mATP) in controls, the vasoconstrictor responses as well as the NA and NPY-LI overflow to nerve stimulation were unmodified. In reserpinized animals, however, the vascular responses and the overflow of NPY-LI were reduced after mATP tachyphylaxis. These data show that both NA and NPY are released upon sympathetic nerve stimulation in the nasal mucosa in vivo and this release seems to be regulated via prejunctional a-adrenoceptors. The lack of effect of mATP tachyphylaxis under control conditions makes it less likely that ATP serves as a major mediator of the large nonadrenergic vasoconstrictor component.     

Endothelium-independent potentiation by neuropeptide Y of vasoconstrictor responses in isolated arteries from rat and rabbit

An endothelium-dependent action of neuropeptide Y (NPY) has been implicated in studies on various vascular beds. In the present study, the requirement of an intact endothelium for NPY-evoked potentiation of the response to sympathetic nerve stimulation was determined in the small mesenteric arteries of the rat and in the central ear artery of the rabbit. Further, NPY-mediated inhibition of relaxing influences was determined in small mesenteric arteries of the rat. Vascular segments were mounted in a double myograph, where one of the two suspended vessels was denuded of endothelium by gently rubbing the intimal surface. Removal of endothelium was verified by en-face silver staining. In both species, the response to bursts of transmural field stimulation eliciting 10% of maximal contraction was potentiated 2-4 times in the presence of 10 nM NPY, whether the endothelium was present or not. In small mesenteric arteries precontracted with noradrenaline, addition of acetylcholine (I microM) caused relaxation only in vessels with an intact endothelium. Subsequent addition of 10 nM NPY enhanced vasoconstriction in both intact and endothelium-denuded vessels. The endothelium-independent beta-adrenergic agonist isoprenaline (I microM) relaxed both intact and denuded small mesenteric arteries, and in both further addition of 10 nM NPY increased the contraction to about the same extent. The results demonstrate that NPY potentiates the responses to sympathetic field stimulation in small mesenteric arteries from the rat and in central ear artery from rabbit whether the endothelium is present or not. NPY inhibits both endothelium-dependent and -independent relaxations in small mesenteric arteries from rat.     

Release and vasoconstrictor effects of neuropeptide Y in relation to non-adrenergic sympathetic control of renal blood flow in the pig

The possible involvement of neuropeptide Y (NPY) in sympathetic control of renal blood flow was investigated in the pig in vivo. Exogenous NPY caused renal vasoconstriction with a threshold effect at an arterial plasma concentration of 164 pmol 6(-1). Stimulation of the renal nerves (0.59, 2 and 10 Hz) in control animals evoked rapid and frequency-dependent reduction in renal blood flow and overflow of NPY-like immunoreactivity (NPY-LI) and noradrenaline (NA) from the kidney, suggesting co-release from sympathetic nerves. Following the administration of the alpha- and beta-adrenoceptor antagonists phenoxybenzamine and propranolol, the vasoconstrictor response to exogenous NA was reduced by 98%, whereas that of NPY was unaltered. The response to nerve stimulation with 0.59 Hz was abolished, whereas relatively slowly developing reductions in renal blood flow by 7 and 28% were obtained upon stimulation with 2 and 10 Hz respectively. The nerve stimulation-evoked overflow of NA at 0.59 and 2 Hz, but not at 10 Hz and not that of NPY-LI, was enhanced after adrenoceptor blockade. Twenty-four hours after reserpine treatment (1 mg kg-1 i.v.) the contents of NPY-LI and NA in the renal cortex were reduced by 80 and 98% respectively. Sectioning of the renal nerves largely prevented the reserpine-induced depletion of NPY-LI, but not that of NA. Nerve stimulation of the denervated kidney with 2 and 10 Hz 24 h after reserpine treatment evoked slowly developing and long-lasting reductions in renal blood flow by 6 and 52% respectively. These responses were associated with overflow of NPY-LI, which was similar to and threefold higher than that observed in controls at 2 and 10 Hz respectively, while no detectable overflow of NA occurred. Repeated stimulation with 10 Hz resulted in a progressive fatigue of the vasoconstrictor response and the associated overflow of NPY-LI, giving a high correlation (r = 0.86, P less than 0.001) between the two parameters. It is concluded that NPY is a potent constrictor of the renal vascular bed. Furthermore, although NA is the likely transmitter mediating most of the responses to low to moderate nerve activation under control conditions, the data suggest that NPY may mediate the non-adrenergic reductions in renal blood flow evoked by high-frequency sympathetic nerve stimulation after reserpine treatment.     

A novel inositol phosphate selectively inhibits vasoconstriction evoked by the sympathetic co-transmitters neuropeptide Y (NPY) and adenosine triphosphate (ATP).

Postganglionic sympathetic nerves release norepinephrine (NE) as their primary neurotransmitter at vascular and other targets. However, much evidence supports involvement of additional messengers, co-transmitters, which are co-released with NE upon sympathetic nerve stimulation and thereby contribute to their actions, e.g., vasoconstriction. Two such putative co-transmitters, neuropeptide Y (NPY) and adenosine triphosphate (ATP) have been of particular interest since they fulfill several neurotransmitter criteria. Importantly, hitherto it has been difficult to antagonize vasoconstriction evoked by either NPY or ATP with agents that are devoid of intrinsic activity. The present study describes the ability of a novel inositol phosphate, D-myo-inositol 1,2,6-trisphosphate (Ins[1,2,6]P3; PP-56) to in vitro potently block vasoconstrictor responses elicited by NPY and ATP, but not by NE, as studied in guinea-pig isolated basilar artery. The action of Ins[1,2,6]P3 does not seem to occur through antagonism at NPY- or ATP-receptor recognition sites, labeled by 125I-peptide YY and 35S-gamma-ATP, respectively, in membranes of rat cultured vena cava vascular smooth muscle cells. However, it does involve inhibition of the influx of Ca2+ induced by either co-transmitter in these same vena cava cells. It is proposed that Ins[1,2,6]P3 may be a useful functional antagonist of non-adrenergic component(s) of the vasoconstrictor response to sympathetic nerve stimulation.     

Effects of cervical sympathetic nerve stimulation and neuropeptide Y (NPY) on cranial blood flow in the cat

Effects of cervical sympathetic nerve stimulation (SNS) at 10 Hz and intravenous infusion of neuropeptide Y (NPY), 10 and 100 pmol x kg body wt-1 x min-1 for 5 min, on regional blood flow in the cat were investigated with radioactive microspheres. Sympathetic nerve stimulation caused significant reductions in blood flows in the facial tissues including the eye. Alpha-adrenoceptor blockade with phenoxybenzamine and combined beta- and alpha-adrenoceptor blockade with propranolol and phenoxybenzamine abolished the effects of sympathetic nerve stimulation in most facial tissues except in the tongue, upper eyelid and masseter muscle. In most cranial tissues, neuropeptide Y reduced regional blood flow and increased vascular resistance. No effect of neuropeptide Y on vascular resistance was observed in the choroid. In the present study, evidence for a non-adrenergic component in sympathetic vasoconstriction was found in the tongue, upper eyelid and masseter muscle but not in the majority of feline facial tissues. Neuropeptide Y was a potent vasoconstrictor in many cranial tissues, while in parts of the uvea, the effects of neuropeptide Y were less pronounced.