EFFECTS OF NEUROPEPTIDE Y ON THE PRESSOR RESPONSES TO PHENYLEPHRINE AND TO ACTIVATION OF THE SYMPATHETIC NERVOUS SYSTEM IN ANAESTHETIZED RATS

1. The effects of neuropeptide Y (NPY) on the pressor responses to intravenous injections of phenylephrine and to reflex activation of the sympathetic nervous system by stimulation of the sciatic nerve were examined in anaesthetized rats. 2. NPY (10-20 micrograms/kg) always potentiated the pressor response to exogenous phenylephrine (by a mean of 28.1 +/- 5.0%). The effect of the same dose of NPY on the pressor response to sciatic nerve stimulation was variable (sometimes inhibition, sometimes potentiation). 3. NPY appears to act by potentiating post-synaptic alpha-adrenoceptor-mediated vasoconstrictor effects. It may also inhibit noradrenaline release by a presynaptic action. Thus the net effect of NPY on sympathetic activation in vivo may depend on the balance between these two opposing actions.     

NEUROPEPTIDE Y IN CORTISOL-INDUCED HYPERTENSION IN MALE VOLUNTEERS

1. Cortisol-induced blood pressure rises in men are not accompanied by increases in plasma catecholamines. The present study examines the effects of cortisol on the sympathetic co-transmitter, neuropeptide Y (NPY). 2. Eight normal men were given cortisol 200 mg/day over 5 days and haemodynamic, metabolic and hormonal measures were taken. Plasma NPY-like immunoreactivity (NPY-LI) concentrations were measured by direct radio-immunoassay. 3. Cortisol significantly increased systolic, diastolic and mean arterial pressure, bodyweight, plasma glucose and total white cell concentration and decreased plasma potassium and total eosinophil count, as in previous studies. Plasma NPY concentrations were not altered significantly during cortisol treatment, but increased following cessation of cortisol treatment (P= 0.006). 4. The essentially unchanged pattern for NPY concentration with cortisol treatment resembles that previously reported for adrenaline and noradrenaline, but the increase in NPY on cortisol withdrawal was not seen for adrenaline or noradrenaline. These data do not support a role for sympathetic activation in the genesis of cortisol-induced hypertension.     

FRUSEMIDE INHIBITION OF SYMPATHETIC VASOCONSTRICTION IN THE RAT IN SITU BLOOD PERFUSED MESENTERY

Mesenteric perfusion pressure was measured in the in situ blood-perfused mesentery of anaesthetized rats. Increases in perfusion pressure were produced by mesenteric periarterial electrical stimulation at 3, 6 and 10 Hz before and after the administration of frusemide 5 mg/kg intravenously (i.v.) or vehicle. Loss of volume due to diuresis was prevented by replacement with intravenous saline. Frusemide did not cause any changes in blood pressure or baseline perfusion pressure. Responses to electrical stimulation were inhibited by frusemide (P less than 0.05) but unchanged by vehicle administration. Acute bilateral nephrectomy or treatment with indomethacin (2 mg/kg i.v.) prevented the inhibitory effect of frusemide on responses to sympathetic nerve stimulation. Responses to sympathetic nerve stimulation were potentiated by an infusion of angiotensin II (12 ng/min) into the mesenteric artery. This infusion did not alter either blood pressure or baseline perfusion pressure. Administration of frusemide 5 mg/kg i.v. attenuated the potentiating effect of angiotensin II on vasoconstrictor responses to electrical nerve stimulation. Frusemide may lead to the release of a prostanoid or prostanoid precursor which inhibits vascular constrictor responses.     

EFFECTS OF HOE 140 ON SYSTEMIC DEPRESSOR RESPONSES TO BRADYKININ AND MESENTERIC VASCULAR REACTIVITY IN VIVO IN PREGNANT WISTAR-KYOTO RATS

1. To examine possible changes in vascular reactivity to exogenous bradykinin (BK) and the possible role of endogenous BK in reduced mesenteric vascular reactivity in pregnant rats. The authors studied the effects of Hoe 140 on systemic depressor responses to BK and on mesenteric vascular reactivity in in situ blood-perfused mesenteric resistance vessels of 18–20 day pregnant and age-matched non-pregnant Wistar-Kyoto rats (WKY). 2. Mean intra-arterial blood pressure (MBP) of pregnant rats was lower than non-pregnant controls. Basal mesenteric perfusion pressure (BPP) was slightly, but not significantly, reduced in the pregnant group. Neither MBP nor BPP was significantly influenced by Hoe 140 (1 mg/kg. s.c.) 3. Systemic depressor responses to BK (1–30 μg/kg, i.v.) were significantly increased in pregnant rats at 1 and 3 μg/kg. Hoe 140 completely abolished systemic depressor responses to BK in either pregnant or non-pregnant animals. 4. Mesenteric vascular responses to regional administration of noradrenaline, electrical stimulation of sympathetic nerve and angiotensin II were overall decreased in pregnant compared with non-pregnant groups, but those responses were not significantly affected by Hoe 140 in either groups. 5. The results suggested that although systemic depressor responses to exogenous BK were increased, endogenous BK does not contribute to decreased mesenteric vascular reactivity in vivo in pregnant WKY.     

INHIBITORY EFFECT OF NEUROPEPTIDE Y ON STIMULATED RENIN SECRETION OF AWAKE RATS

1. (1-36)-NPY is a vasoconstrictor peptide widely distributed in sympathetic nerve terminals. This peptide exerts an inhibitory action on renin release induced by various stimuli. Post-synaptic neuropeptide Y (NPY) receptors show a high affinity for (1-36)-NPY as well as for the agonist (Pro34)-NPY, while presynaptic receptors bind preferentially (13-36)-NPY. 2. This study was undertaken to assess whether the NPY induced renin suppression in awake normotensive rats infused with the beta-adrenoceptor stimulant isoproterenol is mediated by activation of pre- or post-synaptic receptors. 3. Non-pressor doses of (1-36)-NPY and (Pro34)-NPY markedly attenuated the renin secretion triggered by isoproterenol whereas (13-36)-NPY had no effect. This suggests that the effect of NPY on renin release is due to the stimulation of post-synaptic receptors. However it remains unknown whether NPY acts directly on juxtaglomerular cells or indirectly by modifying intraglomerular haemodynamics.     

PLASMA NEUROPEPTIDE Y AND ATRIAL NATRIURETIC PEPTIDE CONCENTRATIONS IN MAN

1. In order to examine the concentration of neuropeptide Y-like immunoreactivity (NPY-LI) and atrial natriuretic peptide (ANP) in the circulation in man, blood was sampled from the iliac vein, the inferior vena cava, the superior vena cava, the pulmonary artery and the femoral artery in 13 patients undergoing cardiac catheterization. 2. Plasma NPY-LI levels were similar at all points sampled and no arteriovenous differences were found. Plasma ANP concentration in the pulmonary artery was greater than in peripheral venous blood but there was a strong correlation between the two. 3. The concentration of NPY-LI and ANP in peripheral venous blood reflects central venous and arterial concentrations.     

AGE-RELATED CHANGES IN NEUROPEPTIDE Y CONTENT IN BRAIN AND PERIPHERAL TISSUES OF SPONTANEOUSLY HYPERTENSIVE RATS

1. This study examined neuropeptide Y (NPY) concentrations in brain regions and peripheral tissues of young (3–4 months) and old (17–18 months) normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). 2. Neuropeptide Y-like immunoreactivity (NPY-LI) was determined in kidney, adrenal, heart ventricles, atria and four brain regions, cerebral cortex, hypothalamus, ventrolateral medulla (VLM) and dorsomedial medulla containing the nucleus tractus solitarius (NTS), by radio-immunoassay following acid extraction. 3. Significant age-related increases in organ weights were observed in atria, ventricle and kidney of both WKY and SHR (P<0.01). In order to take into account tissue hypertrophy, NPY-LI data were analysed as pmol/g tissue as well as total pmol/tissue. 4. At each age, similar NPY-LI concentrations were observed in WKY and SHR in all brain regions. A significant age-induced decrease in NPY-LI concentration and total NPY content was found in the hypothalamus of both WKY and SHR (P<0.01). 5. In the cardiac ventricle, decreases were observed in NPY-LI concentration with ageing, and in SHR relative to WKY; however, no differences were observed in total NPY-LI content. A significant age-related increase in adrenal NPY-LI concentration was observed. No age- or strain-related alterations in atrial or renal NPY-LI were detected, with the exception of an increase in total kidney NPY-LI in WKY with ageing. 6. Thus in the periphery, few changes in NPY-LI were observed with genetic hypertension or with ageing. A significant reduction in hypothalamic NPY-LI concentration occurred with age in both normotensive and hypertensive rats. Thus the previously reported age-related reduction in NPY-LI in the hypothalamus, an area where the peptide influences neuro-endocrine responses and food and water ingestion, is not affected by hypertension.     

NEUROPEPTIDE Y AND CARDIOVASCULAR REGULATION

1. The effects of neuropeptide Y (NPY) and related peptide fragments on blood pressure and vagal action at the heart were compared in the anaesthetized rat. 2. A change in vagal action was taken as a measure of presynaptic activity and a change in blood pressure was taken as a measure of postsynaptic activity. 3. NPY, NPY-(13-36) and a stabilized 13-36 analogue of NPY (ANA NPY) all exerted pressor actions and attenuated vagal action at the heart. 4. On the basis of different potencies demonstrated for the pressor and vagal inhibitory actions of these peptides, the results are consistent with the proposal that there are two populations of NPY receptors.     

ENDOTHELIUM-DEPENDENT INHIBITION OF SYMPATHETIC VASOCONSTRICTION BY FRUSEMIDE ADMINISTRATION TO RATS

1. Segments of the tail artery of the rat were cannulated at both ends and mounted in an organ bath filled with Krebs solution. 2. Using an extracorporeal circuit, blood was pumped at a constant 2 ml/min from the carotid artery of anaesthetized rats to perfuse the segment of tail artery, and returned to the donor rat via the jugular vein. 3. Peri-arterial electrical stimulation of the ex vivo blood perfused tail artery at 5 Hz produced vasoconstriction and an increase in perfusion pressure. 4. The intravenous administration of frusemide 5 mg/kg to the donor rat resulted in an inhibition of the vasoconstrictor responses of the perfused tail artery segment. Diuresis-induced losses of volume and frusemide were prevented by a urinary bladder-venous shunt. 5. Removal of the endothelium from the tail artery segment, by perfusion with dry gas for 4 min, prevented the vasoconstrictor-inhibitory effect of frusemide administration. Removal of the endothelium was confirmed histologically and by the absence of a vasodilator response to acetylcholine. 6. On the basis of these and previous results it is concluded that parenteral frusemide administration releases an unidentified but non-prostanoid hormone from the kidney which produces an endothelium-dependent inhibition of sympathetic vascoconstriction.     

NEUROPEPTIDE Y RADIO-IMMUNOASSAY: CHARACTERIZATION AND APPLICATION

1. A sensitive and specific neuropeptide Y (NPY) radio-immunoassay has been developed. This radio-immunoassay does not detect the NPY-related peptides pancreatic polypeptide or peptide YY. NPY extracted from rat plasma using sequential C18 sorbent and affinity chromatography co-eluted with synthetic rat NPY when applied to high pressure liquid chromatography. 2. The procedure of stabilization of platelets followed by high speed centrifugation reduced basal values of NPY by 60%, and this may be consistent with removal of platelets that release NPY. Administration of the cholinesterase inhibitor physostigmine (0.3 mg/kg), intravenously, produced a small but significant increase (39%) from basal concentrations of NPY. 3. NPY concentrations in young (2-3-month-old) Sprague-Dawley and Fisher 944 rats were similar; however, NPY concentrations were significantly increased (55%) in 2-year-old Fisher 944 rats. Similar to plasma concentrations of noradrenaline, NPY levels increase with age.     

EFFECTS OF NEUROPEPTIDE Y ON BAROREFLEX CONTROL OF HEART RATE AND MYOCARDIAL CONTRACTILITY IN CONSCIOUS RABBITS

1. The effects of intravenous (i.v.) neuropeptide Y (NPY, 10 micrograms/kg bolus) on the stimulus-response curves relating changes in heart period (HP) and in peak left ventricular (LV) dP/dt to acute changes in mean arterial pressure (MAP) were determined in conscious, normotensive rabbits. 2. The relationship between increases and decreases in MAP and the subsequent changes in HP were represented by a sigmoid-shaped curve described by a logistic function. Following NPY administration there was a baroreflex-dependent increase in the maximum slope (sensitivity) at the midpoint of this MAP-HP curve from 7.0 +/- 0.5 to 10.6 +/- 1.3 ms/mmHg (P less than 0.05). NPY caused an upward shift in the whole curve which reflected the NPY-induced bradycardia and was independent of baroreflexes. 3. The relationship between increases in MAP and decreases in peak LV dP/dt was determined during fixed-rate atrial pacing to prevent the effects of the accompanying bradycardia. Increases in MAP and the corresponding reductions in peak LV dP/dt were represented by an exponential function. The slope of the curve, measured at its origin 5-15 min after NPY administration, was reduced from -0.9 +/- 0.2 to -0.4 +/- 0.1 units (P less than 0.05). 4. The effects of NPY are consistent with an action on efferent connections of the arterial baroreceptor reflex, mediated through a reduction in cardiac beta-adrenergic tone. They would also be explained through actions on the afferent or central neural connections of the baroreflex.     

PLASMA NEUROPEPTIDE Y LEVELS RISE IN PATIENTS UNDERGOING EXERCISE TESTS FOR THE INVESTIGATION OF CHEST PAIN

Neuropeptide Y (NPY) is colocalised with noradrenaline in post-ganglionic sympathetic neurons. In order to examine the possibility that activation of the sympathetic nervous system might cause release of NPY into the plasma NPY levels were measured in 16 patients undergoing exercise tests for investigation of chest pain. Plasma NPY concentrations rose in 14 out of the 16 patients, and the mean level of plasma NPY increased from 335 (s.e.m. = 37) to 455 (s.e.m. = 41) pg/ml. Plasma noradrenaline and adrenaline levels increased four- and two-fold respectively. The increase in NPY correlated with the increase in noradrenaline, suggesting that NPY may be released with noradrenaline when sympathetic noradrenergic nerves are activated.     

CARDIOVASCULAR RESPONSIVENESS TO ARGININE VASOPRESSIN IN WATER-LOADED AND SODIUM-DEPLETED PREGNANT AND NON-PREGNANT SHEEP

1. In conscious ewes pregnancy was associated with a significantly increased heart rate and cardiac output, while mean arterial pressure (MAP) and stroke volume were unchanged. 2. The present study examines the effect of arginine vasopressin (AVP) infused at 0.3, 1, 3.0, and 10 micrograms/h, into water-loaded and sodium-depleted ewes, either non-pregnant or during the last third of gestation. 3. In the water-loaded state, MAP rose significantly at the lowest rate of infusion in both pregnant and non-pregnant ewes. Bradycardia occurred first at 0.3 micrograms/h in the pregnant ewes but not until 3.0 micrograms/h in the non-pregnant animals. 4. In sodium deficiency there was no increase in MAP at any rate of infusion in either group. Bradycardia occurred in both groups at 1 microgram/h. 5. This study shows that the pressor effects of AVP are unchanged by pregnancy. However, pregnant ewes are more sensitive to AVP-induced bradycardia when the ewes are water-loaded.     

NEUROPEPTIDE Y INDUCES AND MODULATES VASOCONSTRICTION IN INTRACRANIAL AND PERIPHERAL VESSELS OF ANIMALS AND MAN

• 1 Neuropeptide Y (NPY) has recently been reported to coexist with noradrenaline (NA) in central as well as peripheral noradrenergic nerves. NPY-containing nerve fibres are particularly numerous around blood vessels.
• 2 Studies were performed on isolated pial arteries as well as on arteries and veins from several peripheral vascular beds from rabbit, cat and man. NPY induced a varying degree of direct contraction of the vessels with an E_Am up to 15 mN. Pial arteries were more sensitive than peripheral arteries to NPY (mean EC₅₀ = 7.6 × 10^?9 M). The presence of NPY did not cause any consistent or significant potentiation of the contractile response to NA in any of the vessels tested.
• 3 Transmural electrical stimulation of the perivascular nerves (including blockade with tetrodotoxin) was performed mainly with auricular artery from the rabbit. Blocking experiments confirmed that the neurogenic contraction was mediated by noradrenergic-type fibres. NPY caused a concentration-related potentiation of the neurally evoked contractile response. The peptide also potentiated the tetrodotoxin-resistant probably non-neurogenic contractions obtained during enhanced electrical field stimulation.
• 4 It is concluded that NPY interacts with NA during sympathetic nerve activation primarily through a presynaptic effect.

     

HIGH LEVELS OF EXPRESSION OF NEUROPEPTIDE Y mRNA IN HUMAN PHAEOCHROMOCYTOMAS

1. Neuropeptide Y (NPY) gene expression in human phaeochromocytomas was investigated by measuring the levels of NPY mRNA and NPY-immunoreactivity (NPY-IR) in human phaeochromocytoma tissues in comparison with those in normal human adrenal tissues. 2. The amounts of NPY mRNA and NPY-IR in human phaeochromocytomas were 18 and 93 times higher, respectively, than those in normal adrenal glands. In contrast, beta-actin gene expression was similar in human phaeochromocytomas to that in normal adrenal glands. 3. The amount of NPY mRNA relative to total cellular RNA was 6-fold higher in phaeochromocytoma tissues than in normal human adrenal medulla, suggesting increased NPY gene expression in the tumour cells. 4. Induction of differentiation of PC12 rat phaeochromocytoma cells by compounds, such as dexamethasone and nerve growth factor, resulted in a marked increase in the NPY mRNA level. 5. These findings suggest that NPY gene expression is increased in well-differentiated human phaeochromocytoma cells. Its high level of expression could be responsible for the marked overproduction of NPY by this tumour.     

EFFECTS OF NEUROPEPTIDE Y ON LEFT VENTRICULAR FUNCTION IN THE CONSCIOUS RABBIT

1. Changes in arterial pressure, heart rate and left ventricular contractility induced by intravenous injections of neuropeptide Y (NPY; 1-30 micrograms/kg) were studied in the conscious rabbit. 2. NPY has a brief pressor effect associated with a bradycardia, an increase in left ventricular end diastolic pressure, and a prolonged fall in peak left ventricular dP/dt (LVdP/dt). 3. The haemodynamic changes increase substantially with increasing doses up to 10 micrograms/kg. Beyond 10 micrograms/kg there are only slight effects on heart rate or peak LV dP/dt.     

INHIBITION OF VASOCONSTRICTION BY PLATELET ACTIVATING FACTOR IN THE IN SITU BLOOD PERFUSED RAT MESENTERY

1. Perfusion pressure was measured in the in situ mesentery of anaesthetized rats perfused with blood at a constant 2 mL/min. 2. Increases in perfusion pressure were produced by mesenteric peri-arterial nerve stimulation at 10 Hz for 5 s at 2 min intervals and by bolus intra-arterial injections of the vasoconstrictors noradrenaline, angiotensin II and 5-hydroxytryptamine. 3. The intra-arterial infusion of platelet-activating factor (PAF) to produce a blood concentration of 3 X 10(-10) mol/L inhibited all responses to a similar extent. Intra-arterial prazosin (1-5 X 10(-9) mol/L), however, preferentially reduced responses to nerve stimulation and noradrenaline. 4. PAF at concentrations from 3 X 10(-11) to 10(-9) mol/L produced increasing inhibition of vasoconstrictor responses to nerve stimulation. The dose-response to PAF was shifted to the right by the concomitant intra-arterial infusion of the PAF antagonist SRI 63-441. 5. PAF at very low concentrations in vivo inhibits mesenteric vasoconstriction, produced by sympathetic nerve stimulation or various agonists, by a PAF-receptor mediated vasodilatation of the mesenteric vasculature.     

ENDOTHELIN-1 ENHANCES VASOCONSTRICTOR RESPONSES TO SYMPATHETIC NERVE STIMULATION AND NORADRENALINE IN THE RABBIT EAR ARTERY

1. In rabbit isolated perfused ear arteries denuded of endothelium, a low concentration of endothelin-1 (0.1 nmol/L) that had no direct vasoconstrictor action produced slowly developing enhancements of vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation. The enhancements reached maximal levels after 60 min of exposure to endothelin-1. 2. A higher concentration of endothelin-1 (1 nmol/L), which produced a slow-developing increase in perfusion pressure of 70 mmHg over the course of 1 h, significantly enhanced vasoconstrictor responses to sympathetic nerve stimulation for the first 30 min, after which there was no significant enhancement. Responses to noradrenaline were not enhanced by 1 nmol/L endothelin-1. 3. The enhancing effect of low concentrations of endothelin-1 on vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline may play a physiological role in modulating vasomotor function.     

LACK OF EFFECT OF AGE ON THE CARDIOVASCULAR RESPONSE TO NEUROPEPTIDE Y INJECTION IN THE RAT NUCLEUS TRACTUS SOLITARIUS

1. Neuropeptide Y (NPY) is colocalized with catecholamines in central regions involved in blood pressure regulation and exerts depressor responses in the nucleus tractus solitarius (NTS). Ageing is accompanied by a decline in baroreflex function and a reduction in NPY concentrations in some brain areas. The present study investigated whether the cardiovascular response to NPY microinjection into the NTS and medullary NPY concentrations were conserved in aged rats. 2. Neuropeptide Y (6 pmol in 100 nL) unilaterally injected into the NTS of anaesthetized 3- or 17-month-old male Sprague-Dawley rats produced a prompt 9–10% fall in mean arterial pressure (MAP), which tended to last longer in aged rats. Decreases in heart rate (HR) observed following NPY administration into the NTS were modest but more prolonged than the depressor responses, ANOVA with repeated measures demonstrated no significant effect of age on the MAP or HR response to NPY injection into the NTS. Neuropeptide Y concentrations in the dorsomedial and ventrolateral medulla were not different between the two age groups. 3. Thus, the depressor and bradycardic responses to exogenous NPY administration in the NTS were maintained with age, in keeping with the observation of similar medullary NPY concentrations in adult and aged rats.