The Cardiovascular Effects of Neuropeptide Y (NPY)

NPY (1ng-10/micrograms) had no effect on mean arterial pressure (MAP) or heart rate (HR) when administered into the lateral brain ventricle or cisterna magna of conscious, chronically instrumented normotensive, male Wistar rats. However upon intravenous injection (0.1-5/micrograms) it produced a dose related pressor response accompanied by bradycardia. This pressor effect was not influenced by prazosin infusion (100 micrograms/kg/min). These results suggest that NPY has no direct role in central cardiovascular regulation, but peripherally is a very potent vasoconstrictor peptide.

Elements of cerebral microvascular ischaemia

Although neuronal cells have long been thought to be the prime target of ischaemic insults, events which occur at the blood-vascular-parenchymal interface are necessary for the initiation of ischaemic tissue injury. This cascade of microvascular events includes fibrin accumulation, endothelium expression of leukocyte adhesion receptors, breakdown of the basal laminae with loss of astrocyte and endothelial cell contacts leading to blood-brain barrier disruption and consequently oedema formation and haemorrhagic transformation. Potential stroke treatments have been studied in the clinic and many have not been particularly successful, probably due to the delicate balance between improved outcome and adverse reactions as well as the window of opportunity for drug treatment after symptom onset. The only acute intervention trial demonstrating any benefit in patients was that of intravenous tissue plasminogen activator (tPA), administered within 3 h of the onset of symptoms of ischaemic stroke. Such treatment improved clinical outcome at 3 months, although there was an increased incidence of symptomatic haemorrhage [New Engl. J. Med. 333 (1995) 1581]. The recent progress made in defining the mechanisms involved in the initiation of ischaemic events, as described in this review, may lead to the identification of new strategies for intervention in the ischaemic cascade.

White matter ischaemia

Brain and spinal cord white matter are vulnerable to the effects of ischaemia. Reduction of the energy supply leads to a cascade of events including depolarization, influx of Na(+) and the subsequent reverse operation of the membrane protein the Na(+)/Ca(2+) exchanger which ultimately terminates in intracellular Ca(2+) overload and irreversible axonal injury. Various points along the white matter damage cascade could be specifically targeted as a potential means of inhibiting the development of axonal irreversible injury.

The selectivity of MDL 74,721 in models of neurogenic versus vascular components of migraine

MDL 74,721 (R)-2-(N1,N1-dipropylamino)-8-methylaminosulfonylmethyl-1,2,3,4-te trahydronaphthalene, a sulfonamidotetralin, has been found to exhibit a 10,000-fold greater potency in neurogenic versus vascular models of migraine. Sumatriptan, a relatively pure 5-HT1D/5-HT1B receptor agonist, also showed higher potency versus neurogenic inflammation. However, for sumatriptan the potency difference (100-fold) in the two pathophysiological models was less pronounced than seen for MDL 74,721. The affinity profile of MDL 74,721 at 5-HT1 receptor subtypes may in part explain its ability to differentiate these two physiological responses. MDL 74,721 demonstrated nanomolar affinity for 5-HT1A (12.7 +/- 0.3 nM) and 5-HT1D (41.3 +/- 10.9 nM) but considerably lower affinity for 5-HT1B receptors (> 1000 nM). Serotonin-like activity was seen in in vitro functional assays including inhibition of forskolin-stimulated cAMP accumulation in human 5-HT1D receptor-transfected fibroblasts or eliciting vasoconstriction in isolated human pial arteries. The intrinsic activity (relative to 5 - HT[E(Amax)]) and affinity (pD2) for the human cerebrovascular 5-HT receptors were: 5-HT (100%, 7.51 +/- 0.09), sumatriptan (94%, 6.85 +/- 0.1) and MDL 74,721 (66%, 5.70 +/- 0.23). In anaesthetised cats, treatment with MDL 74,721 resulted in a dose-related reduction in the percentage of carotid flow going through the arteriovenous anastomoses to the lungs, with an ED50 of 0.3 mg/kg i.v., the same as sumatriptan. However, in the guinea-pig neurogenic model, MDL 74,721 inhibited plasma protein extravasation with an ED50 of 0.023 microg/kg compared to 2.5 microg/kg for sumatriptan. MDL 74,721 was also effective in this model (in rats) after oral administration. In conclusion, MDL 74,721 demonstrates a preclinical profile consistent with anti-migraine efficacy. Its marked preference for inhibiting neurogenic inflammation makes this compound a useful tool for assessing the relative contribution of this pathophysiological mechanism to the human disease state.

Preclinical characterization of MDL 27,192 as a potential broad spectrum anticonvulsant agent with neuroprotective properties

The compound 5-(4-chlorophenyl)-2,4-dihydro-4-ethyl-3H-1,2,4-triazol-3-one (MDL 27,192) was evaluated in a variety of rodent models to assess its anticonvulsant profile and its potential neuroprotective activity. MDL 27,192 demonstrated anticonvulsant activity in a wide range of epilepsy models that are genetically-based (audiogenic seizures in the seizure susceptible DBA/2J or Frings mouse; spike wave seizures in genetic absence epilepsy rats of Strasbourg (GAERS), electrically-based (MES seizures in mice and rats, corneally-kindled seizures in rats) and chemically-based (bicuculline, PTZ, picrotoxin, 3-mercaptopropionic acid, quinolinic acid and strychnine). When compared to valproate, orally administered MDL 27,192 was 17-48-fold more potent as an anticonvulsant and showed a safety index one to three-fold greater. Following a timed intravenous administration of PTZ to mice, MDL 27,192, but not phenytoin or carbamazepine, consistently increased the latencies to first twitch and clonus. MDL 27,192 was active in a genetic model of absence epilepsy, the GAERS rat model. These data indicate that MDL 27,192 likely exerts its anticonvulsant action by affecting seizure spread and by raising seizure threshold. MDL 27,192 did not display any signs of tolerance following subchronic (15 day) administration. In tests of neuroprotective potential, MDL 27,192 reduced infarct volume in a permanent middle cerebral artery occlusion model of focal cerebral ischemia in rats and reduced the loss of hippocampal dentate hilar neurons in an animal model of unilateral head injury. In summary, MDL 27,192 possesses a broad-spectrum anticonvulsant profile. The potential for reduced tolerance and neuroprotective activity are additional positive features of MDL 27,192's preclinical profile.

A method using a liquid chromatographic-electrospray-mass spectrometric assay for the determination of antimigraine compounds: preliminary pharmacokinetics of MDL 74,721, sumatriptan and naratriptan, in rabbit

MDL 74,721 (I), sumatriptan(II) and naratriptan(III) are new 5-HT1-like agonists that have potential as a novel treatment for migraine. Liquid chromatographic-electrospray-mass spectrometric (LC-ESI-MS) assay have been developed to compare the pharmacokinetics of these three antimigraine compounds. The concentration of each parent drug was determined using a solid-phase extraction method and LC-ESI-MS analysis demonstrating the high sensitivity and specificity of the methods down to subnanogram levels in rabbit plasma samples. Pharmacokinetic parameters evaluated after administration of single intravenous and oral doses were very similar and the ANOVA analysis did not show any statistically significant differences for t1/2, Cmax, V or AUC (normalised). The pharmacokinetic parameters showed short t1/2 (range 1.14-1.9 h) either after intravenous (i.v.) or oral (p.o.) administration and high total body clearance (CL) after the p.o. dose both probably due to extensive and rapid metabolism of the parent drugs as suggested by the low values for bioavailability (range 13.4-22.8%).

Reduction of traumatic brain injury-induced cerebral oedema by a free radical scavenger

Oxygen derived free radicals have been proposed to be in part responsible for the cerebral oedema resulting from head injury. In the present study the effects of free radical suppression with MDL 74,180 (2,3-dihydro-2,2,4,6,7-pentamethyl-3-(4-methylpiperazino)-methyl-1 - benzofuran-5-ol dihydrochloride), an alpha-tocopherol analogue free radical scavenger, on the development of cerebral oedema resulting from head injury has been assessed. Fluid percussion head injury in rats caused a regional oedema 48 h after injury. Infusion of MDL 74,180 for 2 h after the injury significantly attenuated oedema development in a dose-related manner. Using magnetic resonance imaging, cerebral oedema development was monitored in head injured mice. Oedema was apparent 4 h after head injury and was greatest in the vicinity of the olfactory bulb and surrounding the ventricles. Treatment with MDL 74,180 (1-10 micrograms/kg intravenously, administered 3-5 min after the injury) significantly reduced the oedema development. MDL 74,180 is a potential treatment for the oedema caused as a result of head injury.

MDL 74,180 reduces cerebral infarction and free radical concentrations in rats subjected to ischaemia and reperfusion

The protective effect of MDL 74,180 (2,3-dihydro-2,2,4,6, 7-pentamethyl-3-(4-methylpiperazino)-methyl-1-benzofuran-5-ol dihydrochloride) and alpha-tocopherol analogue free radical scavenger, against cerebral ischaemia and reperfusion in conscious rats has been demonstrated. Tissue damage following middle cerebral artery occlusion (2 h) and reperfusion (8 days) was decreased by MDL 74,180 (0.1 and 1.0 mg/kg per h) infusion beginning 15 min before the onset of reperfusion and continuing for 2 h into the reperfusion period, in a dose-related manner. Nitroxide radical adducts, characterized and quantified by electron spin resonance spectroscopy, were formed on the addition of spin traps to homogenized rat brain tissue previously subjected to global ischaemia and reperfusion. The primary oxidative chain free radicals form diamagnetic intermediates whose slow homolytic decomposition subsequently yields the observed stable spin adducts. Infusion of MDL 74,180 (1-10 mg/kg per h) beginning 15 min before the induction of global cerebral ischaemia (20 min) until the end of reperfusion (5 min), led to a dose-dependent reduction in the final concentration of spin adducts.

Receptor density in the rat heart following ischaemia and prolonged reperfusion

1. The free radical scavenger MDL 73,404 decreases infarct size and improves heart performance after myocardial ischaemia and prolonged reperfusion. In the present study the possibility that changes in receptor density might contribute to the MDL 73,404-induced increase in contractility has been investigated in rats subjected to myocardial ischaemia and 8 days of reperfusion. 2. Both in saline- and MDL 73,404-treated rats a significant decrease in beta1- and increase in beta2-adrenoceptor densities was apparent in the infarcted tissue. 3. In non-infarcted septum and right ventricle tissue ischaemia followed by reperfusion tended to increase the density of beta2-adrenoceptors which was significantly increased after MDL 73,404 treatment. 4. The increased density of beta2-receptors in the non-infarcted tissue may contribute to the MDL 73,404 induced improvement of cardiac performance.

2,3-Dihydro-1-benzofuran-5-ols as analogues of alpha-tocopherol that inhibit in vitro and ex vivo lipid autoxidation and protect mice against central nervous system trauma

A series of alpha-tocopherol analogues was synthesized with potential therapeutic value for such pathological conditions as stroke and trauma. A set of criteria such as the inhibition of in vitro lipid peroxidation, superoxyl radical scavenging, and brain penetration, as measured by ex vivo inhibition of lipid peroxidation, was applied to select the most effective compound. 2,3-Dihydro-2,2,4,6,7-pentamethyl-3-[(4-methylpiperazino)methyl]-1 - benzofuran-5-ol dihydrochloride (22) was selected because of its superior antioxidant properties and better brain penetration. This compound also protected mice against the effects of head injury. The criteria thus turned out to be useful for the characterization of a neuroprotective analogue of alpha-tocopherol.

Myocardial protection by a cardioselective free radical scavenger

Oxygen-derived free radicals are involved in myocardial reperfusion injury. In the present study MDL 74,405 (S-(-)-3,4-dihydro-6-hydroxy-N,N,N-2,5,7,8-heptamethyl-2H-1-benzo pyran-2-ethanaminium 4-methylbenzenesulfonate), a hydrophilic derivative of alpha-tocopherol, has been shown to inhibit lipid peroxidation in rat brain homogenate, ex vivo lipid peroxidation in mouse heart and to accumulate in myocardial tissue. Infused i.v. MDL 74,405 induced a dose-related reduction of myocardial infarct size in pentobarbitone-anaesthetised rats subjected to 60 min coronary artery ligation followed by 30 min reperfusion. Similarly i.v. infusion of MDL 74,405 beginning 10 min before coronary artery occlusion (60 min) until 30 min after the onset of reperfusion (8 days) caused a decrease of infarct size associated with an increase in aortic flow. Plasma levels of creatine phosphokinase were significantly reduced. In isolated infarcted hearts, obtained from MDL 74,405-treated rats after 8 days of reperfusion and perfused according to the Langendorff technique, an increase in the contractility index (+) and (-) dP/dtmax was apparent. In isolated non-infarcted rat hearts subjected to 30 min no-flow global ischaemia, perfusion with MDL 74,405 resulted in an increase in heart rate and the contractility indices (+) dP/dtmax, and left ventricular systolic pressure during reperfusion. In conclusion MDL 74,405, is a cardioselective free radical scavenger, that reduces myocardial infarct size and attenuates post-ischaemic dysfunction.

Protection of infarcted, chronically reperfused hearts by an alpha-tocopherol analogue

Free radicals may cause part of the irreversible injury which occurs during myocardial infarction and reperfusion. In the present study MDL 73404, a hydrophilic, cardioselective, free radical scavenger analogue of alpha-tocopherol, was evaluated for its effects on infarct size as well as on indicators of reperfusion injury. A pentobarbitone-anaesthetised rat model of coronary artery ligation (60 min; followed by 8 days of reperfusion) was used. Intravenous infusion of MDL 73404 (3 mg/kg per h) began 10 min before occlusion until 30 min after the onset of reperfusion. MDL 73404 reduced (P < 0.02) the elevated serum levels of thiobarbituric acid reactive substances and plasma levels of creatine phosphokinase (P < 0.01). An increase in cardiac output and heart rate together with a decrease (P < 0.01) in infarct size was evident in rats that had received MDL 73404, 8 days previously. Isolated infarcted hearts obtained from rats after 8 days of reperfusion had greater (P < 0.01) + dP/dt max, -dP/dt max, left ventricular systolic pressure and coronary flow after MDL 73404 compared to saline-treated controls. Infusion of [14C]MDL 73404, during the time of occlusion resulted in a concentration of 14.5 +/- 2.2 mg eq/g in the non-ischaemic ventricular tissue and a concentration of 3.0 +/- 0.4 mg eq/g in the area at risk. After infusion for the 30 min of reperfusion, 6.4 +/- 0.2 mg eq/g was detected in the non-ischaemic ventricular tissue but only 3.1 +/- 0.5 mg eq/g in the area at risk.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effects of an alpha-tocopherol analogue against myocardial reperfusion injury in rats

Free radicals may cause part of the irreversible injury which occurs during myocardial infarction and reperfusion. In the present study MDL 73404, an alpha-tocopherol analogue which is a free radical scavenger has been evaluated for its effects on infarct size in an anaesthetised rat model of coronary artery ligation (60 min) and reperfusion (30 min). Intravenous infusion of the compound began 10 min before occlusion until the end of reperfusion. MDL 73404 (0.3-3 mg/kg per h) reduced infarct size, although not in a dose-related manner. Infusion of MDL 73404 (3 mg/kg per h) commencing 30 min before reperfusion until the end of reperfusion also induced a significant reduction in infarct size. In the isolated rat heart (Langendorff technique) subjected to 30 min no-flow global ischaemia, pretreatment with MDL 73404 (0.03 and 0.1 mM) in the perfusion buffer and during 30 min of reperfusion resulted in a significant increase in the maximal pressure development (+dP/dt max) and relaxation (-dP/dt max), left ventricular systolic pressure and heart rate during reperfusion, whereas left ventricular diastolic pressure was significantly reduced. In contrast, only one control heart out of five exhibited signs of recovery. Replacement, for 2 min, with a cardioplegic solution before the 30 min period of ischemia resulted in an increased heart rate and contractility during reperfusion compared to hearts that did not receive the cardioplegic solution. The presence of MDL 73404 (0.03 and 0.1 mM) in the perfusion fluid induced an additional increase in left ventricular systolic pressure to the pre-ischaemic levels. MDL 73404 may have potential for cardioprotective use in acute reperfusion of the myocardium following ischaemia.

Cardioselectivity of alpha-tocopherol analogues, with free radical scavenger activity, in the rat

MDL74270 (6-acetyloxy-3,4-dihydro-N,N,N,2,5,7, 8-heptamethyl-2H-1-benzopyran-2-ethanaminium, 4-methylbenzenesulfonate) is a quaternary amine analogue of alpha-tocopherol with free radical scavenger properties. Rats were injected iv with [14C]MDL74270 (0.91 mg/kg), and whole blood and heart tissue were sampled. Five min after drug, the heart tissue/blood ratio (T/B) of radioactivity was 3.5, whereas at 1 hr it was 20.1 and remained at this value up to at least 6 hr. After iv administration the t 1/2 of radioactivity in blood was 6.3 hr, but po blood levels could not be quantified. The 0- to 96-hr urinary elimination of radioactivity was 39.9 +/- 5.7% of the dose after iv and only 1.2 +/- 0.4% after po administration, conversely, 44.7 +/- 5.2% was excreted in feces after iv and 79.1 +/- 17.4% after po administration. These results confirmed poor oral absorption of the compound. Tissue distribution of [14C]MDL74270 was compared with that of its tertiary amine analogue [14C]MDL74366 in rat heart, skeletal muscle, brain, and whole blood, after iv administration (1 mg/kg). The heart T/B was above 20, 1-6 hr after [14C]MDL74270, whereas it was less than 2 after [14C]MDL74366. Over the 1- to 6-hr time interval, skeletal muscle T/B varied from 1.8 to 5 compared with 1.5 to 0.6 for [14C] MDL74366. Brain T/B was higher after the tertiary amine compound. Results showed marked cardioselectivity of radioactivity after [14C] MDL74270. Differential centrifugation of heart homogenates showed that radioactivity was equally distributed between the major subcellular fractions studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a cardioselective alpha-tocopherol analogue on reperfusion injury in rats induced by myocardial ischaemia

Free radicals may cause some of the irreversible injury which occurs during myocardial ischaemia and reperfusion. In the present study the effects of a cardioselective, free radical scavenger, MDL 74270, which is an analogue of alpha-tocopherol, on myocardial infarct size in an anaesthetised rat model of coronary artery ligation (60 min) and reperfusion (30 min) has been evaluated. Infusion of MDL 74270 (0.3-3.0 mg/kg per h) commencing 10 min before occlusion until the end of reperfusion significantly reduced infarct size. The highest dose also caused a significant reduction in serum creatine phosphokinase levels. Similar findings have been obtained with the bromide salt of MDL 74270. Tissue distribution studies with 14C-labelled MDL 74270 and its tertiary amine analogue (MDL 74366) showed heart/blood ratios of total radioactivity, 1-6 h after i.v. administration, greater than 20 after MDL 74270 and around 1 after MDL 74366. The importance of accumulation of total radioactivity in the heart after MDL 74270 is supported by the fact that MDL 74366 was 30 times less potent as a myocardial protector in the ligation/reperfusion studies. It is concluded that MDL 74270 has potential for cardioprotective use in conditions of acute reperfusion.

A cardioselective, hydrophilic N,N,N-trimethylethanaminium alpha-tocopherol analogue that reduces myocardial infarct size

The alpha-tocopherol analogue 3,4-dihydro-6-hydroxy-N,N,N,2,5,7,8- heptamethyl-2H-1-benzopyran-2-ethanaminium 4-methylbenzenesulfonate (1a, MDL 73404) and its O-acetate 1b (MDL 74270) were synthesized. Compound 1a was found to be hydrophilic (log P = -0.60) and to prevent lipid autoxidation in rat brain homogenate with an IC50 of 1.7 +/- 0.9 microM. Tissue distribution studies with [14C]-1b in rats (1 mg/kg iv) showed that radioactivity accumulates in the heart (ratio 20:1 vs blood after 1 h). Infusion of 1 mg/kg per h of 1b bromide reduced infarct size by 54% in rats subjected to coronary artery occlusion for 60 min followed by reperfusion for 30 min, compared to saline-infused controls. By comparison, the tertiary amine analogue 5 was found not to accumulate in heart tissue, to be an equally effective free-radical scavenger in vitro, but to require a higher dose to reduce infarct size in rats. This shows that the cardioselectivity of compound 1 contributes to its potency in salvaging myocardial tissue in rats after ischemia and reperfusion.

The effects of taurine on atherosclerosis development in cholesterol-fed rabbits

The effect of taurine on the development of atherosclerotic lesions in rabbits maintained on a 2% cholesterol diet for a 14 week period has been examined. Taurine (0.2 and 0.5%) administered in the drinking water reduced thoracic aorta involvement. The area covered by atherosclerotic lesions was 58 +/- 15 and 52.5 +/- 12% (P less than 0.05) respectively, compared to 72.4 +/- 19% in the control group. Taurine had no significant effect on serum or tissue cholesterol, calcium, triglyceride or phospholipid concentrations. Nevertheless 0.2% taurine caused an increase in dP/dtmax (measured from the systemic blood pressure) and 0.5% lowered systemic blood pressure. The anti-atherosclerotic effects of taurine appear to be unrelated to a fall in blood pressure. The possibility that taurine is reducing the development of atherosclerotic lesions through a mechanism involving its antioxidant activity is discussed.

The cardiovascular effects of centrally administered taurine in anaesthetised and conscious rats

The effects of pentobarbitone anaesthesia on the cardiovascular changes induced by centrally administered taurine have been investigated in spontaneously hypertensive (SHR) and normotensive rats. Administration of taurine (100-400 micrograms) into the lateral cerebral ventricle (i.c.v.) of anaesthetised SHR and normotensive rats induced a dose-related fall in systemic blood pressure and heart rate, which tended to be of a greater magnitude in the SHR. In anaesthetised rats attached to a ventilator, taurine was not as potent at inducing a fall in systemic blood pressure, approximately double the dose being required to produce the same cardiovascular changes as that which occurred in anaesthetised rats without a ventilator. In conscious normotensive rats taurine had no effect on blood pressure until a dose of 800 micrograms was administered i.c.v., whereas in conscious SHR, a small, but significant depressor response was evident with 400 micrograms. These findings demonstrate that pentobarbitone anaesthesia sensitises the rats to the cardiovascular effects of taurine, partially through a mechanism which involves respiratory depression. Conversely in conscious rats a much higher dose of taurine is required to induce a fall in arterial pressure and heart rate per se.

Comparison between the cardiovascular effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and clonidine in the conscious sino-aortic denervated rat

1. The purpose of this study was to investigate the role of an intact baroreceptor reflex mechanism in the expression of the cardiovascular response to 8-OH-DPAT and to determine whether there are any differences between the activation of central alpha 2-adrenoreceptors and 5-HT1A receptors in this respect. To this end, the effects of 8-OH-DPAT and clonidine have been assessed on blood pressure, heart rate, ECG and cardiac contractility indices in conscious sino-aortic baroreceptor denervated (SAD) rats and their sham-operated controls. 2. In both sham-operated and SAD rats, intravenous (i.v.) administration of 8-OH-DPAT (32 micrograms kg-1) and clonidine (8 micrograms kg-1) produced falls in systemic blood pressure, left ventricular systolic pressure and dP/dtmax. 3. 8-OH-DPAT produced similar bradycardia in each group of rats; in contrast, clonidine had a greater effect in the SAD animals. Increases of the PQ interval mirrored the heart-rate changes with both compounds. 4. No significant changes in end diastolic blood pressure or in the myocardial contractility indices dP/dtmax/P and Vmax were evident. 5. This study provides support for the view that i.v. 8-OH-DPAT lowers blood pressure and heart rate through a central mechanism. The effects occur independently of an intact baroreceptor reflex and are not associated with effects on myocardial contractility. 8-OH-DPAT shows close qualitative similarities to clonidine in this model.

Comparison of the cardiovascular effects of trans-diclofurime with different types of calcium antagonists in conscious spontaneously hypertensive rats

1. Trans-diclofurime has been shown to be a potent group II calcium antagonist in in vitro and in vivo test systems. In contrast to the dihydropyridines, group II calcium antagonists have a reduced propensity to cause reflex tachycardia due to well-balanced inhibitory effects in smooth muscle and heart. Since effects on autonomic reflexes are more reliably assessed in conscious animals, the cardiovascular effects of trans-diclofurime have been examined and compared to those of nifedipine, verapamil and diltiazem in the conscious spontaneously hypertensive rat (SHR). 2. Each SHR had an indwelling catheter in the femoral artery to record mean arterial pressure (MAP) and heart rate (HR) and a cannula in the femoral vein for drug infusion over 1 min. 3. Nifedipine (0.1-3.0 mumol kg-1 i.v.) caused dose-related falls in MAP accompanied by dose-related increases in HR. Trans-diclofurime and verapamil (0.3-3.0 mumol kg-1 i.v.) also caused dose-related decreases in MAP, but significant tachycardia was only seen at 1.0 and 3.0 mumol kg-1. Trans-diclofurime (0.3 mumol kg-1) induced a significant fall in HR. Diltiazem (1.0-10.0 mumol kg-1 i.v.) induced dose-related falls in MAP, significant bradycardia was evident with 1.0 mumol kg-1 and tachycardia with 10 mumol kg-1. Trans-diclofurime and diltiazem induced less tachycardia than nifedipine and verapamil for equivalent falls in MAP. 4. These results suggest that trans-diclofurime is a potent antihypertensive agent in conscious SHR and, like diltiazem, the hypotensive effects are associated with less tachycardia than is usually apparent with calcium antagonists such as nifedipine or verapamil. S. The cardiovascular effects of trans-diclofurime in conscious SHR are those expected of a class II calcium antagonist and are consistent with its proposed mode of interaction with the diltiazem site in the calcium channel.

Effects of lead exposure on rat brain catecholaminergic neurochemistry

1. The effects of lead on catecholaminergic neurotransmission have been investigated. 2. Using the rat as a model, animals were exposed both acutely and chronically to lead. The levels of catecholamines, noradrenaline, adrenaline, and dopamine along with the activities of tyrosine hydroxylase and phenylethanolamine-N-methyl transferase were measured in 5 brain regions--cerebral cortex, brainstem, hippocampus, anterior and posterior hypothalamus. 3. A lead related reduction in the activity of tyrosine hydroxylase was observed in association with alterations in steady-state levels of the catecholamines in the posterior and anterior hypothalamus. 4. Thus, lead exposure, known to result in behavioural changes, is associated with localised neurochemical effects on the hypothalamus.

Influence of lead exposure on catecholamine metabolism in discrete rat brain nuclei

1. Using the rat exposed both acutely and chronically to lead as a model of lead neurotoxicity, various parameters of catecholamine metabolism were investigated. 2. The steady-state concentrations of noradrenaline, adrenaline and dopamine together with the activities of tyrosine hydroxylase and phenylethanolamine N-methyl transferase were measured in discrete brain nuclei--periventricular, paraventricular, median eminence, posterior and anterior hypothalamus, caudate putamen and globus pallidus. 3. Lead exposure resulted in significant fall in the activity of the rate-limiting enzyme of catecholamine synthesis, tyrosine hydroxylase which was associated with alterations in concentrations of catecholamines in the median eminence, periventricular nucleus and anterior hypothalamus. 4. No other brain nuclei investigated exhibited any effect of lead on the catecholaminergic nervous system and, therefore, the effect of lead on rat brain can be considered to be regionally specific.

Comparison between the effects of MDL 72567 and nifedipine on various cardiovascular parameters in conscious sinoaortic-denervated rats and sham-operated controls

In this study the effects of a new calcium entry blocking agent, 2,6-dimethyl-3-methoxycarbonyl-4-(2-nitrophenyl)-5-(2-furoyl)-1, 4-dihydropyridine (MDL 72567), were compared with those of nifedipine on blood pressure, heart rate, ECG, and cardiac contractility indices in conscious sinoaortic baroreceptor-denervated (SA-denervated) rats and their sham-operated controls. In sham-operated rats, the calcium-entry blocking agents (0.1-2 mg/kg i.v.) produced equivalent falls in blood pressure. However, nifedipine caused a much greater reflex tachycardia which was accompanied by a negative inotropic effect, and with the highest dose (2 mg/kg), a prolongation of the PQ interval. MDL 72567 induced an increase in myocardial contractility. In SA-denervated rats, both drugs produced an enhanced fall in blood pressure accompanied by a negative inotropic effect. Nifedipine did not change heart rate in SA-denervated rats, whereas MDL 72567 caused bradycardia. Thus, in these experiments, MDL 72567 caused less reflex tachycardia for a given fall in blood pressure than nifedipine and was less likely to cause myocardial depression. These effects of MDL 72567 may represent valuable, clinically relevant advantages over nifedipine.

MDL 72567, a dihydropyridine calcium-antagonist, that causes vasodilation and direct sinus bradycardia

MDL 72567 (2,6 dimethyl,3 methoxycarbonyl,4-(2-nitrophenyl), 5-(2-furoyl)1,4 dihydropyridine) was a potent antagonist of Ca2+-induced contractions in K+-depolarized taenia preparations from the guinea pig caecum (pA2 8.8 +/- 0.1). MDL 72567 was a potent displacer of [3H]nitrendipine binding from rat cortical membrane preparations (Ki 3.99 nM), indicating an effect at the dihydropyridine binding site, which is consistent with the finding that the inhibitory effects of MDL 72567 in smooth muscle were prevented by the dihydropyridine Ca2+ channel activator Bay K 8644. MDL 72567 slowed spontaneously beating rat atria preparations to a greater extent than did nifedipine, however, for a given negative inotropic effect. Furthermore, in pithed rat preparations infused with angiotensin II to elevate blood pressure, the hypotensive effects of MDL 72567 (3 nmol/kg-3 mumol/kg, intravenously, i.v.) were accompanied by bradycardia, whereas nifedipine, PY 108-068, and nicardipine lowered blood pressure without affecting heart rate. When compared with nifedipine, MDL 72567 caused less reflex tachycardia for a given fall in blood pressure, in anesthetized beagles and in conscious renal hypertensive dogs. In anesthetized dogs, MDL 72567 increased cardiac contractility at all hypotensive doses tested (30-3,000 nmol/kg, i.v.), whereas nifedipine caused profound myocardial depression at higher doses (1,000-3,000 nmol/kg, i.v.) even though the compounds had equivalent vasodilator effects. Thus, although MDL 72567 appears to cause a direct myocardial slowing that can partially offset reflex tachycardia, the compound has negligible negative inotropic effects and may therefore be useful in angina pectoris or even in congestive heart failure.

The cardiovascular effects of oxytocin in conscious male rats

In conscious, chronically instrumented normotensive male Wistar rats intravenous (i.v.) administration of oxytocin (OXT) (greater than or equal to 100 ng) induced a dose-related biphasic change in mean arterial pressure (MAP). This consisted of an initial pressor effect accompanied by bradycardia and a decrease in cardiac output (CO), followed by a more prolonged fall in MAP which reached a maximum 30 min after injection and was accompanied by an increase in CO. The more specific (Thr4,Gly7]OXT analogue (0.01-10 micrograms i.v.) caused a dose-related fall in MAP and a rise in CO which reached a maximum after 15-30 min. Similarly in spontaneously hypertensive rats of the stroke prone strain (SHRSP) an initial pressor effect and delayed fall in MAP were apparent after OXT (0.1 and 10 micrograms i.v.) only the decrease in MAP being evident with the [Thr4,Gly7]OXT analogue. These responses were significantly larger than those observed in Wistar Kyoto controls. The pressor effects are therefore interpreted to be due to vasopressin receptor activation while the depressor effects appear to be oxytocin specific. In sinoaortic denervated rats, OXT (0.1 and 10 micrograms i.v.) induced an enhanced initial pressor effect with a much reduced reflex bradycardia and fall in CO. A larger and more prolonged delayed fall in MAP was apparent with both OXT and [Thr4,Gly7]OXT accompanied by a decrease in CO when compared to sham-operated controls. Intracisternally (i.c.) administered OXT (0.05-10 ng) had no effect on MAP or heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Endorphins and the hypotensive response to stimulation of alpha-receptors in the brainstem by alpha-methylnoradrenaline

Opioid peptide involvement in the fall in blood pressure resulting from stimulation of alpha-receptors in the brainstem has been investigated in the urethane-anaesthetised rat. Unilateral microinjection of alpha-methylnoradrenaline into the nucleus tractus solitarii (NTS) induced a dose-related fall in blood pressure and heart rate. The depressor response induced by the amine was prevented by pretreatment with naloxone, administered either subcutaneously or directly into the nucleus. Pretreatment with antiserum to beta-endorphin, applied locally, also blocked the depressor response, however a similar dilution of antiserum to met-enkephalin was ineffective in this respect. The local application of phentolamine into the n. tractus solitarii caused an initial fall in both blood pressure and heart rate, and blocked the cardiovascular changes induced by alpha-methylnoradrenaline for at least 90 min. Pretreatment with the alpha-receptor antagonist attenuated the fall in blood pressure produced by microinjection of beta-endorphin. These results suggest that the fall in blood pressure observed after administration of alpha-methylnoradrenaline involves a beta-endorphin-like peptide, a probable site of this interaction being the n. tractus solitarii.

Opioid peptides and central control of blood pressure

The role of endogenous opioids in central modulation of baroreceptor reflex function has been assessed in rabbits and in man using stable enkephalin analogues and synthetic opiates with a range of mu, delta and kappa opiate receptor agonist specificity. In addition the effects of naloxone, a mu opiate receptor antagonist, have been studied. In rabbits descarboxy-leu-enkephalin (RX783016) given by intracisternal injection reduced baroreflex sensitivity as assessed by heart rate responses to phenylephrine, sodium nitroprusside and controlled haemorrhage. These effects were prevented by intravenous naloxone. Naloxone alone increased the slope of the heart period: mean arterial pressure relationship and thus increased baroreflex sensitivity. In conscious man essentially similar results were found following intravenous dosing with a stable met-enkephalin analogue (DAMME, FK33824) or naloxone with decreases and increases respectively in the sensitivity of baroreflex responses to sodium nitroprusside. In rabbits and man arterial baroreceptor reflexes mediating heart rate responses can be pharmacologically modified by exogenous opiates and may be under some tonic endogenous opiate peptide influences.

Differential effects of vasopressin (AVP) and oxytocin (OXT) on the baroreceptor reflex in conscious rats

In conscious chronically instrumented, male Wistar rats the intracisternal (ic) administration of AVP (10ng) caused an increase in baroreflex sensitivity in response to both a rise and fall in blood pressure induced by intravenous phenylephrine (1/ug) and sodium nitroprusside (13/ug) respectively. Conversely a synthetic OXT analogue (Thr4 - Gly7) OXT (10ng) injected ic depressed baroreceptor reflex function. It appears that AVP and OXT are involved in the central connections of the baroreceptor reflex arc, where they may act as physiological antagonists.

Enkephalins induce a centrally mediated rise in blood pressure in rats

The cardiovascular effects of enkephalins after administration directly into the nucleus tractus solitarii (NTS) of urethane anesthetized rats were investigated. Unilateral microinjection of Met-enkephalin and its stable analogue D-Ala2-Met5-enkephalin resulted in a dose-related rise in mean arterial pressure which in the case of the analogue was accompanied by tachycardia. The elevation in blood pressure was anatomically specific and restricted to the intermediate third of the NTS, as verified histologically. These cardiovascular changes were prevented by pretreatment with locally applied naloxone (10 ng). A similar pressor effect was obtained with Leu-enkephalin. Antiserum to Met-enkephalin (1:50 dilution) caused a fall in blood pressure on injection into the NTS, and completely blocked the pressor response and tachycardia induced by D-Ala2-Met5-enkephalin. These results suggest that enkephalins have a pressor role in the central nervous system.

Role of opioid peptides in brain mechanisms regulating blood pressure

Beta-endorphin and related opioid peptides are neuropeptides which appear to play a role in cardiovascular regulation which is supported by altered nociceptive responsiveness in hypertensive animals. In spontaneously hypertensive rats the pain threshold for electric stimulation is elevated; these rats show increased response latency time in a hot plate test. The opiate antagonist naloxone reverses these values to that of the normotensive controls. In other forms of experimental hypertension, eg, renal hypertension (one-clip, two-kidney model), no change in pain sensitivity is apparent. Sinoaortic baroreceptor denervation causes a labile hypertension without changes in hot plate response. Administration of beta-endorphin into the nucleus of the solitary tract (NTS) gradually decreases blood pressure and heart rate without affecting respiratory frequency. These cardiovascular effects are blocked by naloxone as well as by an antibody to beta-endorphin. In contrast to the effects of beta-endorphin, microinjection of enkephalins into the NTS increases blood pressure and heart rate. The data suggest the existence of two separate endorphin systems at the level of the NTS, one a depressor and another a pressor system. The depressor influence of beta-endorphin may play a role in the mechanism of action of antihypertensive agents such as methyldopa and clonidine. Our data support a role of endorphins as neuropeptides involved in cardiovascular regulation, exerting a dual influence at the level of the NTS.

Pain sensitivity is not affected by sino-aortic denervation in rats

Genetic hypertension in rats appears to be associated with diminished pain sensitivity. Acute stimulation of the baroreceptors has also been reported to decrease responsiveness to a noxious stimulus. In this study the effect of sino-aortic denervation on pain sensitivity, as measured by means of a hot plate and threshold electric footshock method has been investigated. Deafferentation resulted in a labile hypertension and tachycardia. However, hot plate response latency times were not significantly different in the sino-aortic denervated rats as compared to sham operated controls under subchronic conditions. When tested for pain sensitivity three weeks after surgery using both methods on one occasion only, again the responsiveness to noxious stimuli was not different between the two groups. At the end of the experiments, the efficacy of denervation was confirmed by assessing lability of blood pressure and measuring heart rate responses to pharmacological manipulation of blood pressure. It is concluded that surgical interruption of the baroreceptor nerves in rats does not alter pain sensitivity under subchronic conditions.

The blood pressure changes resulting from the microinjection of fragments of the beta endorphin molecule into the nucleus tractus solitarii (NTS)

Unilateral microinjection of gamma endorphin (1-17) into the nucleus tractus solitarii (NTS) of the urethane anaesthetised rat produces a dose-related pressor response. This effect has been examined in the present study, by means of fragments of the molecule. Unilateral application of the peptides 6-16, 10-16, 6-17 and 10-17 (1 ng) all produced an equivalent rise in blood pressure, however the time course of 6-16 and 10-16 was more rapid than that of gamma endorphin or the fragments containing the 17th amino acid. Naloxone (10 ng) administered into the NTS, did not affect the maximum rise in pressure induced by 6-17 (1 ng). It is concluded that in addition to a depressor region, which is dependent on the N-terminal tyrosine, the beta endorphin molecule (1-31) also has a pressor region, which is present within the amino acid sequence 10-16, and is a non-opiate effect.

Cardiovascular effects of beta-endorphin after microinjection into the nucleus tractus solitarii of the anaesthetised rat

The cardiovascular effects of beta-endorphin after administration directly into the nucleus tractus solitari (NTS) of urethane-anaesthetised rats were investigated. Unilateral injection resulted in a U-shaped dose-response relationship with a fall in mean arterial pressure and heart rate occurring at low doses (less than 10 ng). No change in respiratory frequency was observed at any of the doses examined. The hypotensive effects of beta-endorphin were anatomically specific and restricted to the NTS. The depressor response was prevented and bradycardia reduced by naloxone (1 mg/kg s.c. or 10 ng injected into the NTS) and also by beta-endorphin antiserum (1:50 dilution) but not by antiserum to [Met5]enkephalin (1:50 dilution) applied locally into the NTS. The beta-endorphin antiserum caused a rise in blood pressure when administered alone. Conversely microinjection of antiserum to [Met 5]enkephalin resulted in a brief depressor response. Doses of beta-endorphin larger than 10 ng induced a rise in blood pressure accompanied by variable effects on heart rate. Similarly unilateral administration of Des-tyr-endorphin (100 pg) resulted in a blood pressure increase and [D-Ala2,Met5]enkephalin produced a dose-related pressor response and tachycardia. The results indicate that at least two separate endorphin systems are involved in cardiovascular control at the level of NTS, one being depressor in nature (beta-endorphin-like) and the other pressor ([met5]enkephalin-like).

Central and peripheral alpha-adrenoceptor number and responsiveness after sinoaortic denervation in the rabbit

The effects of bilateral sinoaortic denervation were examined in rabbits in vivo and in vitro. In the in vivo studies the effects of intravenous administration of the alpha-adrenoceptor agonists phenylephrine and guanabenz and the antagonists prazosin and phentolamine on mean arterial pressure, heart rate, and plasma catecholamine levels were investigated in intact sham-operated rabbits and after bilateral baroreceptor deafferentation. In in vitro studies the maximum number of prazosin and clonidine binding sites and their dissociation constants were investigated using radioligand binding techniques. Mean arterial pressure was labile and significantly elevated in the sinoaortic denervated rabbits. Heart rate and basal plasma noradrenaline were also raised, but plasma adrenaline and tissue noradrenaline levels were similar in intact and debuffered animals. alpha-Adrenoceptor antagonists were more effective at lowering mean arterial pressure in the sinoaortic-denervated animals. This was at least in part the result of a lack of compensatory changes in sympathetic activity and heart rate in these animals. Pressor responses to the alpha 1-adrenoceptor agonist and more markedly to the alpha 2-adrenoceptor selective agonist were increased after sinoaortic denervation. These changes could not be related to alteration in alpha 1- or alpha 2-receptor binding in either heart, spleen, or brain tissue as there was no change in number of binding sites or affinity.

The effect of opiates on arterial baroreceptor reflex function in the rabbit

The contribution of exogenous and endogenous opioid peptides to the central modulation of the baroreceptor reflex was investigated in rabbits. Baroreceptor sensitivity was assessed in pentobarbitone anaesthetised animals by measuring heart period in response to rises in arterial pressure after bolus intravenous injections of phenylephrine and falls induced by intravenous sodium nitroprusside and controlled haemorrhage. The slope of the linear relationship between arterial pressure and heart period was used as an index of baroreflex sensitivity. Thirty minutes after the intracisternal administration of 50 microgram/kg RX783016 (a mu-opiate receptor agonist) baroreceptor sensitivity was reduced to all three methods of blood pressure manipulation. Ketazocine (50 microgram/kg) a kappa-opiate agonist and [D-Ala2, D-Leu5] enkephalin (1 microgram/kg) a delta-opiate agonist, 15 min after intracisternal injection caused an increase in baroreflex sensitivity in response to a rise in pressure and a reduction in response to a fall. Intravenous injection of naloxone (80 microgram/kg) caused an increase in varoreflex gain. However, a higher dose (200 microgram/kg) was required to attenuate the effects of RX783016 and [D-Ala2, D-Leu5] enkephalin but not ketazocine. No change in baseline arterial pressure or heart rate occurred after the opiates or naloxone. It appears that exogenous and endogenous opiates modify baroreceptor reflex function, through a mechanism which involves central opiate receptors of the mu- and kappa-types.

An inhibitory role of beta-endorphin in central cardiovascular regulation

The cardiovascular effects of beta-endorphin after administration directly into the nucleus tractus solitarii (NTS) of urethane anaesthetised rats were investigated. Unilateral injection resulted in a dose related fall in mean arterial pressure and heart rate. No change in respiratory frequency was prevented and the bradycardia reduced by pretreatment with locally applied naloxone (10 ng). This dose of the opiate antagonist had no effect on mean arterial pressure or heart rate when administered alone. Antiserum to beta-endorphin (1:50 dilution) caused a rise in pressure and a tendency towards tachycardia on injection into the NTS, while it completely blocked the depressor response and bradycardia induced by beta-endorphin. These results are consistent with the view that a beta-endorphin-like peptide has a depressor role in the central nervous system. The hypotension may result from an effect within the central connections of the baroreceptor reflex arc, probably at the level of the NTS.

Beta-Endorphin, an endogenous depressor agent in the rat?

1. The role of opiates in cardiovascular regulation has been investigated. 2. In urethane-anaesthetized renal hypertensive rats (two-kidney, one-clip Goldblatt model), intracerebroventricular beta-endorphin (10 microgram) caused a greater fall in mean arterial pressure than in sham-operated controls. 3. Unilateral injection of beta-endorphin into the nucleus tractus solitarii of the urethane-anaesthetized rat resulted in a U-shaped dose--response relationship, with a fall in mean arterial pressure and heart rate occurring at low doses. Doses above 10 ng caused a rise in pressure, accompanied by a variable effect on heart rate. 4. The fall in blood pressure and heart rate was prevented by prior subcutaneous administration of naloxone. Naloxone caused an increase in blood pressure when administered alone. 5. These results suggest a depressor role of an endogenous brain opiate, possibly beta endorphin; a site of action is probably the nucleus tractus solitarii.

Captopril and baroreceptor reflex responses to pressor and depressor stimuli in the rabbit

1. The reflex effects of intravenous phenylephrine, sodium nitroprusside or acute haemorrhage in rabbits were examined as indices of baroreceptors reflex function. 2. The slope of the linear regression of heart period: mean arterial pressure relationship was significantly greater (P less than 0.01) when buffering the pressor stimulus than either depressor stress. 3. Captopril (1 mg/kg i.v.) had no effect on the slope of the relationship to either pressor or depressor stimuli. 4. The clinical and experimental observation of hypotension without bradycardia after captopril could not be confirmed in the present study to be a result of changes in baroreflex sensitivity.

Opiate analogs, substance P, and baroreceptor reflexes in the rabbit

The putative transmitters, enkephalins and substance P, and their binding sites have been identified in the nucleus of the solitary tract. Their role in the modulation of baroreceptor reflex activity is the subject of this study in the rabbit. A stable decarboxy analog of leu-enkephalin, RX 783016, which has mu receptor specificity, was used to attenuate the baroreflex sensitivity to intravenous phenylephrine. RX 783016, 50 micrograms/kg intracisternally, did not alter resting heart rate of blood pressure. Intravenous administration of the opiate receptor antagonist, naloxone, prevented the effects of RX 783016. Naloxone given alone significantly increased reflex sensitivity. Substance P given intracisternally in low doses (1 to 10 ng/kg) caused a dose-dependent pressor response, which was reduced by pretreatment with morphine and enhanced by naloxone. Bilateral sinoaortic denervation also enhanced the pressor response to substance P, but after deafferentation, naloxone had no further effect. It is proposed that enkephalin-containing neurons, acting through mu receptors, and substance P neurons influence baroreceptor reflex activity by modulating respectively the primary and second order neurons of the baroreceptor reflex.

Captopril and the maintenance of blood pressure after sinoaortic denervation in the rabbit

1. At a dose of 1 mg/kg intravenously, captopril lowered blood pressure in conscious rabbits, that had undergone denervation of sinoaortic baroreceptors, but had no effect on heart rate. In sham-operated controls, this dose caused only an increase in heart rate. 2. In the same experiments, captopril caused a substantial inhibition of plasma angiotensin converting enzyme (CE) activity of rapid onset and gradual decline over 2 to 3 h. The time course of recovery of blood pressure and plasma CE activity were similar. 3. Saralasin, by intravenous infusion, lowered blood pressure in sinoaortic rabbits but to a smaller extent than captopril. 4. The fall in blood pressure observed after captopril is not dependent on the integrity of baroreceptor afferents.

Effect of baroreceptor deafferentation on central catecholamines in the rat

1. Sinoaortic deafferentation in the rat leads to increased blood pressure and heart rate. 2. Early increases in tyrosine hydroxylase activity both in brain stem and hypothalamus suggest that increased noradrenaline synthesis may contribute to the development of neurogenic hypertension. 3. After 4 weeks, phenylethanolamine-N-methyltransferase activity was reduced in the hypothalamus. 4. Noradrenaline- and adrenaline, but not dopamine-containing neurones may participate in regulation of sympathetic efferent activity.

Participation of adrenergic and noradrenergic neurones in central connections of arterial baroreceptor reflexes in the rat

We examined activities of tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl transferase (PNMT) and concentrations of norepinephrine (NE) in seven brain regions and the spinal cord of Wistar rats with elevated blood pressures 1 week and 4 weeks after denervation of carotid and aortic baroreceptors, and compared them to values in sham-operated control rats. TH activity was increased in the solitary tract nucleus (to 188% of control), parahypoglossal nucleus (to 254%), locus ceruleus (to 191%), and posterior hypothalamus (to 225%) at 1 week but not at 4 weeks after denervation. Similarly, NE concentrations were significantly altered in a number of brain regions at 1 week but not at 4 weeks after denervation. The only change in NE concentration at 4 weeks was in spinal cord where the level was reduced to 80% of control in the denervated rats. In contrast, the only change in PNMT activity 1 week after denervation was in posterior hypothalamus (to 59% of control), whereas at 4 weeks there was in increase in the spinal cord (to 159%) and a decrease in both the anterior and posterior hypothalamus (to 59% and 64% of control, respectively). The experiments suggest that increased activity of noradrenergic neurones in the brain may play a significant role in initiating the increase in pressure produced by baroreceptor denervation, whereas decreased activity of hypothalamic adrenergic neurones may be more important in maintaining the raised pressure. An increase in the activity of both adrenergic and noradrenergic nerves in the spinal cord, however, could contribute to maintaining the increase in blood pressure 4 weeks after denervation.

Catecholamine synthesizing enzymes in brain stem and hypothalamus during the development of renovascular hypertension

The activities of tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) have been measured in brain stem and hypothalamic nuclei during the development of renovascular hypertension. TH activity fell at 72 h in the posterior hypothalamic and peri- and paraventricular nuclei of the hypothalamus, but had returned to control levels by 7 days. PNMT activity was raised in the nucleus of the solitary tract at 7 days and was also elevated in the nucleus of the solitary tract, parahypoglossal nucleus, locus coeruleus and cerebellar cortex at 4 weeks. No change in PNMT was noted in hypothalamus. It appears from these results that both central noradrenergic and adrenergic pathways are involved in the development of this model of experimental hypertension.

Brain catecholamines and catecholamine-synthesizing enzymes in renovascular hypertension in the rat

1. Noradrenaline content of several rat brain stem and hypothalamic nuclei falls transiently at 72 h after initiation of renovascular hypertension (one-kidney Goldblatt model). 2. Tyrosine hydroxylase activity is significantly reduced in posterior, paraventricular and periventricular nuclei of hypothalamus at this time but returns to control value by 7 days. 3. Treatment with hydrallazine, 5 mg/kg intraperitoneally, twice daily or methaoxamine, 5 mg/kg, three times daily for 3 days respectively raises and lowers the noradrenaline content of brain nuclei, suggesting that short-term changes in noradrenaline may be secondary to afferent baroreceptor input. 4. At later times after the development of renovascular hypertension (7 and 28 days) activity of phenylethanolamine-N-methyl transferase is increased in the nucleus of the solitary tract and the locus coeruleus. 5. Brain catecholamines may participate both early in the development and later in the maintenance of renovascular hypertension.