The effects of ageing on neuronal uptake of noradrenaline in the rat

Summary We have examined the effects of ageing on the physiological function of the neuronal noradrenaline uptake system by comparing responses to cocaine in young adult (5 month) and aged (22 month) male Sprague-Dawley rats. In rat atria pre-incubated with [³H]-noradrenaline, cocaine (3–30 mol/l) significantly augmented the 2 Hz stimulation-evoked release of noradrenaline in tissues from young but not from old rats. Cocaine (1 mg/kg) produced a greater increase of the pressor response to noradrenaline in young than in old pithed rats. Cocaine significantly increased the tachycardia to noradrenaline only in young pithed rats, but in old pithed rats the duration of the response to noradrenaline was significantly increased. It is concluded that ageing in the rat is associated with a decreased function of the neuronal noradrenaline uptake system, at least in the cardiovascular system. Send offprint requests to J. R. Docherty at the above address     

Comparative effects of clozapine and alpha-adrenoceptor blocking drugs on regional noradrenaline metabolism in rat brain.

Clozapine increased brain noradrenaline (NA) metabolism, as indicated by changes in 3-methoxy-4-hydroxyphenylglycol sulfate content, in brain regions corresponding to the predominance of alpha- over beta-receptors, i.e., hypothalamus, medulla, midbrain and cortex, but not corpus striatum or cerebellum. Phenoxybenzamine had a stronger effect in the hypothalamus than did clozapine, but did not change cortical NA metabolism within a 60 min treatment time; however, cortical NA metabolism was increased 150 min after phenoxybenzamine. The delayed effect of phenoxybenzamine may be due to either a poor affinity for some central receptors or a slow rate of entry into certain brain regions. Thioridazine and the benzodioxane, dibozane, had regional effects similar to clozapine. The similarity between clozapine and dibozane in ther effects on regional brain NA metabolism may reflect a preference for presynaptic alpha-receptors. It is unlikely that the antipsychotic activity of clozapine is related to a specific adrenolytic effect, but may reflect the combined activity of this drug on several transmitter systems.     

The cardiovascular effects of L-dopa in the pithed rat

1 L-DOPA (1-2 mg i.v.) in the pithed rat reduced the arterial and increased the venous pressure responses to noradrenaline.2 Infusions of dopamine (4-8 mug kg(-1) min(-1)) and noradrenaline (500 ng kg(-1) min(-1)) also reduced the pressor responses to noradrenaline. The pressor response did not recover after stopping dopamine infusions, but it usually did so after stopping noradrenaline infusions.3 The effect of L-DOPA on the response to noradrenaline was prevented by the prior injection of the dopa decarboxylase inhibitor NSD 1024.4 The prior injection of the dopamine-beta-hydroxylase inhibitor diethyldithiocarbamate, only partially prevented the effect of L-DOPA on pressor responses to noradrenaline.5 The perfusion of noradrenaline (400 ng kg(-1) min(-1)) together with Krebs solution (10 ml/min) through an organ bath containing an isolated aortic strip, depressed the response of the strip to doses of noradrenaline after the infusion was stopped. Infusions of dopamine (0.5-8.0 mug kg(-1) min(-1)) had a similar effect. Fifteen minutes after adding L-DOPA (0.5 mg) to the bath and 10 min after washing it out, the response to noradrenaline was depressed in three out of four experiments.6 Infusions of noradrenaline (1 mug kg(-1) min(-1)) in an isolated heart perfused by Langendorf's method blocked the response to injected noradrenaline whilst perfusion was in progress. Infusions of dopamine (4-8 mug kg(-1) min(-1)) or of L-DOPA (200 mug kg(-1) min(-1)) did not have this effect.7 It is concluded that the effect of L-DOPA on pressor responses to noradrenaline in the pithed rat are mediated by its conversion to dopamine and noradrenaline.     

beta-Adrenoceptor blocking drugs and isoprenaline: central effects on cardiovascular parameters.

1. The central hypotensive activity of (+)- and (-)-propranolol (100 microgram), pindolol (100 microgram) and isoprenaline (1 and 4 microgram) injected intracerebroventricularly (i.c.v.) was studied in rats anaesthetized with urethane and chloralose. Blood pressure, cardiac output and heart rate were measured; systolic stroke volume and peripheral vascular resistance were calculated. 2. (+)- and (-)-Propranolol and pindolol induced a fall of blood pressure but (+)-propranolol was less active. The heart rate was reduced more by (-)-propranolol than by (+)-propranolol or (-)-pindolol. The decrease of systolic stroke volume was greater for (-)-propranolol and pindolol than for (+)-propranolol. Peripheral vascular resistance was reduced to the same level but with different time courses, (-)-propranolol having a longer effect than (+)-propranolol and pindolol. 3. Isoprenaline induced a hypotensive effect, while cardiac output and heart rate increased; the systolic stroke volume remained stable but peripheral vascular resistance was significantly decreased. 4. These results suggest that different central regulatory centres are involved in the control of cardiac function and peripheral vascular tone.     

Presynaptic actions of piribedil on the cardiovascular system of the pithed rat

The influence of piribedil on cardiovascular sympathetic responses has been studied in the pithed rat. Piribedil (0.3-1 mg kg-1) inhibited the increases of diastolic blood pressure induced by spinal cord electrical stimulation at the level Th5-L4. This effect was reversed by sulpiride (0.3 mg kg-1) but not by yohimbine (0.3 mg kg-1). The cardiovascular responses induced by noradrenaline were unaffected by piribedil (0.3-1 mg kg-1). However piribedil (0.3-1 mg kg-1) did not modify the heart rate increase induced by spinal cord electrical stimulation at the C7-Th1 level. These results suggest that piribedil inhibits the vascular sympathetic transmission in the pithed rat via stimulation of presynaptic dopamine receptors.     

Responses of the cardiovascular system of the rat to noradrenaline infusions and their modification by adrenoceptor blocking agents.

1 The effects of noradrenaline upon the cardiovascular system of the rat, anaesthetized with pentobarbitone, have been investigated.2 Noradrenaline produces a dose-dependent increase in mean arterial blood pressure (MABP) which is due entirely to an increase in cardiac output; total peripheral vascular resistance (TPR) remains unchanged.3 Following β-adrenoceptor blockade the pressor response to infused noradrenaline is enhanced and is now due mainly to an increase in TPR; the increment in cardiac output is reduced.4 After α-adrenoceptor blockade the pressor response is greatly reduced; the residual increase in MABP is due solely to an increase in cardiac output.5 After ganglion blockade resting cardiac output and TPR both fall, resulting in a reduction in MABP. The pressor response to noradrenaline is enhanced and is now due to increases in both TPR and in cardiac output.6 The cardiovascular response of the anaesthetized rat to noradrenaline can be explained in terms of classical α- and β-adrenoceptor stimulation by the amine; the unusual form of the response may be due to an effective predominance of β-adrenoceptor-mediated effects in this species.7 It is suggested that the failure of exogenous noradrenaline to produce a rise in TPR results from a balance between the α-adrenoceptor-mediated increase and β-adrenoceptor-mediated decrease in this variable. However, this proposed balance is lost if resting vasoconstrictor tone is reduced by ganglion blockade.     

Sympathomimetic effects of amezinium on the cardiovascular system and plasma catecholamines in man

Summary Thirty one in-patients suffering from depression were treated orally with clomipramine (Cl) at various dosage, for 28 days, after a “wash-out” period of three days. In 17 patients receiving 75 mg per day of Cl, steady state plasma levels of Cl were reached at Day 14, and steady state plasma levels of its active metabolite, desmethylclomipramine (DMCl), were reached at Day 21. In contrast, in 7 other patients receiving a dosage increasing to 150 mg per day at Day 7, mean plasma levels of Cl and DMCl continued to rise during the entire treatment period. At the steady state, a correlation was found between Cl dosage expressed as mg kg body weight and the plasma concentration of Cl and DMCl. Factors such as tobacco and alcohol consumption seem to modify the Cl/DMCl ratio. A comparison of clinical response with plasma levels of Cl, DMCl and Cl + DMCl showed a significant negative linear correlation.     

The effects of some β-adrenoreceptor blocking drugs on the uptake and release of noradrenaline by the heart

1. The potencies of some β-adrenoreceptor blocking drugs in reducing noradrenaline uptake by the isolated heart were compared with their potencies in reducing the release of noradrenaline from the heart by tyramine.2. Of the drugs tested, propranolol, pronethalol and dichloroisoprenaline were the most potent in blocking uptake and release of noradrenaline, although none was at potent as cocaine; MJ 1999 and I.C.I. 50172 were only weakly effective.3. Pronethalol and dichloroisoprenaline each reduced release of noradrenaline by tyramine in the concentration range (10^-7-10^-6M) where blockade of responses to tyramine was apparent; with these drugs both reduction of noradrenaline release and β-receptor blockade contribute to the reduction in responses to tyramine.4. Potency of β-receptor blocking drugs in reducing noradrenaline uptake is unrelated to potency in blocking β-receptors; Kö 592 blocks β-receptors without affecting noradrenaline uptake.     

Inhibitory dopamine receptors on sympathetic neurons innervating the cardiovascular system of the pithed rat

Summary Additional experimental evidence was obtained for an inhibitory function of prejunctional ₂-adrenoceptors and/or dopamine receptors located on noradrenergic neurons innervating the heart and resistance vessels of the pithed normotensive rat. Mixed ₂-adrenoceptor receptor agonists, differing in selectivity towards either receptor type, i.e. N,N-di-n-propyldopamine (DPDA), 2-N, N-di-n-propylamino-6, 7-dihydroxy-1,2,3,4-tetrahydronaphthalene (DP-6,7-ADTN), B-HT 920 and B-HT 933 (azepexole) were used.In pithed normotensive rats, DPDA (30 and 100 g/kg/min) dose-dependently inhibited the electrical stimulation-induced increase in diastolic pressure, but did not significantly affect the stimulation-evoked increase in heart rate. The inhibition exerted by DPDA was blocked by haloperidol and sulpiride (0.3 mg/kg of each), but not by yohimbine (1 mg/kg), indicating the involvement of dopamine receptors. In this respect, sulpiride and haloperidol were found approximately equipotent.DP-6,7-ADTN (10 and 30 g/kg/min) impaired both tachycardic and vasoconstrictor responses in a dose-dependent manner. Sulpiride (0.3 mg/kg) only partially restored the DP-6,7-ADTN-depressed stimulation-evoked increase in diastolic pressure, whereas yohimbine (1 mg/kg) alone was without effect. The combination of both antagonists completely prevented the inhibition caused by DP-6,7-ADTN. On the other hand, yohimbine (1 mg/kg), but not sulpiride (0.3 mg/kg), selectively antagonized the DP-6,7-ADTN-induced inhibition of stimulation-evoked tachycardia.B-HT 920 (1, 3 and 10 g/kg/min) very effectively reduced the increase in diastolic pressure and heart rate caused by electrical stimulation. Inhibitory dopamine as well as ₂-adrenoceptors participated in the vascular effects of B-HT 920, whereas ₂-adrenoceptors were only involved in the cardioinhibitory response to this agonist.B-HT 933 (0.6 and 1 mg/kg/min) dose-dependently reduced the stimulation-evoked increase in arterial pressure through selective stimulation of inhibitory ₂-adrenoceptors, dopamine receptors not taking a part.The results confirm and extend the observations that in addition to ₂-adrenoceptors inhibitory dopamine receptors are located on the sympathetic neurons connected with the arterial vasculature of the pithed normotensive rat. The sympathetic nerves innervating the rat heart do not contain inhibitory dopamine receptors; their activity only can be modulated by ₂-adrenoceptor stimulation. In the pithed normotensive rat, activation of prejunctionally located ₂-adrenoceptors more effectively inhibits the sympathetic activity directed to the heart than that to the resistance vessels.     

Differential effects of hypothermia on neuronal efflux,release and uptake of noradrenaline

Summary Isolated rabbit hearts were perfused at 34° (control), 24° or 12°C. The neuronal efflux of noradrenaline after perfusion with the amine for 1 h was depressed at 24° C (Q ₁₀ about 5) in the presence or absence of desipramine; at 12°C the efflux was below the limit of estimation. Moderate reduction of the temperature (24° C) decreased the removal of perfused noradrenaline to about 60% of the control value and caused a 1.7-fold increase of the output of noradrenaline evoked by sympathetic nerve stimulation. It is concluded that the extremely temperature-dependent efflux of noradrenaline across the axonal membrane is not part of the release of noradrenaline evoked by nerve stimulation.     

The effect of renal failure on the disposition and neuromuscular blocking action of pancuronium bromide

Summary Plasma concentrations of pancuronium following single dose administration in six patients, and following multiple dose administration in four patients, all undergoing renal transplantation surgery, were measured using a fluorimetric method. A two-compartment open model was used in the pharmacokinetic analysis of the data. Comparison of the pharmacokinetic findings with data previously obtained for patients undergoing elective surgery but having normal renal function indicated that the clearance of the drug was reduced significantly in the patients with renal failure, and that in these individuals the half-life was increased significantly. Measurement of the evoked mechanical twitch response concurrently with plasma concentration monitoring of pancuronium confirmed that the prolongation of half-life in the patients with renal failure was often but not always associated with an extended duration of neuromuscular blockade and furthermore that the rate of recovery from block might also be prolonged. The clinical implications of these findings are discussed.     

The TiPS lecture: functional aspects of the neuronal uptake of noradrenaline.

For various amine transmitters (noradrenaline, dopamine, 5-HT) re-uptake into the releasing varicosity limits the transmitter's life span in the biophase. In the second TiPS Lecture, given at this year's FASEB meeting in Atlanta, Georgia, Ullrich Trendelenburg summarized the evidence relating to the function of the neuronal noradrenaline carrier (uptake1), why it is absolutely dependent on Na+ and Cl- and how it functions as a metabolizing system, hand in hand with intraneuronal monoamine oxidase and vesicular storage. This carrier clears noradrenaline from the extracellular space very efficiently. Hence, loading of incubated organs with exogenous substrates of uptake1 results in a very heterogeneous distribution of the amine. Moreover, under certain experimental and pathophysiological conditions the carrier is able to transport axoplasmic noradrenaline out of the varicosity, a 'release' mechanism operating, for instance, in cardiac ischaemia.     

Inhibition of synaptosomal [3H]noradrenaline uptake by beta-adrenoceptor blocking drugs: influence of lipophilicity

Ten beta-adrenoceptor blocking drugs varying in lipophilicity and beta-adrenoceptor blocking potency were examined for inhibitory effects on synaptosomal [3H]noradrenaline uptake. All compounds produced a concentration-dependent inhibition of noradrenaline uptake, but were at least one order of magnitude less potent than desmethylimipramine and cocaine. The order of potency was pronethalol greater than propranolol greater than betaxolol greater than alprenolol greater than oxprenolol greater than practolol greater than metoprolol greater than acebutolol greater than sotalol greater than atenolol, with IC50 values ranging from 4.0 X 10(-6) to 2.2 X 10(-3) M. Uptake inhibition was unrelated to beta-adrenoceptor blocking potency, but was highly correlated with drug lipophilicity. (+)-Propranolol was an effective uptake inhibitor, as was the local anaesthetic procaine. Kinetic analysis of uptake inhibition by propranolol, oxprenolol, metoprolol and procaine revealed a mixed inhibition for all four agents examined. It is suggested that this effect of beta-adrenoceptor blockers may be mediated, at least in part, by an action on membrane phospholipids associated with the noradrenaline carrier protein, and that noradrenaline uptake inhibition may underlie certain central side-effects observed with some drugs in this group.     

The effects of some slow channel blocking drugs on high affinity serotonin uptake by rat brain synaptosomes

The effects of some slow channel blocking drugs were investigated on high affinity serotonin uptake into crude rat brain synaptosomes. Serotonin uptake was sodium-dependent and competitively inhibited by imipramine (IC50 0.6 microM, Ki 0.26 microM). Bepridil, verapamil and diltiazem produced an apparent competitive inhibition of serotonin uptake with respective IC50 of 4.8, 5.2 and 308 microM. Nitrendipine and the sodium channel blocker, lidocaine, were without effect, even at 100 microM. The mechanism of the inhibitory effect is unknown but may involve an allosteric interaction with the sodium-dependent transporter.     

Effect of ageing on the number of neuronal noradrenaline uptake sites in the rat vas deferens

Summary Previous work has shown an age-related reduction in neuronal uptake of noradrenaline in the prostatic, but not in the epididymal portion of the rat vas deferens. In the present paper, the influence of ageing on the number of [³H]desipramine binding sites and on the effect of lithium on neuronal [³H]noradrenaline uptake were studied in the prostatic and epididymal portions of vasa deferentia from 4- and 20-month-old rats. The affinity for [³H]desipramine (K _d values) in the epididymal and prostatic portions did not change with age. However, ageing reduced the maximal number of [³H]desipramine binding sites (B_max values) in the prostatic, but not in the epididymal portion. Lithium potentiated neuronal [³H]noradrenaline uptake only in the prostatic portion and this potentiation was not changed by ageing. The results showed differences in neuronal noradrenaline uptake between the two portions of the vas deferens. Furthermore, the data suggest that the age-related reduction in neuronal uptake in the prostatic portion is due to a reduction in the number of neuronal uptake sites for noradrenaline. Correspondence to R. Pekelmann Markus at the above address