The uptake of noradrenaline by the isolated perfused rat heart

The uptake of noradrenaline by the isolated perfused rat heart was studied after perfusion with a medium containing various concentrations of (±)-[³H]-noradrenaline. Simultaneous measurement of the uptake of [³H]-noradrenaline and of the net increase in the noradrenaline content of the heart showed that [³H]-noradrenaline entering the heart both increased the tissue content and exchanged with endogenous noradrenaline. A large part (about 75%) of the endogenous noradrenaline pool, however, exchanged very slowly if at all with exogenous noradrenaline. The initial rates of noradrenaline uptake satisfied Michaelis-Menten kinetics with a Km for (±)-noradrenaline of 6.64×10^-7 M. Further analysis of the uptake process indicated that noradrenaline entered into at least two intracellular pools at different rates. Measurement of the initial rates of noradrenaline uptake during perfusion with various concentrations of nonradioactive (+)- and (-)-noradrenaline showed that the uptake process exhibited stereochemical specificity. The Km values for (+)- and (-)-noradrenaline were 13.9×10^-7 and 2.66×10^-7 M respectively. Cocaine acted as a potent competitive inhibitor of noradrenaline uptake. This finding suggested that diffusion did not play any significant role in the entry of noradrenaline into the tissue.     

Effect of metavanadate on the uptake and release of noradrenaline in rat brain cerebral cortex slices

The effect of vanadium (as VO3-) on the uptake and release of tritiated noradrenaline ([3H]NA) was studied in vitro in rat cerebral cortex slices. Vanadate inhibited [3H]NA uptake and the inhibition was dependent upon concentration and on incubation time. The IC50 value (20 min incubation) was 8 X 10(-5) M of vanadate. Inhibition of Na+, K+ -ATPase activity by VO3-, chelation of noradrenaline or autooxidation of catecholamine by this oxyanion might contribute to the decrease of [3H]NA uptake. Vanadate inhibited also the release of [3H]NA in a time- and concentration-dependent fashion.     

Effect of phentolamine on noradrenaline uptake and release

Summary The influence of phentolamine on the uptake of exogenous noradrenaline infused into the aortic cannula and on the overflow of endogenous noradrenaline caused by sympathetic nerve stimulation was investigated in the isolated perfused rabbit heart. 10^–6 M phentolamine doubled the overflow of endogenous noradrenaline, but did not change noradrenaline uptake. 10^–5 M phentolamine increased the stimulation-induced overflow of noradrenaline 4-fold and inhibited amine uptake by about 50%. 10^–4 M phentolamine elevated the overflow of noradrenaline less than 10^–5 and 3×10^–5 M did. The augmentation of transmitter overflow was only partly reversed by 13 min perfusion with drug-free medium.Pretreatment of hearts with 1.5×10^–5 M cocaine or with 10^–7 or 10^–6 M desipramine did not change the effect of phentolamine on the overflow of noradrenaline evoked by nerve impulses. Pretreatment of hearts with 10^–5 M, but not with 10^–6 M, phentolamine prevented the increase of transmitter overflow by cocaine.It is concluded that low concentrations of phentolamine potentiate the overflow of noradrenaline during nerve stimulation by a mechanism different from that of cocaine, i.e. different from inhibition of neuronal re-uptake. The nature of this mechanism is discussed.This work was supported by the Deutsche Forsehungsgemeinschaft. We have the pleasure to thank Mrs. Ch. Arts, Miss B. Piel and Mr. E. Hagelskamp for skilful technical assistance.     

ACTH increases noradrenaline release in the rabbit heart

Summary The effects of ACTH on the release of noradrenaline and the increase of heart rate produced by sympathetic nerve stimulation (1 Hz) were studied in isolated perfused rabbit hearts. ACTH-(1–24) 0.1–100 nmol/l increased the stimulation-evoked overflow of noradrenaline concentration-dependently, reversibly and up to two-fold. The basal outflow of noradrenaline, the basal heart rate and the stimulation-evoked increase in heart rate were not changed. Human ACTH-(1–39) also increased the evoked overflow of noradrenaline. The effect of ACTH-(1–24) 0.3 nmol/l persisted after blockade of -adrenoceptors with propranolol and blockade of neuronal catecholamine uptake by cocaine. ACTH-(1–24) 3 nmol/l did not change the removal of noradrenaline from the perfusion fluid, when hearts were perfused with medium containing 59 nmol/l noradrenaline. The results show that ACTH increases the action potential-evoked release of noradrenaline from cardiac postganglionic sympathetic neurones, probably by activating specific presynaptic ACTH receptors. The high potency of ACTH suggests that these presynaptic receptors may be activated in vivo by circulating ACTH under certain pathophysiological conditions.Send offprint requests to B. Szabo at the above address     

Effects of phenoxybenzamine on the uptake and metabolism of noradrenaline in the rat heart and vas deferens

1. In the isolated rat heart perfused with various concentrations of (+/-)-(3)H-noradrenaline ((3)H-NA) the addition of phenoxybenzamine, cocaine or desipramine to the perfusion medium resulted in an inhibition of (3)H-NA uptake which appeared on kinetic analysis to be of a competitive nature.2. Phenoxybenzamine also blocked the formation of labelled metabolites of (3)H-NA in the heart at all perfusion concentrations of (3)H-NA; this effect appeared to be unrelated to the inhibition of the neuronal uptake of NA produced by phenoxybenzamine, since no blockade of (3)H-NA metabolism was produced by cocaine in similar experiments.3. In slices of rat vas deferens incubated with various concentrations of (3)H-NA, cocaine and desipramine and phenoxybenzamine were also shown to act as competitive inhibitors of NA uptake. Cocaine and phenoxybenzamine were less potent inhibitors of uptake in the vas deferens than they were in the heart; desipramine was equally potent in both tissues.4. When vas deferens slices were incubated in a medium containing phenoxybenzamine for 30 min before the addition of (3)H-NA, the resulting inhibition of (3)H-NA uptake was increased and changed to a non-competitive type of interaction.5. In hearts or vasa deferentia taken from animals pretreated in vivo with phenoxybenzamine (20 mg/kg), a significant inhibition of (3)H-NA uptake was found when the tissues were exposed to low concentrations of (3)H-NA but not when higher concentrations of (3)H-NA were used.     

The lymphocytic absorption of p,p'-DDT and some structurally-related compounds in the rat.

The lymphatic absorption of a series of 14C-labelled compounds structurally related to p,p'-DDT has been examined in thoracic duct-cannulated rats. The compounds (p,p'-DDT, p,p-DDT, P,P'-DDD, p,p-DDD, DDE, DDA and 2,4-D) varied markedly in lipid solubility, as well as in extent of lymphatic absorption, urinary and biliary excretion, and localization in body fat. More than 12% of the dose was recovered in 24-hour thoracic duct lymph of rats treated with the p,p'- and o,p-isomers of DDT and DDD and with DDE, and this percentage was increased by concomitant fat absorption. A strict parallel between lipid solubility and lymphatic absorption was not observed in this series of compounds, possibly because of differences in their rates and routes of excretion.     

Inhibition of noradrenaline uptake 2 in the isolated rat heart by steroids, clonidine and methoxylated phenylethylamines

Various steroids (corticosterone, testosterone, progesterone, β-oestradiol) produced a dose-dependent inhibition of the uptake of ³H-noradrenaline by the uptake₂ mechanism in the isolated perfused rat heart. These compounds were potent and selective inhibitors of uptake₂, with the exception of β-oestradiol which also inhibited noradrenaline accumulation in sympathetic nerves by the uptake₁ mechanism. The hypotensive drug, clonidine, was a moderately potent and highly selective inhibitor of uptake₂. Uptake was also inhibited by certain methoxylated phenylethylamine derivatives, but was unaffected by cocaine, desipramine, amitriptyline, angiotensin, histamine, ethanol or prostaglandins.     

The effects of acute administration of propranolol and practolol on the uptake,content, release,and turnover of noradrenaline in the rat heart in vivo

Summary The effect of the acute administration of (±)-, (+)-, and (–)-propranolol and practolol, respectively, on the cardiac turnover of noradrenaline was studied in male Wistar rats kept under controlled conditions of environmental lighting. Propranolol caused no or only minor effects on the turnover in concentrations of 0.001–0.1 mmoles/kg, whereas higher doses (0.2–0.4 mmoles/kg) decreased the noradrenaline turnover in rat hearts concomitantly with toxic signs from the CNS. The endogenous noradrenaline content was not changed by any concentration of propranolol. Practolol (0.1–0.4 mmoles/kg) did not influence the cardiac noradrenaline turnover, but, like an indirectly acting sympathomimetic drug, released noradrenaline.Supported by a grant of the Deutsche Forschungsgemeinschaft.     

Receptor-mediated effects of serotonin and 5-methoxytryptamine on noradrenaline release in the rat vena cava and in the heart of the pithed rat

In segments of the rat inferior vena cava preincubated with 3H-noradrenaline, it was examined whether presynaptic serotonin (5-HT) receptors exist on the postganglionic sympathetic nerves of the circulatory system; for this purpose the effects of 5-HT receptor agonists and antagonists on the electrically evoked 3H overflow were studied. Furthermore, vagotomized pithed rats (treated with atropine and captopril) were used to investigate the effects of these drugs on heart rate and on the tachycardia induced by electrical stimulation of the preganglionic sympathetic nerves (C7--T1) via the pithing rod; these experiments were carried out to provide evidence that the presynaptic 5-HT receptors are operative in vivo. 5-HT and 5-methoxytryptamine (5-OCH3-T) concentration-dependently inhibited the electrically evoked 3H overflow from the vena cava. The inhibitory effect was more pronounced at 0.66 Hz than at 2 Hz. 8-Hydroxy-2-(di-n-propylamino)tetralin did not alter the evoked overflow. The inhibitory effect of 5-HT or 5-OCH3-T was antagonized by metitepin but not affected by ketanserin or rauwolscine. In pithed rats 5-HT and 5-OCH3-T by themselves dose-dependently increased heart rate. The positive chronotropic effect of 5-HT 10 mumol/kg, which was not affected by ketanserin, was considerably decreased by desipramine, indicating that 5-HT at least at this high dose acts predominantly by a tyramine-like indirect sympathomimetic effect.(ABSTRACT TRUNCATED AT 250 WORDS)