Release of 3H-noradrenaline from the rat vas deferens under various in vitro conditions

Intracellular Ca2+ regulates amphetamine-induced dopamine efflux and currents mediated by the human dopamine transporter

Although it is clear that amphetamine-induced dopamine (DA) release mediated by the dopamine transporter (DAT) is integral to the behavioral actions of this psychostimulant, the mechanism of this release is not clear. In this study, we explored the requirement for intracellular Ca(2+) in amphetamine-induced DA efflux and currents mediated by the human DAT. The patch-clamp technique in the whole-cell configuration was used on Na(+) and DA-preloaded human embryonic kidney 293 cells stably transfected with the human DAT (hDAT cells). Chelation of intracellular Ca(2+) by inclusion of 50 microM BAPTA in the whole-cell pipette reduced the voltage-dependent amphetamine-induced hDAT current, with the greatest effect seen at positive voltages. Likewise, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) reduced amphetamine-induced DA efflux as measured by amperometry. Furthermore, preincubation of the cells with 50 microM BAPTA acetoxy methyl ester (AM) or thapsigargin also blocked amphetamine-induced release of preloaded N-methyl-4-[(3)H]phenylpyridinium from superfused hDAT cells. BAPTA-AM also reduced DA release from striatal synaptosomes. Amphetamine also led to an increase in intracellular Ca(2+) that was blocked by prior treatment with 5 microM thapsigargin or 10 microM cocaine. These studies demonstrate that amphetamine-induced DAT-mediated currents and substrate efflux require internal Ca(2+) and that amphetamine can stimulate dopamine efflux by regulating cytoplasmic Ca(2+) levels through its interaction with DAT.

Amphetamine reverses or blocks the operation of the human noradrenaline transporter depending on its concentration: superfusion studies on transfected cells

Whether amphetamine enhances noradrenergic activity by uptake blockade or a releasing action is still a matter of debate. In order to gain insight into the interaction of amphetamine with the noradrenaline transporter its cDNA was transfected into COS-7 cells (NAT-cells) or cotransfected with the cDNA of the vesicular monoamine transporter (NAT/VMAT-cells); cells were loaded with [3H]noradrenaline, superfused and the efflux analysed for total tritium and [3H]noradrenaline. In NAT-cells amphetamine stimulated [3H]noradrenaline efflux concentration-dependently when added to the superfusion buffer at 0.01, 0.1 and 1 microM. By contrast, 10 or 100 microM amphetamine stimulated efflux to a smaller extent or not at all; however, on switching back to amphetamine-free buffer a prompt increase of efflux was observed. Cocaine did not increase efflux per se and blocked the amphetamine-induced efflux. In NAT/VMAT-cells amphetamine stimulated efflux in a concentration-dependent manner. The effect showed saturation at 1 microM and was not suppressed at higher concentrations. Cocaine also elicited efflux from NAT/VMAT-cells concentration-dependently; the maximum was reached at approximately 1 microM and amounted to only about half of the amphetamine-induced efflux. It is concluded that amphetamine can induce noradrenaline transporter mediated release only at high nanomolar to low micromolar concentrations. At higher concentrations it blocks the noradrenaline transporter; in this case, the releasing action of amphetamine, like that of cocaine, is dependent on a vesicular pool of noradrenaline.

Evidence for uptake1-mediated efflux of catecholamines from pulmonary endothelial cells of perfused lungs of rats

Previous pharmacological studies have demonstrated that pulmonary endothelial cells and noradrenergic neurones possess the same transporter for inward transport of catecholamines, uptake1. In noradrenergic neurones, it has been shown that uptake1 is also involved in the carrier-mediated outward transport, or efflux, of noradrenaline and dopamine. The aim of the present study was to examine the efflux of noradrenaline and dopamine from perfused lungs of rats to determine whether uptake1, in addition to diffusion, mediates efflux of catecholamines from pulmonary vascular endothelial cells. The effects of reducing the cellular sodium gradient and of substrates and inhibitors of uptake1 on the efflux of 3H-noradrenaline and 3H-dopamine from rat lungs were measured. Isolated perfused lungs of rats (monoamine oxidase and catechol-O-methyltransferase inhibited) were loaded with 3H-(-)-noradrenaline or 3H-dopamine for 10 min followed by perfusion with either (1) a low sodium, amine-free Krebs solution, in which NaCl was replaced by either Tris.HCl or LiCl, for 15 or 10 min, respectively or (2) amine-free Krebs solution for 30 min in the absence or presence of a substrate or inhibitor of uptake1 for the last 15 min. The rate constants for spontaneous efflux of noradrenaline and dopamine from the lungs were 0.0163 min-1 and 0.0466 min-1, respectively. When NaCl was replaced by Tris.HCl during efflux, the rate constants for efflux of noradrenaline and dopamine were increased 2.5-fold and 3-fold, respectively, whereas, when NaCl was replaced by LiCl, the rate constants were increased 8-fold and 4-fold, respectively. The uptake1 substrates, dopamine (1 and 3 mumol/l) and adrenaline (40 mumol/l), both caused a rapid and marked increase in the efflux of noradrenaline, while noradrenaline (4 mumol/l) had a similar effect on the efflux of dopamine. The uptake1 inhibitors, imipramine (3 and 10 mumol/l) and nisoxetine (50 nmol/l), caused small and gradual increases in the efflux of noradrenaline and dopamine from rat lungs. These results demonstrate that efflux of noradrenaline and dopamine from rat lungs is affected by alterations in the normal sodium gradient across the cell and by drugs that interact with the uptake1 transporter. Thus, it can be concluded that the spontaneous efflux of catecholamines from pulmonary vascular endothelial cells is mediated predominantly by uptake1. In addition, efflux of catecholamines from the lungs has a diffusional component, which, combined with inhibition of reuptake, accounts for the small increase in amine efflux by inhibitors of uptake1.

Sympathomimetic actions of methylenedioxymethamphetamine in rat and rabbit isolated cardiovascular tissues

The aim of this study was to investigate the actions of methylenedioxymethamphetamine (MDMA) in several isolated cardiovascular tissues. In spontaneously beating rat atria, concentration-dependent positive chronotropic responses to MDMA and amphetamine were blocked by the neuronal-uptake inhibitor desipramine (1 microM) and the beta-adrenoceptor antagonist propranolol (1 microM). In atria incubated with [3H]noradrenaline to label transmitter stores, 10 microM MDMA and 1 microM amphetamine increased the resting outflow of radioactivity, while 1 microM desipramine had no effect on resting outflow. The MDMA- and amphetamine-induced release of radioactivity were blocked by 1 microM desipramine. MDMA, amphetamine and desipramine each enhanced the electrical stimulation-induced (2 Hz, 30-s train) release of radioactivity; the enhancing effects of MDMA and amphetamine were blocked by 1 microM desipramine. In rat isolated perfused hearts, MDMA (1 and 10 microM) increased heart rate by a similar amount to the increase caused by noradrenaline (10 and 50 nM). MDMA also induced dysrhythmias in 7 out of 11 rat isolated perfused heart preparations. In rabbit isolated perfused and superfused ear arteries preloaded with [3H]noradrenaline, MDMA increased the resting release of radioactivity by 230 +/- 18% (n = 6) of control resting release; the increase was accompanied by a rise in perfusion pressure of 17 +/- 7 mmHg (n = 6). MDMA also facilitated the vasoconstrictor responses to noradrenaline (3-9 ng) and perivascular nerve stimulation (1-5 Hz, 10-s train). MDMA-induced vasoconstriction and the facilitation of vasoconstrictor responses to noradrenaline and electrical stimulation were blocked by 1 microM desipramine.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac sympathetic activity in myocardial ischemia: release and effects of noradrenaline

Sympathetic overactivity in myocardial ischemia is closely associated with the progression of myocyte injury and the incidence of malignant arrhythmias. Adrenergic stimulation of the ischemic myocardium is predominantly due to increased local noradrenaline concentrations in the heart, whereas plasma catecholamine levels are of minor relevance. During the first few minutes of ischemia, efferent sympathetic nerves are activated. Excessive accumulation of noradrenaline, however, is prevented since adenosine, formed in the ischemic myocardium, suppresses exocytotic noradrenaline release, and released noradrenaline is rapidly removed as long as catecholamine reuptake is functional. With progression of ischemia to more than 10 min, the myocardium is no longer protected against excess catecholamine accumulation in the interstitial space, since local metabolic release mechanisms become increasingly important. This release, which is independent of central sympathetic activity and from extracellular calcium, occurs in two steps: First, noradrenaline escapes from its intracellular storage vesicles and accumulates in the cytoplasm of the neuron. In a second, rate-limiting step, noradrenaline is transported across the plasma membrane into the interstitial space, using the neuronal uptake carrier in reverse of its normal transport direction. As a consequence of local metabolic catecholamine release, extracellular noradrenaline reaches 1000 times the normal plasma concentration within 20 min of ischemia. Studies using acute and chronic sympathetic denervation and antiadrenergic agents demonstrate that local metabolic, rather than centrally induced noradrenaline release is critically involved in the progression of ischemic cell damage within the occurrence of ventricular fibrillation in early ischemia. Myocardial ischemia results in a temporary supersensitivity of the myocytes to catecholamines. This is due to a twofold increase of alpha 1- and a 30% increase of beta-adrenergic receptor number at the cell surface. The sensitization of adenylate cyclase during the first 20 min of total ischemia is followed by a rapid inactivation of the enzyme. The beta-adrenergic hyperresponsiveness to catecholamines is therefore limited to the first few minutes of ischemia. The deleterious combination of extremely high noradrenaline concentrations with a temporarily enhanced responsiveness to catecholamines of the tissue is thought to accelerate the propagation of the wavefront of irreversible cell damage within the ischemic myocardium. Moreover, the inhomogenous distribution of catecholamine excess within the heart is considered to promote malignant arrhythmias by unmasking and enhancing electrophysiological disturbances in early ischemia.

The role of co-transported sodium in the effect of indirectly acting sympathomimetic amines

The adrenergic nerve endings of vasa deferentia of either untreated or reserpine (R) and/or pargyline (P) pretreated rats were loaded with 3H-noradrenaline; COMT was inhibited by U-0521 (U). After 100 min of wash-out with Ca2+-free solution, the efflux of tritium (and of 3H-noradrenaline) from the tissue was largely of neuronal origin and remained constant with time (when expressed as fractional rate of loss; FRL). After 110 min of wash-out the effect of inhibition of the Na+,K+-ATPase (by low K+ or ouabain) on basal and on sympathomimetic amine-induced efflux of tritium (or 3H-noradrenaline, under the condition U) was studied in paired experiments. Inhibition of the Na+,K+-ATPase caused a time-dependent increase in the efflux of tritium (or 3H-noradrenaline) which was inhibited by desipramine. Inhibition of the Na+,K+-ATPase also caused a time-dependent reduction of the initial rate of neuronal uptake of 3H-noradrenaline. The effectiveness of the sympathomimetic amines tyramine and amphetamine in inducing "release" (i.e., outward-transport) of noradrenaline depended on the experimental condition: it was most pronounced under the condition RPU, followed by the condition PU and lowest under the condition U (i.e., in tissue of untreated rats). Inhibition of the Na+,K+-ATPase caused an early and transient enhancement of the "release" of noradrenaline induced by tyramine or amphetamine. This enhancement was seen already within the first min after inhibition of the ATPase, i.e., before a pronounced inhibition of uptake (of noradrenaline) and before a pronounced increase of the basal efflux was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of endogenous catecholamines in the ischemic myocardium of the rat. Part A: Locally mediated release

The accumulation of endogenous catecholamines within the extracellular space of the ischemic myocardium has been studied in the isolated perfused (Langendorff) heart of the rat subjected to various periods of complete ischemia, with subsequent collection of the reperfusate. Catecholamines and deaminated metabolites were measured by radioenzymatic methods, or high pressure liquid chromatography. Ischemic periods of less than 10 minutes are not associated with an increased overflow of catecholamines or metabolites. Longer periods of ischemia are accompanied by the overflow of noradrenaline and its deaminated metabolite 3,4-dihydroxyphenylglycol. This overflow increases with lengthening of the preceding ischemic period (10 minutes: 2.5 +/- 0.6, 20 minutes: 209.8 +/- 17.2, 60 minutes: 1270.5 +/- 148.1 pmol noradrenaline/g heart). Noradrenaline concentration is highest during the first minute of reperfusion, suggesting that the noradrenaline detected during reperfusion is released into the extracellular space of the myocardium during ischemia and is subsequently eluted. Experiments with variation of extracellular calcium concentration and with neuronal uptake (uptake1) blocking agents suggest that different mechanisms of catecholamine release are acting during the course of ischemia. A calcium-independent carrier-mediated efflux of noradrenaline from the nerve terminals is of major importance, using the same carrier as is normally responsible for transporting noradrenaline from the synaptic clefts into the neuronal varicosities. Thus, various uptake1-blocking agents diminish the noradrenaline overflow following ischemic periods of between 10 and 40 minutes. The noradrenaline overflow following longer periods of ischemia is unaffected by uptake1-blocking agents, and additional noradrenaline release at this time is probably consequent upon dissolution of cell membranes. Overflow of adrenaline and dopamine occurs to a minor degree (less than 5% of the corresponding noradrenaline overflow), and only after ischemic periods of more than 15 minutes.

Estrogen inhibition of noradrenaline release in the rabbit oviduct

In the present study, we investigated the influence of estrogen on 3H-noradrenaline (3H-NA) release induced in the oviductal isthmus by electrical stimulation, potassium and calcium. The fractional release of 3H-NA was measured in oviducts isolated from ovariectomized rabbits and from ovariectomized rabbits treated with estradiol cypionate, 70 micrograms/kg im 72 h before an experiment. Electrical field stimulation of the intramural nerves induced muscle contraction and augmented the release of labelled NA from the muscle. The 3H-NA release was reduced after estrogen treatment when reuptake of NA into the nerve terminals was blocked by desipramine, 10(-6) M. Estrogen also reduced the 3H-NA release evoked by exposure of the oviducts to 121 mM KC1 in the presence of calcium (2.5 mM) and in a high potassium, calcium-free medium upon the addition of 2.5 mM calcium. In the presence of desipramine a small fraction of 3H-NA was released in high potassium, calcium-free medium. This release was unaffected by estrogen. These results suggest that estrogen reduces the release of NA from the adrenergic nerves within the oviduct and that this action is exerted primarily on the calcium-dependent release. It therefore might be due to a reduction in the entry of calcium into the nerve terminal.

KCl contractions in the rat intact and bisected vas deferens: contribution of endogenous noradrenaline release

1. KCl produced a biphasic contraction in the intact rat vas deferens. Both components were larger and the initial rapid phasic component was faster in the prostatic portion than the epididymal portion. In some experiments the epididymal phasic response was a single slow contraction, while in others it had a mixture of fast and slow responses. 2. Phentolamine reduced the phasic response but not the tonic response of the intact vas deferens. This effect was not observed after denervation produced by chronic guanethidine treatment. 3. Both phases of the response to KCl 160 mmol/l were substantially reduced by phentolamine in the epididymal portion. In the prostatic portion phentolamine produced only slight inhibition of the phasic component and had no effect on the tonic component. 4. Isoprenaline had no effect on the response to KCl 160 mmol/l but reduced both phases of the response to KCl 50 mmol/l. This effect was antagonized by propranolol. 5. It is concluded that part of the phasic component of the response to KCl in the rat vas deferens is due to the release of noradrenaline from intramural nerves.

Ionophore (A23187)-induced efflux of [3H]norepinephrine and endogenous norepinephrine in the rat vas deferens

The calcium ionophore, A23187, produced a concentration-dependent increase in the release of norepinephrine from nerves in the rat vas deferens. Maximum response to A23187 (10(-6) - 10(-5) M) was delayed in onset, occurring 60-80 min after initiation of continuous superfusion with A23187. In fact. after tissue exposure to A23187 (10(-5) M) for only 5 min with subsequent superfusion in A23187-free buffer, a significant but delayed increase in norepinephrine efflux occurred. The A23187-induced increase in efflux of norepinephrine was not altered when neuronal sodium conductance was blocked with tetrodotoxin (3.1 X 10(-7) M) or when Na+, K+ -stimulated ATPase was blocked with ouabain (10(-4) M). Release of norepinephrine by A23187 was calcium-dependent since A23187-induced efflux of norepinephrine was diminished (approximately 50%), although not abolished, in calcium-free buffer. Thus, one component of A23187 action was calcium independent. A23187 caused an increased efflux of both norepinephrine formed endogenously and [3H]norepinephrine taken up into neuronal stores. However, the effects of A23187, both on rate and maximum amount of release were greater for [3H]norepinephrine than for endogenous norepinephrine. The present studies demonstrate that neurotransmitter efflux can be induced by carboxylic ionophores in a calcium-dependent process, and this approach may prove useful in studies evaluating factors that modulate neurotransmitter release processes.

A neurally mediated inotropic effect of veratridine and cevadine on isolated guinea-pig papillary muscle

The positive inotropic effect of veratridine and cevadine was investigated in the isolated, isometrically contracting guinea-pig papillary muscle. The increase of the force of the rested-state contraction, elicited after incubation of the resting muscle with veratridine or cevadine, served as a measure of the neurally mediated, sympathomimetic effect of these alkaloids. Concentrations exceeding 5 mumol/l veratridine or 15 mumol/l cevadine produced concentration-dependent increases of the force of the rested-state contraction. These concentrations are larger than those causing the maximum positive inotropic effect on muscles contracting continually at a rate of 1 Hz. The positive inotropic effect of 30 mumol/l veratridine as manifested by the rested-state contraction was absent in the presence of 100 nmol/l tetrodotoxin and in muscles from reserpine-pretreated animals. It was significantly inhibited by 50 nmol/l (-)-propranolol, but not by the same concentration of (+)-propranolol. The effect of 30 or 60 mumol/l cevadine was likewise absent in catecholamine-depleted preparations. It is concluded that the indirect inotropic effect of veratridine or cevadine, which is attributed to their noradrenaline-releasing effect on intracardiac nerves, requires higher concentrations than the direct positive inotropic effect, which is a consequence of the increased transsarcolemmal influx of Na ions into the myocardial cell.