Effect of veratridine on the fluxes of 3H-noradrenaline and 3h-bretylium in the rat vas deferens in vitro

Analysis of high intracellular [Na+]-induced release of [3H]noradrenaline in rat hippocampal slices

Our aim was to investigate the mechanisms involved in the high intracellular sodium-induced transmitter release in the CNS through the characterisation of the veratridine-evoked (40 microM) noradrenaline release from rat hippocampal slices. The response to veratridine was completely inhibited by tetrodotoxin (1 microM), indicating that the effect is due to the activation of sodium channels. Omission of Ca2+ from the superfusion fluid inhibited the veratridine-evoked release by 72%, showing that the majority of release results from external Ca2+-dependent exocytosis. The residual Ca2+-independent release was not blocked by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester (100 microM) suggesting that intracellular Ca2+ stores are not involved in this component of veratridine effect. The noradrenaline uptake blockers, desipramine (10 microM) and nisoxetine (10 microM), inhibited the external Ca2+-independent release by 50 and 46%, respectively, indicating that the release partly originates from the reversal of transporters (carrier-mediated release). In contrast to uptake blockers, lowering the temperature, another possibility to inhibit transporter function, completely inhibited the effect of veratridine in the absence of Ca2+. Further experiments revealed that low temperature (20 and 12 degrees C) reduces the veratridine-induced increase of intracellular sodium concentration ([Na+]i) in rat cortical synaptosomes (68 and 78% inhibition, respectively). The clinical relevance of our data is that during ischemia a massive release of transmitters occurs mainly due to the elevation of [Na+]i, which contributes to the development of ischemic brain injury. Our results show that low temperature may be a better therapeutic approach to the treatment of ischemia because it has a dual action on this process. Firstly, it inhibits the function of uptake transporters and hence reduces the carrier-mediated outflow of transmitters. Secondly, it inhibits the sodium influx and therefore prevents the unwanted elevation of [Na+]i. Our data also suggest that veratridine stimulation can be a suitable model for ischemic conditions.

Neuronal sodium homoeostatis and axoplasmic amine concentration determine calcium-independent noradrenaline release in normoxic and ischemic rat heart

Calcium-independent noradrenaline release was studied in the isolated perfused rat heart under conditions of normoxia, cyanide intoxication, and ischemia. The release of endogenous noradrenaline and dihydroxyphenylglycol were determined by high-performance liquid chromatography. The release of dihydroxyphenylglycol, the main neuronal noradrenaline metabolite, was used as an indicator of the free axoplasmic amine concentration. When storage function of neuronal vesicles was disturbed by Ro 4-1284 or trimethyltin, high dihydroxyphenylglycol release was observed without concomitant overflow of noradrenaline. If, however, these agents were combined with inhibition of Na+K+-ATPase or with veratridine-induced entry of sodium into the neuron, both dihydroxyphenylglycol and noradrenaline were released. Noradrenaline release was independent of extracellular calcium and was suppressed by blockade of neuronal catecholamine uptake (uptake1), indicating nonexocytotic noradrenaline liberation from the sympathetic nerve ending. This release critically depended on two conditions: 1) increased cytoplasmic concentrations of noradrenaline within the sympathetic neuron and 2) intraneuronal sodium accumulation. Both conditions together were required to induce noradrenaline efflux across the plasma membrane using the uptake1 carrier in reverse of its normal transport direction. A disturbed energy status of the sympathetic neuron, induced by cyanide intoxication or ischemia, likewise caused calcium-independent noradrenaline release by interfering with both vesicular storage function and neuronal sodium homoeostatis. Again, release was sensitive to uptake1 blockade. Since neuronal sodium accumulation was the rate-limiting step, release was further accelerated when residual Na+,K+-ATPase activity was inhibited. Na+-H+ exchange was identified as the predominant pathway of sodium entry into the sympathetic nerve ending in ischemia, and its inhibition by amiloride and ethylisopropylamiloride markedly suppressed ischemia-induced noradrenaline release.

Sodium-dependence of the potency of inhibitors of the neuronal noradrenaline carrier in the rat vas deferens

Vasa deferentia obtained from reserpine-pretreated rats were incubated (monoamine oxidase and catechol-O-methyltransferase inhibited) in media containing various concentrations of 3H-(-)noradrenaline and Na+ and initial rates of the neuronal uptake of 3H-noradrenaline measured both in the absence and presence of uptake inhibitors after 1 min of incubation. When rates of uptake were determined at various 3H-noradrenaline (1.0-12.2 mumol/l) and two fixed Na+ concentrations (25 and 140 mmol/l), the inhibition of uptake produced by (+)amphetamine, (-)metaraminol, desipramine, nomifensine and cocaine was competitive with respect to 3H-noradrenaline at both Na+ concentrations. While the Ki for (+)amphetamine, (-)metaraminol desipramine and nomifensine increased when the Na+ concentration was lowered, that for cocaine decreased. When the Na+ concentration was varied (10-140 mmol/l) and the 3H-noradrenaline concentration held constant (1.2 mumol/l), (+)amphetamine, (-)metaraminol, nomifensine and desipramine acted as mixed-type inhibitors with respect to Na+, and the inhibition of uptake produced by these drugs was the more pronounced, the higher the Na+ concentration. On the other hand, cocaine was competitive with Na+ and the inhibition produced by this drug was the more pronounced, the lower the Na+ concentration. It is concluded that the inhibitors of neuronal uptake tested here act in dependence on the external Na+ concentration. Desipramine and nomifensine resemble alternative amine substrates in being more potent at high than at low Na+ concentrations. On the other hand, cocaine is more potent at low than at high Na+ concentrations.

Sodium-dependence of the saturability of carrier-mediated noradrenaline efflux from noradrenergic neurones in the rat vas deferens

The carrier-mediated transport of 3H-noradrenaline out of noradrenergic neurones was studied in vasa deferentia obtained from rats after pretreatment with reserpine and pargyline (to inhibit vesicular storage and monoamine oxidase, respectively). The tissue was first preincubated with various concentrations of 3H-noradrenaline (0.3--100 mumol/1; 30 min) and then washed out for 110 min with amine-free medium. During the last 10 min of washout, carrier-mediated neuronal efflux of 3H-noradrenaline was elicited by exposure to either Na+-free medium or 100 mumol/l veratridine; it was measured at 1-min intervals. While the peak rates of carrier-mediated 3H-noradrenaline efflux elicited by Na+-free medium were linearly related to the 3H-noradrenaline content of the tissue (which cannot be raised beyond a certain maximal value, since uptake is saturable), those evoked in response to veratridine approached saturation as the 3H-noradrenaline level in the tissue was raised. Hence, saturation of 3H-noradrenaline outward transport was demonstrated at high (exposure to veratridine), but not at low (exposure to Na+-free medium) intraneuronal Na+ concentrations. The results indicate that the Km for the mediated outward transport of noradrenaline across the plasma membrane of noradrenergic neurones is inversely related to the internal Na+ concentration, just as the Km for the mediated inward transport of noradrenaline (i.e., the neuronal noradrenaline uptake) is inversely related to the external Na+ concentration.

Cardiac glycosides, calcium and the release of neurotransmitter from peripheral noradrenergic nerves

The requirement for external Ca (Cao) of neurotransmitter release evoked by cardenolides has been investigated in canine saphenous vein. Basal efflux of 3H-compounds from saphenous veins pre-loaded with 3H-noradrenaline was the same in the absence as in the presence of Cao; Cao is not required for basal efflux of neurotransmitter. Efflux of 3H-compounds was increased by cardenolides. Both ACS and ouabain caused a similar maximum net efflux of 3H suggesting that each evokes release from the same pool of 3H compounds. The similarity of the effects obtained with cardenolides to those obtained during exposure of saphenous vein preparations to potassium-free media suggests that 3H-efflux is the result of Na,K-ATPase inhibition. With ACS (ca. EC50) the net efflux of 3H-compounds early (less than 60 min) in the release period was greater in the absence of Cao than in its presence whereas at longer times the reverse was true; net efflux was less in the absence of Cao than in its presence. The difference in the 3H-efflux pattern was paralleled qualitatively throughout by efflux of 3H-noradrenaline. The ACS-evoked efflux of 3H-compounds in the presence and absence of Cao derives from sympathetic, noradrenergic nerves; 3H present in extraneuronal tissues was not released by the cardenolide. With ouabain (less than EC50) the total efflux of 3H over a 75 min period was greater in the absence of Cao than in its presence. The reverse was found with ouabain (greater than EC50): the total efflux of 3H was less in the absence of Cao than in its presence.(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.

Tetrodotoxin-sensitive and -resistant effects of veratridine on the noradrenergic neurone of the rat vas deferens

The veratridine-induced release of 3H-noradrenaline from noradrenergic neurones was examined in the isolated vas deferens of either untreated or reserpine plus pargyline-pretreated rats. The rat vas deferens, whose catechol O-methyltransferase was inhibited, was first incubated with 0.4 mumol/l 3H-(-)noradrenaline (30 min) and then washed repeatedly with amine-free solution. After 120 min (i.e., well after the efflux of tritium from the tissue had reached a steady level and was predominantly of neuronal origin), washout was continued in the presence of veratridine for further 10-15 min. In vasa deferentia of untreated rats, veratridine (1-100 mumol/l) caused a concentration-dependent increase in the efflux of tritium. At high concentrations of the drug (30 or 100 mumol/l), this increase in efflux was peak-like during the first 3 min ("peak response") and then fell to a plateau ("plateau response"). In the presence of veratridine, unchanged 3H-noradrenaline accounted for about 75% of the tritium efflux (the rest being represented by deaminated 3H-catechol metabolites). The "peak response" to veratridine (100 mumol/l) was abolished by tetrodotoxin (TTX; 1 mumol/l) or the absence of external Ca2+. Cocaine (10 mumol/l) affected neither the "peak response" as such nor the contribution by 3H-noradrenaline to the efflux of tritium during that response. Hence, the "peak response" was due to exocytotic release of 3H-noradrenaline from the neurone. The "plateau response" to veratridine (100 mumol/l) was unaffected by the absence of external Ca2+, largely resistant to TTX (1 mumol/l) and moderately reduced by cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

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

The release of 3H-(-)-noradrenaline (NA) from rat vas deferens in vitro was examined under various experimental conditions. It was found that in normal and reserpinized vas deferens the release of NA evoked by (+)-amphetamine (5 X 10(-6) M) or low external Na+ (26 mM) was antagonized by imipramine methiodide and desipramine, inhibitors of the NA uptake, but was not dependent on the presence of Ca2+ in the medium and was not antagonized by the potent local anaesthetic agent bethoxycaine. The release evoked by veratridine in reserpinized tissue was antagonized by the uptake inhibitors but was in normal tissue only partially inhibited in presence of Ca2+ but almost completely in absence of Ca2+. The release by high K+ (117 mM)+low Na+ (26 mM) in normal tissue was dependent on the presence of Ca2+ and was antagonized by the muscarinic agonists carbacholine and metacholine and by high concentrations of desipramine. In the reserpinized vasa the corresponding release was not dependent on Ca2+ and was not antagonized by the muscarinic agents but was inhibited by high concentrations of desipramine.