Comparison of uptake of dopamine in rat striatal chopped tissue and synaptosomes

Uptake of dopamine (DA) by chopped tissue prepared from rat corpus striatum has been examined to determine whether one or two kinetically distinct uptake sites exist. Two methods were used: direct measurement of accumulated [3H]DA and determination of the rate of formation of 3,4-dihydroxyphenylacetic acid (DOPAC) after exposure to DA. The rate of formation of DOPAC in the latter experiments is a direct function of the rate of DA uptake. The rates of [3H]DA uptake and DOPAC formation are both linear with time in the presence of 10 microM substrate. Studies of [3H]DA accumulation into chopped tissue reveal two apparent components with Km values of 160 nM and 3.8 microM, whereas similar experiments with striatal homogenate or synaptosomes yield a single uptake component with a Km equivalent to the lower value found in chopped tissue. Evaluation of DA uptake via the rate of DOPAC formation gave a Km value of 2.3 microM. (High substrate values were used, so a lower value for Km is not apparent in the data.) The high Km-value component was absent in animals with a lesioned striatum induced by prior nigral injections of 6-hydroxydopamine. Several pharmacologic agents (benztropine, amfonelic acid, bupropion, nomifensine and ouabain) were evaluated. All reduced the uptake of DA in chopped tissue, but with reduced potencies compared with the effect of these agents in synaptosomes. The high Km activity in chopped tissue, as well as the apparent reduced potency of uptake inhibitor, appears to arise from the diffusional barrier present inside more intact tissue. This barrier is not present in homogenates or synaptosomes, and, thus, a single uptake process is seen.     

Effect of nitric oxide donors on endogenous dopamine release from rat striatal slices

Summary— Incubation of striatal slices with sodium nitroprusside (SNP) or hydroxylamine (HA) for 60 min caused a dose-dependent increase in dopamine (DA) release. This effect was inhibited completely by tetrodotoxin (TTX) (1 //M) if low concentrations of SNP (1 fiM) or HA (10 and 100 /JM) were tested. Although higher concentration of SNP (10 and 100 /JM) and HA (500 /JM) were still effective in stimulating DA release, increases observed under these conditions were less than the values found in the absence of TTX. Verapamil (10 /JM), but not co-conotoxin (100 nM), significantly reduced DA release stimulated by high concentrations of SNP or HA. When verapamil was combined with TTX, moreover, SNP and HA failed to stimulate DA release. If striatal slices were incubated in the presence of nomifensine (10 /JM), SNP and HA did not enhance DA release. SNP and HA-induced depletions in tissue DA levels were also protected by nomifensine. Inhibition of guanylate cyclase with 10 fjM of methylene blue could not reduce the effects of NO-donors. SNP and HA also failed to alter endogenous glutamate release from striatal slices. Similarly, SNP and HA-induced increases in DA release were not affected by kynurenic acid and MK-801. These results indicate that NO-donors SNP and HA stimulate DA release by facilitating reverse DA transport. This effect seems to be dependent on the activation of both voltage dependent sodium channels and L-type of calcium channels. Results presented here also indicate that neither endogenous glutamate nor guanylate cyclase activation plays an intermediary role in stimulatory effects of NO-donors on DA release from rat striatal slices.     

Microdialysis studies on 3,4-methylenedioxymethamphetamine-induced dopamine release: effect of dopamine uptake inhibitors.

The effect of dopamine (DA) and serotonin (5-HT) uptake inhibitors on 3,4-methylenedioxymethamphetamine (MDMA)-induced increase in DA efflux was studied using in vivo microdialysis. MDMA was infused directly into the anterolateral striatum via the dialysis probe. The local administration of MDMA produced a dose- and time-dependent increase in the extracellular concentration of DA in the striatum. Peripheral administration of the DA uptake blockers, mazindol (5 mg/kg, i.p.) or GBR 12909 (10 mg/kg, i.p.), produced a slight but significant increase in the extracellular concentration of DA. Moreover, pretreatment with either mazindol or GBR 12909 30 min before the infusion of MDMA (10 microM) significantly attenuated the MDMA-induced increase in the extracellular concentration of DA. Pretreatment with fluoxetine (10 mg/kg, i.p.), a 5-HT uptake blocker, 30 min before the infusion of MDMA produced a slight but significant inhibition of MDMA-induced increase in DA concentration. In contrast, pretreatment with the 5-HT2/1C antagonist, ketanserin (3 mg/kg, i.p.), had no significant effect on the increase in DA concentration produced by the local administration of MDMA. These data are suggestive that MDMA increases the concentration of DA in the striatum, in part, via a carrier-mediated mechanism which is largely independent of its effects on 5-HT release.     

Effects of phencyclidine, amphetamine and related compounds on dopamine release from and uptake into striatal synaptosomes

The ability of phencyclidine (PCP), amphetamine and other substances to stimulate dopamine release from and inhibit dopamine uptake into rat striatal synaptosomes was examined in a continuous superfusion system. Inhibition of uptake was measured by determining inhibition of [3H]dopamine displacement by unlabeled dopamine ([1H]dopamine). The displacement of [3H]dopamine by 10(-7) M [1H]dopamine was temperature- and sodium-sensitive and calcium-independent. [1H]Dopamine was an order of magnitude more potent than serotonin or norepinephrine in displacing [3H]dopamine. The concentrations of reserpine required to inhibit [3H]dopamine uptake and [3H]dopamine displacement by [1H]dopamine were similar. Nomifensine, benztropine, PCP and amphetamine also inhibited the displacement of [3H]dopamine by [1H]dopamine at concentrations which have been shown previously to inhibit the uptake of [3H]dopamine, suggesting that the mechanism behind displacement and uptake are very similar. PCP, at 10(-7) to 10(-5) M, significantly inhibited [3H]dopamine displacement by 10(-7) M [1H]dopamine, PCP was less potent than nomifensine or benztropine in inhibiting [3H]-dopamine displacement by 10(-7) M [1H]dopamine, but was equipotent to amphetamine. Superfusion of the synaptosomes for 6 min with PCP, 10(-6)M, induced increases in the spontaneous release of dopamine. In this regard, PCP was less potent than amphetamine, reserpine, flupenthixol, or benztropine. Upon initial exposure of the synaptosomes to amphetamine at 10(-7) to 10(-5) M, a substantial calcium-dependent release of dopamine was induced. In contrast, PCP did not stimulate the early calcium-dependent release of dopamine. These results indicate that PCP is less potent than amphetamine at releasing dopamine and may affect dopamine metabolism in the striatum primarily by inhibiting the reuptake of this catecholamine.     

Modulation of dopamine release in rat striatal slices by delta opiate agonists

The effects of various opiates on the spontaneous release of [3H]dopamine ([3H]DA) continuously formed from [3H]tyrosine has been studied in rat striatal slices. Morphine (5 x 10(-6) M), fentanyl (5 x 10(-8) M) and the tripeptide Tyr-D-Ala-Gly-NH-CH(CH3)CH2-CH(CH3)2 (10(-6) M) were without effect, whereas D-Ala2-Met-enkephalinamide (3 x 10(-7) to 10(-5) M), D-Ala2-D-Leu2-enkephalin (5 x 10(-9) M) and the hexapeptide Tyr-D-Ser-Gly-Phe-Leu-Thr (10(-7) M) enhanced [3H]DA release in vitro. The D-Ala2-Met-enkephalinamide stimulation of [3H]DA release was not accompanied by any increase in [3H]DA synthesis and persisted in the presence of tetrodotoxin (5 x 10(-7) M). Naloxone (5 x 10(-7) M) completely blocked the effect of Tyr-D-Ser-Gly-Phe-Leu-Thre (10(-7) M) on [3H]DA release. However, the opiate antagonist did not affect the action of the hexapeptide or of D-Ala2-Met-enkephalinamide when used at a concentration equal to or lower than the agonist. This suggests that both peptides act on opiate receptors having a low sensitivity to naloxone. According to these various results and to the pharmacological characteristics of the opiates tested as described in peripheral organs or in the brain, it is concluded that opiates acting on delta opiate receptors may presynaptically regulate the release of DA in the striatum.     

Recombinant cytochrome P450 2D18 metabolism of dopamine and arachidonic acid

The function of cytochrome P450 (P450) in the mammalian brain is not well understood. In an effort to further this understanding, this study identifies two endogenous substrates for P450 2D18. Previous reports have shown that this isoform is expressed in the rat brain, and the recombinant enzyme catalyzes the N-demethylation of the antidepressants imipramine and desipramine. By further examining the substrate profile of P450 2D18, inferences can be made as to potential endogenous P450 substrates. Herein we demonstrate the metabolism of the central nervous system-acting compounds chlorpromazine and chlorzoxazone with turnover numbers of 1.8 and 0. 9 nmol/min/nmol, respectively. Because the four aforementioned pharmaceutical substrates work by binding to neurotransmitter receptors, binding assays and oxidation reactions were performed to test whether dopamine is a substrate for P450 2D18. These data indicate a K(S) value of 678 microM and that P450 2D18 can support the oxidation of dopamine to aminochrome through a peroxide-shunt mechanism. We also report the P450 2D18-mediated omega-hydroxylation and epoxygenation of arachidonic acid, primarily leading to the formation of 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids, compounds that have been shown to have vasoactive properties in brain, kidney, and heart tissues. The data presented herein suggest a possible role for P450 involvement in membrane and receptor regulation via epoxyeicosatrienoic acid formation and a potential involvement of P450 in the oxidation of dopamine to reactive oxygen species under aberrant physiological conditions where the sequestering of dopamine becomes compromised, such as in Parkinson's disease.     

Lipoxygenase pathway in islet endocrine cells. Oxidative metabolism of arachidonic acid promotes insulin release

Metabolism of arachidonic acid (AA) via the cyclooxygenase pathway reduces glucose-stimulated insulin release. However, metabolism of AA by the lipoxygenase pathway and the consequent effects on insulin secretion have not been simultaneously assessed in the endocrine islet. Both dispersed endocrine cell-enriched pancreatic cells of the neonatal rat, as well as intact islets of the adult rat, metabolized [(3)H]AA not only to cyclooxygenase products (prostaglandins E(2), F(2alpha), and prostacyclin) but also to the lipoxygenase product 12-hydroxyeicosatetraenoic acid (12-HETE). 12-HETE was identified by coelution with authentic tritiated or unlabeled 12-HETE using four high performance liquid chromatographic systems under eight mobile-phase conditions and its identity was confirmed by gas chromatography/mass spectrometry using selected ion monitoring. The predominant effect of exogenous AA (5 mug/ml) was to stimulate insulin release from pancreatic cells grown in monolayer. This effect was concentration- and time-dependent, and reversible. The effect of AA upon insulin release was potentiated by a cyclooxygenase inhibitor (indomethacin) and was prevented by either of two lipoxygenase inhibitors (5,8,11,14-eicosatetraynoic acid [ETYA] and BW755c). In addition, glucose, as well as two structurally dissimilar agents (the calcium ionophore A23187 and bradykinin), which activate phospholipase(s) and thereby release endogenous AA in several cell systems, also stimulated insulin secretion. The effects of glucose, glucagon, bradykinin and high concentrations of A23187 (5 mug/ml) to augment insulin release were blocked or considerably reduced by lipoxygenase inhibitors. However, a lower concentration of the ionophore (0.25 mug/ml), which did not appear to activate phospholipase, was resistant to blockade. Exogenous 12-HETE (up to 2,000 ng/ml) did not alter glucose-induced insulin release. However, the labile intermediate 12-hydroperoxy-ETE increased insulin release. Furthermore, diethylmaleate (which binds intracellular glutathione and thereby impedes conversion of the lipoxygenase intermediates hydroperoxy-ETE and leukotriene A(4) to HETE and leukotriene C(4), respectively) potentiated the effect of glucose and of exogenous AA. Finally, 5,6-epoxy, 8,11,14-eicosatrienoic acid (a relatively stable epoxide analogue of leukotriene A(4)) as well as two other epoxy-analogues, potentiated glucose-induced insulin release. We conclude that dual pathways of AA metabolism exist in islet endocrine cells and have opposing regulatory effects on the beta cell-an inhibitory cyclooxygenase cascade and a stimulatory lipoxygenase cascade. Labile products of the latter pathway may play a pivotal role in stimulus-secretion coupling in the islet.     

Dopamine synthesis in rat brain striatal synaptosomes. I. Correlations between veratridine-induced synthesis stimulation and endogenous dopamine release.

The effects of depolarizing concentrations of veratridine were studied to determine the degree to which dopamine synthesis stimulation was correlated with stimulated endogenous dopamine release in rat brain striatal synaptosomes. Incubations included cocaine and pargyline to prevent dopamine reuptake and metabolism, respectively. Veratridine produced a 44% increase in dopamine release in 10 minutes and a similar increase in synthesis rate. Preincubation with tetrodotoxin prevented the increase in both release and synthesis, while incubation in a calcium-free medium antagonized both responses approximately 50%. Addition of 1 mM ethylene glycol bis(beta-aminoethylether)-N, N'-tetraacetic acid to the calcium-free incubation medium further inhibited the release response, but not the synthesis stimulation. These results indicate that, in general, the degree of synthesis stimulation produced by depolarizing agents such as veratridine can be correlated with their stimulation of dopamine release, suggesting either 1)that transmitter release itself is a signal for synthesis stimulation, possibly by removal of feedback inhibition of tyrosine hydroxylase or 2) that depolarizing agents increase synthesis through a secondary metabolic change that is either caused by, or at least accompanied by, transmitter release.     

Effect of novel alpha-conotoxins on nicotine-stimulated [3H]dopamine release from rat striatal synaptosomes

Nicotine's action on the midbrain dopaminergic neurons is mediated by nicotinic acetylcholine receptors (nAChRs) that are present on the cell bodies and the terminals of these neurons. Previously, it was suggested that one of the nAChR subtypes located on striatal dopaminergic terminals may be an alpha3beta2 subtype, based on partial inhibition of nicotine-stimulated [(3)H]dopamine release by alpha-conotoxin MII, a potent inhibitor of heterologously expressed alpha3beta2 nAChRs. More recent studies indicated that alpha-conotoxin MII also potently blocks alpha6-containing nAChRs. In the present study, we have examined the nAChR subtype(s) modulating [(3)H]dopamine release from striatal terminals by using novel alpha-conotoxins that have 37- to 78-fold higher selectivity for alpha6-versus alpha3-containing nAChRs. All of the peptides partially (20-35%) inhibit nicotine-stimulated [(3)H]dopamine release with IC(50) values consistent with those obtained with heterologously expressed rat alpha6-containing nicotinic acetylcholine receptors. These results, together with previous studies by others, further support the idea that alpha6-containing nicotinic receptors modulate nicotine-stimulated dopamine release from rat striatal synaptosomes.     

Endothelium-dependent relaxation is independent of arachidonic acid release.

Endothelium-dependent relaxation is mediated by the release from vascular endothelium of an endothelium-derived relaxing factor (EDRF). It is not clear what role arachidonic acid has in this process. Inhibition of phospholipase A2, and diacylglycerol lipase in cultured bovine aortic endothelial cells caused a marked reduction in agonist-induced arachidonic acid release from membrane phospholipid pools, and complete inhibition of prostacyclin production. EDRF release, assayed by measuring endothelium-dependent cGMP changes in mixed endothelial-smooth muscle cell cultures, was not inhibited under these conditions. In fact, EDRF release in response to two agonists, melittin and ATP, was actually increased in cells treated with phospholipase A2 inhibitors. In addition, pretreatment of rats with high-dose dexamethasone, an inhibitor of PLA2, did not attenuate endothelium-dependent relaxation in intact aortic rings removed from the animals, or depressor responses in anesthetized animals induced by endothelium-dependent vasodilators. In summary, inhibition of arachidonic acid release from membrane phospholipid pools does not attenuate endothelium-dependent relaxation in rats, or the release and/or response to EDRF in cultured cells.     

Diacylglycerol metabolism in mast cells: a potential role in membrane fusion and arachidonic acid release

Purified rat peritoneal mast cells stimulated with the polycationic histamine-releasing agent compound 48/80 demonstrated a two- to four- fold increase in cellular levels of 1,2-diacylglycerol (DAG) within 1 min as detected by radioactive labeling and direct quantitation experiments. When 2-[1-14C]arachidonoyl-DAG was incubated in the presence of mast-cell homogenates, a rapid conversion to free arachidonate, and to a lesser extent, to monoacylglycerol, triglyceride, and phospholipid was observed. The release of arachidonate was proportional to the amount of broken-cell preparation added and the time of incubation, was prevented by preheating mast-cell preparations, and did not occur when 1-[1-14C]arachidonoyl- phosphatidylcholine was used as substrate, suggesting that the degradation was mediated by an enzyme with Dag-lipase activity. Although much work remains to be done to clarify the precise role of DAG in mast cells, DAG metabolism may be involved in secretion by generating substances which may faciliate membrane fusion and also in arachidonic acid-derived mediator formation by liberating esterified arachidonic acid from mast-cell lipids. Taken together, these studies indicate that the formation of DAG may play a central role in mast-cell function.     

The influence of environmental temperature on the transient effects of methamphetamine on dopamine levels and dopamine release in rat striatum.

When male rats were injected four times (once every 2 hr) with 5 mg/kg methamphetamine (METH) at an environmental temperature of 23 degrees C, transient changes occurred in the levels of striatal dopamine (DA) and the regulation of striatal DA release. Striatal DA levels were minimally affected 1 day after METH treatment, but 3 days after METH treatment, striatal DA levels decreased to approximately 40% of control. DA levels returned to 70% of control 2 weeks after METH. Similarly, striatal tyrosine hydroxylase (TH) activity decreased to approximately 50% of control activity 3 days after METH treatment at 23 degrees C, but did not differ from controls at 1 or 14 days after METH treatment. No changes in striatal DA levels were observed in rats treated with four doses of 5 mg/kg METH at an environmental temperature of 4 degrees C. Striatal DA levels decreased modestly to approximately 70% of controls 3 days after treatment with four doses of 10 mg/kg METH at 4 degrees C, but DA levels returned to control levels 14 days after METH treatment. Furthermore, striatal TH activity was not affected by 10 mg/kg METH at 4 degrees C. Thus, a cold environmental temperature (4 degrees C) reduced the effects of METH on striatal DA levels and striatal TH activity. Changes in the presynaptic regulation of DA release after either 5 mg/kg (23 degrees C) or 10 mg/kg (4 degrees C) METH treatment were determined in vitro using striatal slices.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neurotensin modulates differently potassium, veratridine and 4-aminopyridine-evoked release of dopamine in rat striatal slices

Summary— We have studied the effects of neurotensin (NT) on the release of [³H]dopamine ([³H]DA) evoked by terminal depolarization with either K+, veratridine or 4-aminopyridine (4-AP). NT (1–1000 nM) induced a net potentiation (up to 170%) of the K+ (25 mM) -evoked release of [³H]DA. The capacity of NT to potentiate the effect of K+ ions decreased as the K+ concentration rose from 25 to 50 mM and totally disappeared at this high K+ concentration. NT (100 nM; 1000 nM) had no significant effect on the veratridine (1.5; 5μM) or 4-AP (20 μM) -evoked release of [³H]DA. The relevance of these experimental models of DA release to physiological transmitter release remains to be established. Those data highlight the complexity of the modulation of evoked neurotransmitter release by pharmacological agents.     

Relaxing effects of 15-lipoxygenase products of arachidonic acid on rat aorta

In the presence of indomethacin, arachidonic acid relaxes precontracted rings of rat aorta only when the endothelium is intact. Arachidonate-induced, endothelium-dependent relaxation is potentiated by superoxide dismutase. In contrast, linoleic acid (LA) contracts endothelium-intact and -denuded rings. Arachidonate is metabolized in endothelial cells by both cyclo-oxygenase and 15-lipoxygenase. Therefore, we determined the vasodilatory effect of 15-lipoxygenase products. The products generated by soybean lipoxygenase (SLO) from arachidonate in the bioassay bath relax precontracted, de-endothelialized ring segments of rat aorta. This relaxation is potentiated by superoxide dismutase and is more prominent when high concentrations of SLO are used. The main metabolites recovered from the bioassay bath were 5,15-dihydroperoxyeicosatetraenoic acid and 8,15-dihydroperoxyeicosatetraenoic acid. At lower concentrations of SLO the degree of relaxation is less and the major product is 15-hydroperoxyeicosatetraenoic acid. LA is metabolized by SLO to 13-hydroperoxyoctadecadienoic acid. The relaxation induced by the incubation of LA with SLO in endothelium-denuded rings is less than that obtained with arachidonic acid. In endothelium-denuded rings that were precontracted with phenylephrine authentic 15-hydroperoxyeicosatetraenoic acid did not induce clear effect (at 40 microM 15-hydroperoxide caused relaxation, whereas at 15 microM induced small contraction) and 13-hydroperoxyoctadecadienoic acid of LA induced contraction. Neither 5,15-dihydroperoxyeicosatetraenoic acid nor 8,15-dihydroperoxyoctadecadienoic acid (1-15 microM) induced a well defined relaxation. This study indicates that arachidonic acid is metabolized by SLO to a vasodilatory compound(s) that is possibly derived from 15-hydroperoxyeicosatetraenoic acid.     

Inhibitory effects of amphetamine on potassium-stimulated release of [3H]dopamine from striatal slices and synaptosomes

Amphetamine, 10(-7) M or greater, evoked the release of [3H]dopamine ([3H]DA) and inhibited subsequent K+-evoked [3H]DA release from striatal synaptosomes superfused at a flow rate (1 ml/min) that prevented reuptake. Amphetamine inhibited the K+-evoked release of [3H]DA to a lesser extent in striatal slices or in synaptosomes superfused at a flow rate (0.35 ml/min) that allowed reuptake. The observed decrease in amphetamine inhibition of K+-evoked release was primarily due to amphetamine blocking [3H]DA reuptake. Interneuronal interactions may account for some of the inhibitory effects of amphetamine on K+-evoked release in the slice. Inhibition of K+-evoked release from either slices or synaptosomes was still evident when 10(-6) M amphetamine was removed from the superfusion buffer and the spontaneous release had returned to control levels. The presence of Ca++ during amphetamine exposure was required for subsequent inhibition of K+-evoked release in synaptosomes. Amphetamine in the presence of Ca++ did not affect the subsequent release of [3H]DA evoked by the Ca++ ionophore, A23187. Therefore, amphetamine inhibition of the K+-evoked release of [3H]DA cannot be explained by prior depletion of Ca++-releasable pools. Nifedipine, 1 microM, failed to block either the Ca++-dependent release of [3H]DA or the inhibition of K+-evoked release by amphetamine. However, 1 mM cobalt inhibited the Ca++-dependent release of [3H]DA by amphetamine and antagonized the inhibition of K+-evoked release after amphetamine exposure. This suggests that amphetamine may open voltage-dependent Ca++ channels sensitive to cobalt but not nifedipine. Amphetamine may desensitize these voltage-dependent Ca++ channels and inhibit their activation by K+ depolarization.     

Amphetamine-haloperidol interactions on striatal and mesolimbic tyrosine hydroxylase activity and dopamine metabolism

Rat striatal and mesolimbic dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid levels and the activity of tyrosine hydroxylase (TOH) were determined as a function of dose of amphetamine (AMPH) or haloperidol (HAL). AMPH increases DA levels with an ED50 of 0.6 mg/kg and decreases DOPAC and homovanillic acid levels with an ED50 of 1.5 mg/kg, suggesting two sites of action for the drug. HAL increases DOPAC and homovanillic acid levels and activates TOH all with an ED50 near 0.05 mg/kg and maximal activation at 0.2 mg/kg. AMPH administered subsequent to 0.2 mg/kg of HAL has no effect on mesolimbic TOH activity or DA metabolism. In contrast AMPH after HAL still increases striatal DA levels and, in addition, partially reverses the HAL-induced increases in both DOPAC levels and TOH activity, all with an ED50 near 0.6 mg/kg. The maximal reversal by AMPH is near 25% of the HAL-induced increases with doses of AMPH to 10 mg/kg. The ED50 for AMPH reversal and the extent of reversal are independent of HAL dose. The equivalent reversal by AMPH of HAL-induced increases in striatal TOH activity and DOPAC levels suggest a neuronally mediated phenomenon involving a noncompetitive interaction between the two drugs on the nigrostriatal pathway. The data are discussed in terms of sites of action for AMPH in both the striatum and the substantia nigra.     

Urinary dopamine in man and rat: effects of inorganic salts on dopamine excretion.

1. Plasma and urine free dopamine (3,4-dihydroxyphenethylamine) were measured in six normal male volunteer subjects and the urinary clearance of dopamine was calculated for each subject. 2. The excretion rates for free dopamine in man were greater than could be explained by simple renal clearance. It was concluded that free dopamine must, therefore, be formed in the kidney. 3. Changes in urinary dopamine excretion were studied in four groups of rats initially maintained on low sodium diet and then given equimolar dietary supplements of NaCl, NaHCO3, KCl or NH4Cl, to study the specificity of the previously observed increase in dopamine excretion after increased dietary NaCl. 4. The mean dopamine excretion increased significantly in rats given NaCl, KCl and NH4Cl, whereas dopamine excretion decreased in those given NaHCO3. 5. The failure of dopamine excretion to rise in response to loading with NaHCO3 was unexpected, and argues against a simple effect of volume expansion by the sodium ion. The increase in dopamine excretion with KCl and NH4Cl showed that this response was not specific to the sodium ion.     

Cationic modulation of human dopamine transporter: dopamine uptake and inhibition of uptake.

Effects of cations on dopamine (DA) uptake into cells expressing the human dopamine transporter and on inhibition of DA uptake by various substrates and inhibitors were investigated by using rotating disk electrode voltammetry. The Na(+) dependence of DA uptake varied with Na(+) substitutes, hyperbolic with Li(+), almost linear at 1 microM DA but hyperbolic at 8 microM DA with choline, and sigmoidal with K(+). With Na(+) substituted by Li(+), K([DA]) decreased and V(app) remained constant with increasing [Na(+)], whereas K([Na+]) decreased and V(app) increased with increasing [DA], suggesting an ordered sequence with Na(+) binding before DA. Similar trends for the Na(+)-DA interactions were observed in the presence of cocaine. Cocaine inhibited DA uptake solely by increasing K([DA]), with its K(i) not significantly different at 55 and 155 mM [Na(+)], whereas it inhibited Na(+) stimulation by reducing V(app) more than K([Na+]) at 1 microM DA, and V(app) only and less potently at 8 microM DA. Thus, cocaine may compete with DA, not with Na(+), for the transporter, and might not follow a strictly ordered reaction with Na(+). With Na(+) substituted by K(+), K([DA]) or K([Na+]) became insensitive to Na(+) or DA. K(+) impaired the DA uptake mainly by reducing V(app,) but affected cocaine inhibition by elevating K(i). Despite their different patterns for inhibiting DA uptake, nontransportable inhibitors cocaine, methylphenidate, mazindol, and 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenyl-2-propyl)piperazi ne (GBR12909) showed similarly modest Na(+) dependence in their K(i) values. In contrast, substrates DA, m-tyramine, and amphetamine displayed a similarly stronger Na(+) requirement for their apparent affinities.