Effects of des-tyr1 -γ-endorphin on dopamine release from various rat brain regions in vitro

In rat striatum, nucleus accumbens and frontal cortex slices 6×10^?8M of the potential neuroleptic peptide des-Tyr-γ-endorphin (DTγE) did not affect basal dopamine release but depressed K⁺-evoked release. Haloperidol at 5×10^?6M increased both basal and K⁺-induced release in striatal and nucleus accumbens slices whereas it increased only basal dopamine release in frontal cortex slices. At 5×10^?8M haloperidol, however, had no effect. It is concluded that DTγE may decrease dopaminergic activity in the brain by depressing depolarization-induced dopamine release, possibly via a presynaptic mechanism.     

Comparative Effects of Hydrogen Sulfide-Releasing Compounds on [<Superscript>3</Superscript>H]D-Aspartate Release from Bovine Isolated Retinae

We investigated the pharmacological actions of a slow-releasing H₂S donor, GYY 4137; a substrate for the biosynthesis of H₂S, l-cysteine and its precursor, N-acetylcysteine on potassium (K⁺; 50 mM)-evoked [³H]D-aspartate release from bovine isolated retinae using the Superfusion Method. GYY 4137 (10 nM–10 µM), l-cysteine (100 nM–10 µM) and N-acetylcysteine (10 µM–1 mM) elicited a concentration-dependent decrease in K⁺-evoked [³H]D-aspartate release from isolated bovine retinae without affecting basal tritium efflux. At equimolar concentration of 10 µM, the rank order of activity was as follows: l-cysteine?>?GYY 4137?>?N-acetylcysteine. A dual inhibitor of the biosynthetic enzymes for H₂S, cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE), amino-oxyacetic acid (AOA; 3 mM) reversed the inhibitory responses caused by GYY 4137, l-cysteine and N-acetylcysteine on K⁺-evoked [³H]D-aspartate release. Glibenclamide (300 µM), an inhibitor of K_ATP channels blocked the inhibitory action of GYY 4137 and l-cysteine but not that elicited by N-acetylcysteine on K⁺-induced [³H]D-aspartate release. The inhibitory effect of GYY 4137 and l-cysteine on K⁺-evoked [³H]D-aspartate release was reversed by the non-specific inhibitor of nitric oxide synthase (NOS), l-NAME (300 µM). Furthermore, a specific inhibitor of inducible NOS (iNOS), aminoguanidine (10 µM) blocked the inhibitory action of l-cysteine on K⁺-evoked [³H]D-aspartate release. We conclude that both donors and substrates for H₂S production can inhibit amino acid neurotransmission in bovine isolated retinae, an effect that is dependent, at least in part, upon the intramural biosynthesis of this gas, and on the activity of K_ATP channels and NO synthase.     

INFLUENCES OF COLCHICINE, VINBLASTINE AND CYTOCHALASIN ON THE RELEASE OF 5-HYDROXYTRYPTAMINE FROM RAT BRAIN SYNAPTOSOMES

Rat brain synaptosomes preincubated with [³H]5-HT. were further incubated and the release of [³H]5-HT from the preparation was studied. The spontaneous release consisted of an initial rapid phase followed by slower release. Incubation with 60 mM-KCl increased the release while 60 mw-NaCl did not affect it. The effect of KG was abolished when NaCl was omitted from the medium. The potassium-induced release was Ca²⁺ -dependent while that induced by tyramine (10^?5-10^?4M) and the spontaneous release did not depend on Ca²⁺. Vinblastine (10^?5–2.5 X 10^?4 M) caused an increase in the spontaneous release and an decrease in the potassium-induced release, while it completely inhibited the release by tyramine at 2.5 X 10^?4 M. Colchicine (5 X 10^?5–10^?3M) and cytochalasin D (10^?5, 10^?4 M) failed to produce any change in the release. Cytochalasin B (10^?5, 10^?4M) increased the spontaneous release and decreased the potassium-induced release but it did not affect the release by tyramine. Colchicine (10^?3 M). vinblastine (10^?4 M) and cytochalasin B (10^?4 M) did not affect significantly Na⁺.K⁺-. Mg²- and Ca²⁺ -ATPase activities. These results suggest that none of microtubules. microfilaments and contractile protein participates in the mechanism of [³H]5-HT release from synaptosomes, in vitro.     

Effects of dexamethasone on K(+)-evoked glutamate release from rat hippocampal slices

Dexamethasone (DEX) at physiologically elevated (stress) concentration (1 µM) decreased K⁺-evoked glutamate release from rat hippocampal slices under superfusion in the presence of Ca²⁺. On the contrary 10 µM DEX increased this K⁺-evoked glutamate release while 0.1 µM DEX had no effect. The glucocorticoid antagonist for the classic receptor, RU 486, completely reversed the effect of 1 µM DEX. Actinomycin D had no effect. Dexamethasone at 1 µM had no effect on the Ca²⁺-independent (10 µM Mg²⁺ replacing 1 mM Ca²⁺) K⁺-evoked glutamate release. Dexamethasone at 1 µM or 10 µM had no effect on the phosphate-activated glutaminase—the key enzyme for the biosynthesis of neurotransmitter glutamate. These results suggest that the effect of DEX on K⁺-evoked glutamate release: (i) depends on its concentration; (ii) is exerted on the Ca²⁺-dependent (neurotransmitter release), at least at physiological stress concentrations; and (iii) is exerted via the classical receptor but is nongenomic.     

Anticonvulsants and trifluoperazine inhibit the evoked release of GABA from cerebral cortex of rat at different sites

The action of anticonvulsant drugs, phenytoin, diazepam, clonazepam and phenobarbitone, was tested on the release of [¹⁴C]-GABA from tissue slices of rat cerebral cortex. All drugs caused a significant dose-dependent depression of the 33mM-K⁺-evoked release of [¹⁴C]-GABA but had little effect on the resting release of [¹⁴C]-GABA, except at high concentrations. The IC₅₀ values for inhibition of K⁺-evoked release of [¹⁴C]-GABA were 4.7 × 10^?5, 7 × 10^?5, 28 × 10^?5 and 7.9 × 10^?4M for diazepam, clonazepam, phenytoin and phenobarbitone respectively. Trifluoperazine also caused a similar and complete inhibition of [¹⁴C]-GABA release with an IC₅₀ of 1 × 10^?5M. The effect of diazepam and trifluoperazine were additive. The inhibition by trifluoperazine could be overcome by addition of exogenous calmodulin, whereas that of diazepam, phenytoin or phenobarbitone was not overcome. It is proposed that the anticonvulsants tested inhibit calcium-dependent transmitter release at a site distal to the formation of a calcium-calmodulin complex, which is presumably activated by this complex. Trifluoperazine, on the other hand, acts by reducing the availability of calmodulin.     

Presynaptic Control of the Synthesis and Release of Dopamine from Striatal Synaptosomes: A Comparison Between the Effects of 5-Hydroxytryptamine, Acetylcholine, and Glutamate

The effects of 5-HT and glutamate on dopamine synthesis and release by striatal synaptosomes were investigated and compared with the action of acetylcholine, which acts presynaptically on this system. 5-HT inhibited (28%) synthesis of [¹⁴C]dopamine from L-[U-¹⁴C]tyrosine, at 10-⁵M and above. This contrasts with the action of acetylcholine, which stimulated [¹⁴C]-dopamine synthesis by 24% at 10-⁴ M. Tissue levels of GABA were unaffected by either 5-HT or acetylcholine up to concentrations of 10-⁴ M. The inhibitory action of 5-HT (5 × 10^?5 M and 2 × 10^?4 M) on [¹⁹C]dopamine synthesis was completely abolished by methysergide (2 × 10^?6 M). Higher concentrations of methysergide (10^?4 M) or cyproheptadine (10^?5 M) inhibited [¹⁴C]dopamine synthesis by 28% and 25%, respectively, when added alone to synaptosomes. However, only methysergide prevented the further inhibition of synthesis caused by 5-HT. At concentrations of 2 × 10^?5 M and above, 5-HT stimulated [¹⁴C]dopamine release. This releasing action differed from that of acetylcholine, which occurred at lower concentrations (e.g., 10^?6 M). Methysergide (up to 10^?4 M) or cyproheptadine (2 × 10^?4 M) did not reduce the 5-HT (5 × 10^?5 M)-induced release of [¹⁴C]dopamine, but methysergide (10^?4 M) showed a potentiation (49%) of this increased release. The stimulatory effects of 5-HT (2 × 10^?5 M) and K⁺ (56 mM) on [¹⁴C]dopamine release were additive, indicating that two separate mechanisms were involved. However, when both agents were present the stimulatory effect of K⁺ (56 mM) on [¹⁴C]dopamine synthesis was not seen above the inhibitory effect of 5-HT. Glutamate (0.1-5 mM) did not affect [⁴C]dopamine release or its synthesis from L-[U-¹⁴C]tyrosine. It is concluded that 5-HT modulates the synthesis of dopamine in striatal nerve terminals through a presynaptic receptor mechanism, an action antagonised by methysergide. The releasing action of 5-HT apparently occurs through a separate mechanism which is also distinct from that involved in the response to K⁺ depolarisation.     

Interleukin-2 Modulates Evoked Release of [3H]Dopamine in Rat Cultured Mesencephalic Cells

Abstract: Mesencephalic cell cultures were used as a model to investigate the effects of interleukin-2 (IL-2) on evoked release of [³H]dopamine ([³H]DA) and γ-[³H]-aminobutyric acid ([³H]GABA). At low concentrations (10^?13-10^?12M), IL-2 potentiated [³H]DA release evoked by the excitatory amino acids N-methyl-D-aspartate (NMDA) and kainate, whereas higher IL-2 concentrations (10^?9-10^?8M) had no effect. IL-2 (10^?14-10^?8M) modulated K⁺-evoked [³H]DA release in a biphasic manner, with low concentrations (10^?12-10^?11M) of IL-2 potentiating and higher concentrations (10^?9-10^?8M) inhibiting K⁺-induced [³H]DA release. IL-2 (10^?14-10^?8M) by itself failed to alter spontaneous [³H]DA release. The inhibition by IL-2 of K⁺-evoked [³H]DA release was reversible and not due to neurotoxicity, as preexposure to IL-2 (10^?8M) had no significant effect on the subsequent ability of dopaminergic cells to take up and to release [³H]DA. Under our experimental conditions, IL-2 (10^?8 M) did not alter Ca²⁺-independent [³H]GABA release evoked by either K⁺ or NMDA. The results of this study indicate that IL-2 is able to potentiate [³H]DA release evoked by a number of different stimuli, including K⁺ depolarization and activation of both NMDA and non-NMDA receptor subtypes in mesencephalic cell cultures. IL-2 is active at very low concentrations, a finding that indicates a potent effect of IL-2 on dopaminergic neurons and implicates a physiological role for this cytokine in the modulation of DA release.     

The ionophore X537A (Lasolocid) transiently increases acetylcholine release from rat brain invitro

The effect of X537A on acetylcholine (ACh) release was examined $\text{in vitro}$ in superfused slices of rat cerebrum and striatum. The ionophore (30 μM) induced a transient release of ACh which was not dependent on calcium in the medium. Also in contrast to K⁺-stimulated release, X537A-induced release was not sustained by 10^?5M choline in the superfusion medium and not inhibited by 5 × 10^?4M pentobarbital. The ionophore did not transport ACh or choline from an aqueous to an organic phase. Both K⁺ and X537A inhibited 1 μM (³H) choline uptake into striatal synaptosomes but this effect of X537A was more extensive and less reversible than that caused by K⁺. X537A did not inhibit choline acetyltransferase activity.     

Nicardipine-sensitive enhancement of high K+-evoked dopamine release in PC12 cells pretreated with 12-O-tetradecanoylphorbol 13-acetate

The effects of the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA) on stimulus-evoked dopamine release were studied in PC12 cells. Pretreatment of the cells with TPA resulted in an enhancement of dopamine release which could be further stimulated by high concentrations of K⁺, A23187, but not with carbamylcholine. TPA-dependent, high-K⁺-evoked enhancement of dopamine release was studied in detail: a maximum release was observed (169% of control) in response to 50 mM KCl upon treatment with 10⁻⁷ M TPA for 5 min at 37°C. This enhancement of dopamine release was associated with the concomitant reduction of the concentration rise of intracellular Ca²⁺ ([Ca²⁺]_i) induced by a high concentration of K⁺ monitored by a fluorescent indicator, fura2. Thus, these data provide an example for alteration in the efficiency of stimulus-secretion coupling as pointed out in our previous paper. Moreover, we have shown that nicardipine, CdCl₂, and CoCl₂ inhibit high-K⁺-evoked dopamine release more effectively in TPA treated cells than that of untreated cells, and that the TPA-dependent, high-K⁺-evoked dopamine release observed in TPA treated cells is completely abolished by the presence of nicardipine, Cd²⁺ or Co²⁺, but is only partially inhibited in the presence of verapamil. These relevant findings suggest the possible involvement of protein kinase C in regulating the efficiency of a high-K⁺-evoked dopamine release through the modification of nicardipine-sensitive Ca²⁺ channels.     

K+-evoked taurine efflux from cerebellar astrocytes: On the roles of Ca2+ and Na+

The ionic requirements for K⁺-evoked efflux of endogenous taurine from primary cerebellar astrocyte cultures were studied. The Ca²⁺ ionophore A23187 evoked taurine efflux in a dose-dependent fashion with a time-course identical to that of K⁺-induced efflux. The Ca²⁺-channel antagonist nifedipine had no effect upon efflux induced by 10 or 50 mM K⁺. In addition, verapamil did not antagonize 50 mM K⁺-evoked efflux except at high, non-pharmacological concentrations (>100 M), and preincubation with 2 M -conotoxin had no effect on 50 mM K⁺-evoked efflux. Similarly, preincubation with 1 mM ouabain had no effect on the amount of taurine released by K⁺ stimulation, but did accelerate the onset of efflux by 2–4 min. Although 2 M tetrodotoxin had no effect on K⁺-evoked release, replacing Na⁺ with choline abolished the taurine efflux seen in response to K⁺ stimulation. Together, these findings suggest that neuronal N- and L-type Ca²⁺- and voltage-dependent Na⁺-channels are not involved in the influx of Ca²⁺ which appears to be necessary for K⁺-evoked taurine efflux, and that in addition to Ca²⁺, extracellular Na⁺ is also required.     

Stimulation-evoked release of purines from the rabbit retina

The evoked release of purines from rabbit retinae preloaded with [³H]adenosine was studied in vitro. Potassium (8.6–43.6 mM) and ouabain (1 or 10 μM) increased the release of radioactivity in a concentration-dependent manner. The K⁺-evoked release was significantly reduced when the superfusion was carried out at 2–4°C. The effect of K⁺ (8.6, 13.6 and 23.6 mM) and of ouabain (1 μM) were completely abolished when the retinae were superfused with a Ca²⁺-free medium containing 0.1 mM EGTA. Calcium removal only partially reduced the effect of higher K⁺ and ouabain concentrations (43.6 mM and 10 μM, respectively). Further, the effect of K⁺ was found to be independent of extracellular Ca²⁺ when retinae were pretreated with ouabain for 30 min. Stimulation of the retina with light flashes induced a small, persistent increase in the release of radioactivity observable for several minutes after the end of stimulation.The superfusate contained mainly hypoxanthine and inosine. There were no significant changes in the relative proportions of the different purine compounds released before or in response to either K⁺ (23.6 mM) or ouabain (10 μM) stimulation. Potassium stimulation significantly increased the release of adenosine, inosine and hypoxanthine. Addition of the adenosine deaminase inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), significantly increased the relative proportions of released endogenous adenosine and inosine.The results indicate that K⁺ stimulation induces the release of purines from the rabbit retina by a Ca²⁺- and energy-dependent process. Light flashes also induce a purine release. The results suggest an active role for adenosine in retinal neurotransmission.     

Activation of adenosine A2 receptors enhances high K+-evoked taurine release from rat hippocampus: A microdialysis study

Summary The present study was designed to examine which type of adenosine receptors was involved in enhancement of high K⁺-evoked taurine release fromin vivo rat hippocampus using microdialysis. Perfusion with 0.5 or 5.0 mM adenosine enhanced high K⁺-evoked taurine release. Perfusion with 2M R(–)-N⁶-2-phenylisopropyladenosine (PIA), a selective adenosine A₁ receptor agonist, did not modulate taurine release. Perfusion with 1M 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective adenosine A₁ receptor antagonist, increased taurine release. On the other hand, perfusion with 20M 2-[4-(2-carboxyethyl)phenethylamino]-5-N-ethyl-carboxamideadenosine (CGS21680), a selective adenosine A_2A receptor agonist, enhanced taurine release, while perfusion with 1 mM 3,7-dimethyl-propagylxanthine (DMPX), an adenosine A₂ receptor antagonist, did not affect taurine release. These results demonstrate that adenosine enhances high K⁺-evoked taurine release via activation of adenosine A_2A receptors from both neurons and glial cells ofin vivo rat hippocampus.     

Microinjection of arginase into enzyme-deficient cells with the isolated glycoproteins of sendai virus as fusogen

Bumetanide is a potent diuretic drug which has some structural features in common with furosemide. The steady-state exchange of K⁺ and Cl^? was investigated in Ehrlich ascites tumor cells treated with bumetanide. This agent did not alter the cellular content of K⁺ or Cl^? but the self-exchange of both ions was depressed. K⁺ self-exchange was inhibited by 55% at bumetanide concentrations as low as 10^?6 M. Cl^? self-exchange was less sensitive to this drug but at low concentrations (between 10^?6 and 10^?3 M) bumetanide was a more effective inhibitor of Cl^? transfer than furosemide. The steady-state K⁺ flux of cells equilibrated in NO₃^? media was compared with the K⁺ flux in cells treated with 10^?4 or 10^?3 M bumetanide; the Cl^? -sensitive K⁺ exchange was equivalent to the bumetanide-sensitive K⁺ exchange. Since the results suggested that a bumetanide-sensitive (Cl^?, K⁺) cotransport could be operative in steady-state cells, the stoichiometry of the bumetanide-sensitive fluxes was determined by measuring Cl^? and K⁺ fluxes simultaneously in the same cell suspension. At $\text{5 · 10}{}^{\mathrm{?4}}$ and 10^?3 M bumetanide concentrations, the ratio of these fluxes was $\text{0.98 ? 0.07 (}\text{S.E.}\text{)}\text{and}\text{1.04 ? 0.06}$, respectively, consistent with the postulated cotransport mechanism. At 10^?4 and 10^?5 M, however, the ratio of the bumetanide-sensitive Cl^?/K⁺ flux was significantly less than 1.0. Since the magnitude of the bumetanide-sensitive K⁺ flux at 10^?4 M was close to that of the Cl^?-sensitive flux, a ratio of less than 1.0 at this drug level indicates that Cl^? sensitivity and drug sensitivity may not reflect inhibition of the same process under all circumstances.     

A microdialysis study in rat brain of dihydrokainate,a glutamate uptake inhibitor

Microdialysis in neostriatum of anaesthetized rats was performed to study effects on amino acid efflux of the glutamate uptake-inhibitor dihydrokainate (DHK). Both basal and K⁺-evoked (100 mM) efflux of glutamate increased in the presence of DHK. The increase in the basal glutamate efflux occurred at lower DHK concentrations than during K⁺-depolarization (when the extracellular glutamate concentration was several-fold higher), confirming that DHK is a competitive inhibitor. The increase in basal efflux caused by DHK did not exhibit Ca²⁺-dependency, whereas ∼50% of the increase in glutamate efflux during K⁺-depolarization was Ca²⁺-dependent. The Ca²⁺-dependent efflux is related to transmitter release, whereas the Ca²⁺-independent efflux is probably due to metabolic events and/or transport of DHK into cells in exchange for glutamate. Taurine efflux in response to DHK increased both during basal conditions and K⁺-depolarization, probably secondary to the increase in glutamate concentration, whereas aspartate, GABA, glutamine and alanine effluxes did not change.     

Activation of superoxide production and differential exocytosis in polymorphonuclear leukocytes by cytochalasins A,B, C,D and E: Effects of various ions

All of the common cytochalasins activate superoxide anion release and exocytosis of β-N-acetylglucosaminidase and lysozyme from guinea-pig polymorphonuclear leukocytes (neutrophils) incubated in a buffered sucrose medium. Half-maximal activation of both processes is produced by approx. 2 μM cytochalasin A, C >μM cytochalasin B ? 4–5 μM cytochalasin D, E. While maximal rates of O₂^? release and extents of exocytosis require extracellular calcium (1–2 mM), replacing sucrose with monovalent cation chlorides is inhibitory to neutrophil activation by cytochalasins. Na⁺, K⁺ or choline inhibited either cytochalasin B- or E-stimulated O₂^? production with IC₅₀ values of 5–10 mM and inhibition occurs whether Cl^?, NO₃^? or SCN^? is the anion added with Na⁺ or K⁺. Release of β-N-acetylglucosaminidase in control or cytochalasin B-stimulated cells is inhibited by NaCl (IC₅₀ ≈ 10 mM), while cytochalasin E-stimulated exocytosis is reduced less and K⁺ or choline chloride are ineffective in inhibiting either cytochalasin B- or E-stimulated exocytosis. Release of β-glucuronidase, myeloperoxidase or acid phosphatase from neutrophils incubated in buffered sucrose is not stimulated by cytochalasin B. Stimulation of either O₂^? or β-N-acetylglucosaminidase release by low concentrations of cytochalasin A is followed by inhibition of each at higher concentrations. It appears that all cytochalasins can activate both NAD(P)H oxidase and selective degranulation of neutrophils incubated in salt-restricted media and that differential inhibition of these two processes by monovalent cations and/or anions is produced at some step(s) subsequent to cytochalasin interaction with the cell.     

Insensitivity of lepidopteran tissues to ouabain: Physiological mechanisms for protection from cardiac glycosides

Neuronal tissues from Manduca sexta, the tobacco hornworm, Hyalophora cecropia, the silkmoth and Danaus plexippus, the Monarch Butterfly, contain Na⁺K⁺-ATPase which is sensitive to cardiac glycoside (ouabain). The K_m for K⁺ stimulation of Na⁺K⁺-ATPase in M. sexta and D. plexippus is 2.2 mM and for Na⁺ stimulation in D. plexippus, 6.0 mM. In vitro ouabain concentrations of 1.0 × 10^?5 M and 5.0 × 10^?5 M in the presence of 7.5 mM K⁺ inhibited Na⁺K⁺-ATPase activity in H. cecropia and M. sexta by 50% respectively. Na⁺K⁺-ATPase from D. plexippus was approximately 300 times less sensitive. High concentrations (10^?3 M in haemolymph) of ouabain had no effect on M. sexta in vivo. This is largely explained by haemolymph K⁺ (>; 30 mM) antagonizing the binding of ouabain to Na⁺K⁺-ATPase. As demonstrated in vitro, 30 mM K⁺ totally protects Na⁺K⁺-ATPase from inhibition by 7.5 × 10^?3 M ouabain in D. plexippus and protects the enzyme by 65% in M. sexta. At least part of the physiological burden incurred in utilization of cardiac glycoside ingestion and storage for protection from predation, however, is probably related to the toxic effects of cardiac glycosides on neuronal Na⁺K⁺-ATPase.     

Valinomycin inhibition of insulin release and alteration of the electrical properties of pancreatic B cells

The effects of valinomycin on insulin release and rubidium efflux from perifused isolated rat islets were investigated and correlated with its effects on the electrical properties of mouse B cells studied with microelectrode techniques. Valinomycin produced a (1 · 10^?9–1 · 10^?6 M) dose- and time-dependent inhibition of (10–15 mM) glucose-stimulated insulin release but did not affect basal secretion. This inhibitory effect rapidly followed addition of the ionophore and equally affected the two phases of glucose-stimulated secretion. It was not reversible by simple washing of the islets, but could be reversed transiently by tetraethylammonium or high extracellular potassium ion levels. At low or high glucose, valinomycin rapidly augmented the rate of ⁸⁶Rb⁺ efflux from preloaded islets. Amplitude and rapidity of this effect were dose-dependent and it was antagonized by tetraethylammonium. Glucose metabolism by islet cells was reduced only slightly (15%) by 1 · 10^?7 M valinomycin. During the first 6 to 8 min of valinomycin addition, the membrane potential of B cells augmented slowly but the typical bursts of spikes disappeared rapidly. Later on, B cells hyperpolarized more quickly to a stable value of approx. ?70 mV. Increasing extracellular K⁺ immediately depolarized B cells and the linear relationship found between the logarithm of K⁺ concentration and the membrane potential was characterized by a slope of 58 mV for a ten-fold increase in extracellular K⁺.These results suggest that valinomycin interferes with the insulin releasing effect of glucose by increasing the potassium permeability of the B cell membrane.     

Direct measurement of the effect of potassium,calcium, veratridine,and and amphetamine on the rate of release of dopamine from superfused brain tissue

The rate of release of endogenous DA from rat brain striatal minces has been measured using a rapid superfusion apparatus. The apparatus provides immediate, continuous readout of easily oxidized substances in the perfusate using an amperometric detector. Subsequent analysis of the perfusate (which contains pargyline) by liquid chromatography shows that the major substance detected is DA. DA release is induced by a 30 s exposure to 60 mM K⁺ and is Ca²⁺-dependent. Similar results are obtained with veratridine (10^?4 M). The time resolution of the perfusion system permits discrimination of the decreased rate of release induced by veratridine (10^?4 M) and amphetamine (10^?5 M) as opposed to 60 mM K⁺. Repetitive stimulation of the striatal mince with 60 mM K⁺ results in a decreased amount and rate of DA release. Subsequent exposure of the striatal mince to exogenous DA results in a restoration of the K⁺-induced, Ca²⁺-dependent release, indicating uptake of DA is operant under these conditions.     

Fluxes and compartmentation of K+, Na+ and Cl−, and action of auxins in suspension-cultured Petroselinum cells

Transport of ⁸⁶Rb⁺/K⁺, ²²Na⁺, ³⁶Cl^?, and [³H]indole acetic acid (IAA) has been studied on suspension-cultured cells of the parsley, Petroselinum crispum (Mill) Nym. By compartmental analysis two intracellular compartments of K⁺, Na⁺, and Cl^? have been identified and ascribed to the cytoplasm and vacuole; half-times of exchange were around 200 s and 5 h, respectively. According to the Ussing-Teorell flux equation, active transport is required for the influx into the cytoplasm at the plasmalemma (K⁺, Cl^?) and the tonoplast (K⁺, Na⁺, Cl^?). The plasmalemma permeability pattern, P_K:P_Na:P_Cl=1.00:0.24:0.38, features an increased chloride permeability compared with cells from higher plant tissues. IAA uptake showed an exponential timecourse, was half-maximal after 10 min, and a linear function of the IAA concentration from 10^?9 to 10^?5 M. IAA and 2,4-dichlorophenoxy acetic acid reduce the apparent influx of K⁺, Na⁺, Cl^? during the initial 30 min after addition and subsequently accelerate both in- and efflux of these ions. We discuss that auxins could affect the ion fluxes in a complex way, e.g. by protonophorous activity and by control of the hypothetical proton pump.     

The ontogeny of apomorphine-induced alterations of neostriatal dopamine release: Effects on potassium-evoked release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on K⁺-evoked dopamine release were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10–11, 21–22, 35–36 days of age, and adults, and quantified by high performance liquid chromatography with electrochemical detection. Apomorphine attenuated K⁺-evoked dopamine release in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine was observed in all age groups except at 5 and 10 days of age. Absolute levels of extracellular dopamine were significantly lower at 5 and 10 days of age compared with the other ages, and the effectiveness of a high-K⁺ artificial cerebrospinal fluid to evoke dopamine release increased with age.