The effect of ouabain on the release of [14C]acetylcholine and other substances from synaptosomes

Antinociceptive Synergistic Interaction Between Clonidine and Ouabain on Thermal Nociceptive Tests in the Rat

The antinociceptive effect produced by spinal injection of clonidine (an alpha(2)-adrenergic agonist) is mediated by a cholinergic mechanism. We aimed in the current study to evaluate the antinociceptive interaction between intrathecally administered ouabain, an inhibitor of Na(+), K(+)-ATPase, and clonidine. We used rats chronically implanted with lumbar intrathecal catheters to examine the ability of intrathecal clonidine and ouabain and the mixtures of clonidine-ouabain to alter tail-flick latency. To characterize the interaction, isobolographic analysis was performed. Intrathecal clonidine (0.5-10 microg) and ouabain (0.1-5 microg) produced significant dose- and time-dependent antinociception in the tail-flick tests. The median effective dose (ED(50)) values for intrathecally administered ouabain and clonidine were 2.3 microg and 4.7 microg, respectively. The experimental point for the ouabain-clonidine combination decreased significantly (P < .05) below the lines of additivity. Isobolographic analysis exhibited a synergistic interaction after the coadministration of ouabain and clonidine. No motor impairment was observed in the animals after intrathecal administration of the combination of ouabain and clonidine or clonidine alone. Intrathecal pretreatment with atropine but not yohimbine blocked the antinociceptive effect of ouabain and attenuated its interaction with spinal clonidine. These results suggest that the synergistic interaction of ouabain and clonidine were probably mediated, at least in part, via an enhancement of cholinergic transmission in the spinal nociceptive processing system.

PERSPECTIVE

Although intrathecal clonidine produces pronounced analgesia, antinociceptive doses of intrathecal clonidine produce several side effects, including hypotension, bradycardia, and sedation. This article presents antinociceptive synergistic interaction between clonidine and ouabain on thermal nociceptive tests in the rat.

Comparative effects of aluminum and ouabain on synaptosomal choline uptake, acetylcholine release and (Na+/K+)ATPase

Closing the gap between adverse health effects of aluminum and its mechanisms of action still represents a huge challenge. Cholinergic dysfunction has been implicated in neuronal injury induced by aluminum. Previously reported data also indicate that in vivo and in vitro exposure to aluminum inhibits the mammalian (Na(+)/K(+))ATPase, an ubiquitous plasma membrane pump. This study was undertaken with the specific aim of determining whether in vitro exposure to AlCl(3) and ouabain, the foremost utilized selective inhibitor of (Na(+)/K(+))ATPase, induce similar functional modifications of cholinergic presynaptic nerve terminals, by comparing their effects on choline uptake, acetylcholine release and (Na(+)/K(+))ATPase activity, on subcellular fractions enriched in synaptic nerve endings isolated from rat brain, cuttlefish optic lobe and torpedo electric organ. Results obtained show that choline uptake by rat synaptosomes was inhibited by submillimolar AlCl(3), whereas the amount of choline taken up by synaptosomes isolated from cuttlefish and torpedo remained unchanged. Conversely, choline uptake was reduced by ouabain to a large extent in all synaptosomal preparations analyzed. In contrast to ouabain, which modified the K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions, AlCl(3) induced reduction of stimulated acetylcholine release was only observed when rat synaptosomes were challenged. Finally, it was observed that the aluminum effect on cuttlefish and torpedo synaptosomal (Na(+)/K(+))ATPase activity was slight when compared to its inhibitory action on mammalian (Na(+)/K(+))ATPase. In conclusion, inhibition of (Na(+)/K(+))ATPase by AlCl(3) and ouabain jeopardized the high-affinity (Na(+)-dependent, hemicholinium-3 sensitive) uptake of choline and the Ca(2+)-dependent, K(+) depolarization evoked release of acetylcholine by rat, cuttlefish and torpedo synaptosomal fractions. The effects of submillimolar AlCl(3) on choline uptake and acetylcholine release only resembled those of ouabain when rat synaptosomes were assayed. Therefore, important differences were found between the species regarding the cholinotoxic action of aluminum. The variability of (Na(+)/K(+))ATPase sensitivity to aluminum of cholinergic neurons might contribute to their differential susceptibility to this neurotoxic agent.

Brain Na+,K+-ATPase isozyme activity and protein expression in ouabain-induced hypertension

In normotensive rats, chronic infusion of exogenous ouabain causes hypertension involving central mechanisms. To determine whether ouabain-induced hypertension is associated with specific changes in brain Na+,K+-ATPase activity and expression, we assessed brain Na+,K+-ATPase isozyme activity and protein expression in rats treated with ouabain (50 microg/day s.c. or 10 microg/day i.c.v. for 14 days). Resting mean arterial pressure (MAP) was higher in s.c.- and i.c.v.-ouabain-treated animals vs. control (124+/-2 vs. 105+/-2 and 130+/-2 vs. 109+/-2, respectively, p<0.01). Ouabain infused s.c. or i.c.v. for 14 days had no effect on Na+,K+-ATPase isozyme activity in hypothalamic, pontine/medullary or cortical microsomes. However, the percent increase in total Na+,K+-ATPase activity produced in vitro by antibody Fab fragments that bind ouabain with high affinity (Digibind) was two-fold greater in s.c.- and i.c.v.-ouabain-treated rats vs. control, but only in hypothalamic microsomes. Thus, ouabain infused s.c. or i.c.v. does appear to directly inhibit Na+,K+-ATPase activity in the hypothalamus. On the other hand, in the hypothalamus, s.c.- and i.c.v.-ouabain infusions tended to increase alpha3 (by 30-44%), but had no effect on alpha1 or alpha2 Na+,K+-ATPase isozyme protein expression. In addition, ouabain was found to partially dissociate from the Na+,K+-ATPase enzyme following sample processing. Thus, the inability to detect a decrease in enzyme activity in the hypothalamus in response to ouabain may be due, in part, to an increase in enzyme expression and the dissociation of ouabain during sample processing.

Regulation of somatostatin gene expression by veratridine-induced depolarization in cultured fetal cerebrocortical cells

The stimulatory effect of veratridine (VTD) depolarization upon somatostatin mRNA (SS mRNA) levels in primary cultures of fetal cerebrocortical cells was analyzed. Depolarizing stimuli, such as 100 microM VTD exposure for 30 min, elicited an increase in immunoreactive somatostatin (IR-SS) release to the media without affecting SS mRNA levels. These levels increased when exposure to depolarization stimuli was prolonged up to 3 or more hours. At this time, veratridine acted as a secretagogue, stimulating somatostatin secretion, but was also effective in stimulating somatostatin mRNA levels. These changes were blunted by the Na+ channel blockade tetrodotoxin (TTX), and partially abolished by the Ca2+ channel antagonist, verapamil (VPM). To study whether VTD may affect mRNA stability we determine the rate of disappearance of SS mRNA after inhibition of transcription by actinomycin D and demonstrated that VTD stimulation did not stabilize the SS mRNA. These results indicate that the induction of SS mRNA expression by VTD involves the modulation of Ca2+ and Na+ channels. The time course study confirmed that the VTD-induced SS mRNA accumulation is time-dependent, and requires a prolonged exposure to stimulate SS gene expression. VTD stimulation does not modify the SS mRNA rate of degradation.

Excitatory actions of dehydroabietic acid on mammalian synaptosomes

The presynaptic effects of dehydroabietic acid were investigated using mouse brain synaptosomes as the in vitro model. At concentrations ranging from 10 to 100 microM, dehydroabietic acid depolarises the synaptosomal membrane and causes pronounced release of the neurotransmitters gamma-aminobutyric acid, acetylcholine and L-glutamate. The effects on membrane potential and transmitter release occur predominantly at concentrations below which any lytic actions of this compound can be detected. Dehydroabietic acid-induced depolarisation and release of neurotransmitters are not influenced by tetrodotoxin and are associated with only marginal inhibition of membrane-associated ATPase activity. When synaptosomes are challenged with dehydroabietic acid in calcium-free saline, a partial reduction in the stimulated release of transmitters is observed. These results provide clear evidence that dehydroabietic acid is neuroactive and capable of causing substantial increases in the release of both excitatory and inhibitory neurotransmitter substances. The mechanism which underlies the neuroexcitatory effects of dehydroabietic acid remains to be resolved however it is proposed that release of transmitters from central nerve endings may be a contributory factor in the toxicity of resin acids.

Brain lesions induced by specific and non-specific inhibitors of sodium-potassium ATPase

The cytotoxicity in the brain of potent selective and non-selective inhibitors of Na+/K+ ATPase has been assessed. Following injection of cardiac glycosides into the dorsal hippocampus of rats, the extent of neuronal loss roughly paralleled their potencies as inhibitors of the enzyme. Dihydroouabain was less potent than ouabain as a cytotoxin by an order of magnitude, similar to their relative affinities for Na+/K+ ATPase. The non-specific inhibitors, melittin, erythrosin B and zinc (chloride) were less neurocytotoxic than the selective inhibitors having equivalent potencies. The toxicity of a low dose of ouabain appeared to be selective for neuronal perikarya as staining for acetylcholinesterase (present on the nerve terminals of the afferent cholinergic innervation) was unaffected. A higher dose of ouabain caused a non-specific necrosis including damage to the neuropil and a loss of cholinesterase staining. Concurrently, there was an invasion of the tissue by cells resembling foaming macrophages. Other inhibitors of the enzyme caused a mixed pathology with both types of responses evident. It is suggested that the pathological response may depend on the relative degrees to which the glial and neuronal activities of the enzyme are affected.

Mobilization of a vesamicol-insensitive pool of acetylcholine from a sympathetic ganglion by ouabain

These experiments investigate the release of transmitter from the perfused superior cervical ganglia of cats induced by ouabain in the absence or presence of 2-(4-phenylpiperidino)cyclohexanol (vesamicol), a blocker of acetylcholine (ACh) uptake. Ouabain, perfused through the ganglia, released ACh in a Ca(2+)-dependent way. Vesamicol caused some inhibition of the release of ACh by ouabain; however, under this condition, the Na+,K(+)-ATPase inhibitor released five times more transmitter than did preganglionic stimulation at 5 Hz. Also, when ganglia exposed to vesamicol were depleted of the impulse-releasable pool of ACh, subsequent perfusion with ouabain released ACh, and this included ACh newly synthesized in the presence of vesamicol; this phenomenon could be inhibited by the lack of Ca2+ and presence of EGTA, and was completely abolished by perfusion with a medium containing 18 mM Mg2+. To test whether the release of this vesamicol-insensitive Ca(2+)-dependent pool by ouabain is associated with a decrease in the number of synaptic vesicles, ganglia treated with the ATPase inhibitor after the depletion of the impulse-releasable pool of ACh were fixed for electron microscopy. In the presence of Ca2+, coincident with the release of the vesamicol-insensitive pool of ACh, nerve terminals were almost depleted of synaptic vesicles; ganglia treated similarly, but with medium containing 18 mM Mg2+ instead of Ca2+, were not depleted of synaptic vesicles. These results suggest that ouabain releases a vesamicol-insensitive pool of ACh from the sympathetic ganglion and also support the notion that this compartment is vesicular and its exocytosis depends on extracellular Ca2+. It is suggested that empty-vesicle recycling in the presence of vesamicol restricts mobilization of full vesicles to release sites.

Lack of involvement of [Ca2+]i in the external Ca(2+)-independent release of acetylcholine evoked by veratridine, ouabain and alpha-latrotoxin: possible role of [Na+]i

Synaptosomes were challenged by veratridine, ouabain and alpha-latrotoxin, and the release of 14C-acetylcholine (ACh) was measured in the absence of external Ca2+. We wished to test whether Ca2+ mobilized from internal stores triggered the ACh release that was independent of external Ca2+. We found that none of the agents altered the [Ca2+]i in a Ca(2+)-free medium. Buffering the intracellular Ca2+ concentration with BAPTA did not prevent the increase in release of 14C-ACh by veratridine or ouabain in the absence of Ca2+, however, it greatly reduced the release evoked in a Ca(2+)-containing medium. In parallel samples the release of ACh and the change in the internal Na+ concentration ([Na+]i) were measured. It was found that veratridine, ouabain and alpha-latrotoxin all enhanced [Na+]i in a concentration-dependent manner and a good quantitative relationship existed between the increase in [Na+]i and the release of ACh.

On the mechanism of ouabain-induced release of acetylcholine from synaptosomes

Ouabain (5 x 10(-8)-5 x 10(-4) M) was confirmed to cause a dose-dependent increase in [3H]acetylcholine ([3H]ACh) release, cytosolic free Ca2+ concentration ([Ca2+]i), and 22Na+ uptake in cerebrocortical synaptosomes of rats in the presence of extracellular Ca2+. Ouabain also caused a dose-dependent decrease in membrane potential. In a low-Na+ (10 mM) medium, ouabain failed to increase [3H]ACh release and [Ca2+]i. Tetrodotoxin (10(-6) M) had no effect on the ouabain-induced increase in both [3H]ACh release and [Ca2+]i but abolished the increase in 22Na+ uptake and partially inhibited the depolarizing effect. Verapamil (10(-6)-5 x 10(-4) M) inhibited the ouabain-induced increase in both [3H]ACh release and [Ca2+]i in a dose-dependent manner. Removal of extracellular Ca2+ abolished the effect of ouabain on [Ca2+]i but not on [3H]ACh release and 22Na+ uptake, regardless of the presence or absence of EGTA. In the absence of extracellular Ca2+, 10 mM Mg2+ blocked ouabain-induced [3H]ACh release, which was resistant to verapamil. These results suggest that ouabain can increase ACh release from synaptosomes without the preceding increases in intracellular Ca2+ and/or Na+ content. It seems likely that the removal of extracellular Ca2+ unmasks mechanisms of ouabain action different from those operating in the presence of Ca2+.

Inhibition of sodium-potassium-ATPase: a potentially ubiquitous mechanism contributing to central nervous system neuropathology

Direct and indirect evidence suggests that Na+/K(+)-ATPase activity is reduced or insufficient to maintain ionic balances during and immediately after episodes of ischemia, hypoglycemia, epilepsy, and after administration of excitotoxins (glutamate agonists). Recent results show that inhibition of this enzyme results in neuronal death, and thus a hypothesis is proposed that a reduction and/or inhibition of this enzyme contributes to producing the central neuropathy found in the above disorders, and identifies potential mechanisms involved. While the extent of inhibition of Na+/K(+)-ATPase during ischemia, hypoglycemia and epilepsy may be insufficient to cause neuronal death by itself, unless the inhibition is severe and prolonged, there are a number of interactions which can lead to a potentiation of the neurotoxic actions of glutamate, a prime candidate for causing part of the damage following trauma. Presynaptically, inhibition of the Na+/K(+)-ATPase destroys the sodium gradient which drives the uptake of acidic amino acids and a number of other neurotransmitters. This results in both a block of reuptake and a stimulation of the release not only of glutamate but also of other neurotransmitters which modulate the neurotoxicity of glutamate. An exocytotic release of glutamate can also occur as inhibition of the enzyme causes depolarization of the membrane, but exocytosis is only possible when ATP levels are sufficiently high. Postsynaptically, the depolarization could alleviate the magnesium block of NMDA receptors, a major mechanism for glutamate-induced neurotoxicity, while massive depolarization results in seizure activity. With less severe inhibition, the retention of sodium results in osmotic swelling and possible cellular lysis. A build-up of intracellular calcium also occurs via voltage-gated calcium channels following depolarization and as a consequence of a failure of the sodium-calcium exchange system, maintained by the sodium gradient.

Ca2+o-independent veratridine-evoked acetylcholine release from striatal slices is not inhibited by vesamicol (AH5183): mobilization of distinct transmitter pools

The effect of 2-(4-phenylpiperidino)cyclohexanol (AH5183 or vesamicol), a compound known to block the uptake of acetylcholine (ACh) into cholinergic synaptic vesicles, on the release of endogenous and [14C]ACh from slices of rat striatum was investigated. ACh release was evoked either by electrical stimulation or by veratridine. The effect of electrical stimulation was entirely dependent on external Ca2+. By contrast, veratridine (40 microM) also enhanced ACh release in the absence of Ca2+. Indeed, with veratridine two components were clearly distinguished: one dependent on external Ca2+ and the other not. Vesamicol inhibited [14C]ACh release evoked by both veratridine and electrical stimulation in the presence of external Ca2+, provided it was added to the tissue prior to loading with [14C]choline. With the same treatment vesamicol only slightly affected the release of endogenous ACh. Under the same conditions the Ca2(+)-independent [14C]ACh release evoked by veratridine was not prevented by vesamicol. The differential responsiveness to vesamicol suggests that ACh pools involved in Ca2+o-dependent ACh release are different from those mobilized during Ca2+o-independent ACh release.

In vivo mechanisms underlying dopamine release from rat nigrostriatal terminals: I. Studies using veratrine and ouabain

The in vivo mechanisms underlying the dopamine (DA)-releasing actions of veratrine and ouabain in the striatum of halothane-anaesthetised rats have been investigated using brain microdialysis. Relevant catecholamines and indoleamines were separated and quantified using HPLC combined with an electrochemical detection system. Veratrine (10 micrograms/ml-1 mg/ml) and ouabain (10 microM-1 mM) were added to the medium perfusing the dialysis probes. Both compounds increased dialysate DA content in a dose-related manner. Dialysate levels of the DA metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid were reduced by both veratrine and ouabain. Veratrine-induced DA efflux was maximal in the first 20-min sample collected after drug infusion began, whereas the maximal effect of ouabain was not observed until 20-40 min after administration began. Veratrine-induced DA efflux was unaffected by systemic injection of the DA uptake inhibitor nomifensine but was inhibited by either coperfusion of tetrodotoxin (TTX) or removal of calcium from the perfusing buffer. These data suggest that veratrine induces release of DA via a carrier-independent mechanism, perhaps involving an exocytotic release process. In contrast, ouabain-induced DA release was reduced by nomifensine but was inhibited to a lesser degree by calcium depletion and TTX. Detailed analyses of these data suggest that although ouabain initially induces release of DA via a carrier-dependent mechanism, an exocytotic process may also be involved. The finding that ouabain-induced DA efflux exhibits a degree of TTX and calcium sensitivity suggests that membrane depolarisation caused by Na+,K(+)-ATPase blockade opens voltage-gated sodium channels and initiates an exocytotic release of DA. The intracellular pools of DA involved in the release of DA induced by veratrine and ouabain were also examined. Depletion of vesicular pools of DA by pretreatment with reserpine reduced the amount of DA release induced by both agents, although this effect was only significant in the case of veratrine. However, in reserpinised animals the residual amount of DA release induced by veratrine was inhibited by nomifensine, a result suggesting that DA may be released via a carrier-dependent process in the absence of vesicular DA. Newly synthesised pools of DA were also depleted by pretreatment with the DA synthesis inhibitor alpha-methyl-p-tyrosine. Under these conditions, both veratrine- and ouabain-induced DA efflux was reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

[3H]acetylcholine release and the change in cytosolic free calcium level induced by high K+ and ouabain in rat brain synaptosomes

High K+ (50 mM) increased both [3H]acetylcholine ([3H]ACh) release and cytosolic free calcium level ([Ca2+]i) in rat brain synaptosomes in the presence of extracellular Ca2+. Ouabain (5 x 10(-8) to 5 x 10(-4) M) also caused a dose-dependent increase in [3H]ACh release, but not in [Ca2+]i, in the absence of Ca2+. The effects of high K+ and ouabain on [3H]ACh and/or [Ca2+]i, were inhibited by the intracellular Ca2+ antagonist TMB-8 (10(-4) M). These results suggest that unlike high K+, ouabain increases transmitter release from nerve endings through a mechanism which is independent of [Ca2+]i, but sensitive to TMB-8.

Characterization of the effect of monensin on gamma-amino-n-butyric acid release from isolated nerve terminals

The action of the polyether antibiotic monensin on the release of gamma-[3H]amino-n-butyric acid [( 3H]GABA) from mouse brain synaptosomes is characterized. Monensin enhances the release of this amino acid transmitter in a dose-dependent manner and does not modify the efflux of the nontransmitter amino acid alpha-[3H]aminoisobutyrate. The absence of external Ca2+ fails to prevent the stimulatory effect of monensin on [3H]GABA release. Furthermore, monensin is less effective in stimulating [3H]GABA release in the presence of Ca2+. The releasing response to monensin is absolutely dependent on external Na+. The blockade of voltage-sensitive Na+ or Ca2+ channels does not modify monensin-induced release of the transmitter. Also, the blockade of the GABA uptake pathway fails to prevent the stimulatory effect of monensin on [3H]GABA release. Although monensin markedly increases Na+ permeability in synaptosomes, these data indicate that the Ca2+-independent monensin-stimulated transmitter release is not mediated by the Na+-dependent uptake pathway. It is concluded that the entrance of Na+ through monensin molecules inserted in the presynaptic membrane might be sufficient to initiate the intraterminal molecular events underlying transmitter release.

Ouabain induces acetylcholine release from pure cholinergic synaptosomes independently of extracellular calcium concentration

We have studied the correlation between [3H]ouabain binding sites, (Na+ + K+)ATPase (EC 3.6.1.3) activity and acetylcholine (ACh) release in different subcellular fractions of Torpedo marmorata electric organ (homogenate, synaptosomes, presynaptic plasma membranes). Presynaptic plasma membranes contained the greater number of [3H]ouabain binding sites, in good agreement with the high (Na+ + K+)ATPase activity found in this fraction. Blockade of this enzymatic activity by ouabain dose-dependently induced ACh release from pure cholinergic synaptosomes, either in the presence or absence of extracellular calcium ions. We suggest that one of the mechanisms involved in the ouabain-induced ACh release in the absence of Ca2+o may be an increase in Na+i that could (a) evoke Ca2+ release from internal stores and (b) inhibit ATP-dependent Ca2+ uptake by synaptic vesicles.

The packing of acetylcholine into quanta at the frog neuromuscular junction is inhibited by increases in intracellular sodium

Pretreatment with hypertonic solutions, insulin, or adrenaline increases the size of quanta at the frog neuromuscular junction, as determined by measurements of miniature end plate potentials or currents (Van der Kloot and Van der Kloot 1985, 1986). The increase in quantal size apparently is due to an increase in acetylcholine (ACh) content of individual quanta. These treatments, therefore, can be used to study the packaging of ACh. Previously, I reported that increases are blocked by an inhibitor of active ACh uptake into vesicles (Van der Kloot 1986b, 1987b). The present study shows that the increases in quantal size were antagonized by inhibiting the Na+-K+ exchange pump with 100 microM ouabain, 10 microM dihydroouabain, or K+-free solutions. The increases in quantal size were also antagonized by 10 microM monensin, a Na+ ionophore, or by 5 microM aconitine, which opens Na+ channels at normal resting potentials. Apparently a rise in intracellular [Na+] inhibits the addition of ACh to quanta. The mechanism by which a rise in intracellular Na+ inhibits ACh packing is unknown, but apparently it is not due to inhibition of choline reuptake into the terminals. Also consistent with the above hypothesis is that the increase in quantal size following depolarization for 2 h in elevated [K+]out was substantially enhanced when tetrodotoxin (TTX) was present, suggesting that in the absence of TTX there is a rise in [Na+]in that antagonizes the incorporation of additional ACh into the quanta.

The neurosecretory effect of ouabain on isolated rabbit ileal mucosa

Ouabain, when added to fluid bathing rabbit ileal mucosa mounted in a flux chamber, transiently increases short circuit current, implying a paradoxical secretory response. To determine the cause of this change, we studied unidirectional fluxes of 36Cl and 23Na and the effects of ion substitution, of reduced Ca concentration, verapamil, tetrodotoxin and atropine. Ouabain 0.1 mM, transiently increased the serosal to mucosal flux of Cl and Na, increased Isc and PD and reduced ion conductance. The Isc response to ouabain was diminished by reducing the bath fluid concentration of Cl, of Ca, and by adding verapamil. Tetrodotoxin both delayed and reduced the maximal Isc response; atropine had no effect. We conclude that ouabain acts by releasing a neurotransmitter of unknown identity and by increasing the serosal to mucosal flux of Cl.

Unsuitability of the 86Rb+ uptake method for estimation of (Na+ + K+)-ATPase activity in innervated tissues

(Na+ + K+)-ATPase activity was estimated by 86Rb+ uptake in dog saphenous vein to determine the validity of the technique in tissues that have a sympathetic innervation. When saphenous vein rings were incubated at 37 degrees C in Krebs' solution containing 86Rb+, the cardenolide acetylstrophanthidin caused a concentration-dependent inhibition of Rb+ uptake. The threshold for inhibition was approx. 10 nM acetylstrophanthidin and the maximum effect was obtained at 9 microM. In the upper part of this concentration range (greater than 1 microM) acetylstrophanthidin released noradrenaline from the sympathetic nerve terminals associated with the tissue. In this upper part of the acetylstrophanthidin concentration range the alpha-adrenoceptor antagonist phentolamine (8 microM) reduced, by up to 25%, the degree of 86Rb+ uptake inhibition caused by the cardenolide. In other experiments, saphenous vein strips were loaded with 86Rb+ and perifused with Krebs' solution containing acetylstrophanthidin. At concentrations which release noradrenaline, acetylstrophanthidin increased the efflux of 86Rb+. Phentolamine (8 microM) prevented the acetylstrophanthidin-evoked efflux of the isotope as did prior in vitro denervation of 86Rb+ loaded strips with 6-hydroxydopamine. Exogenous noradrenaline (1-100 microM) added to the perifusing fluid also caused an efflux of 86Rb+ that was attenuated by phentolamine. The data indicate for dog saphenous vein that with low concentrations of acetylstrophanthidin the extent of 86Rb+ accumulation might accurately reflect prevailing (Na+ + K+)-ATPase activity. At higher concentrations of acetylstrophanthidin, however, noradrenaline is released from the nerve endings and causes 86Rb+ efflux from the smooth muscle cells consequent upon alpha-adrenoceptor activation. Since this efflux reduces the extent of Rb+ accumulation, measurement of the latter does not adequately reflect uptake mediated by the activity of (Na+ + K+)-ATPase. This is significant because in most applications of the 86Rb+ uptake method it is the estimate of Rb+ accumulation made in the presence of a high concentration of cardenolide that forms the basis of all subsequent calculations with respect to (Na+ + K+)-ATPase activity.

Calcium uptake of rat brain synaptosomes as a function of membrane potential under different depolarizing conditions

The uptake of 45Ca2+ was measured in brain synaptosomes under conditions designed to depolarize the membranes. Membrane potential was estimated from the distribution of 86Rb+ between the intra- and extracellular compartments. K+ depolarization (8-60 mM) only increased "a2+ uptake beyond a threshold depolarization of about 10 mV, whereas veratridine (5-40 microM) induced an increased Ca2+ uptake at a concentration which depolarized the membrane less than this threshold. Ouabain did not enhance Ca2+ uptake, but the depolarization it produced did not reach threshold. Ca2+ influx already stimulated by K+ depolarization can be further enhanced by veratridine without any parallel change in membrane potential. Only the pathway mediating Ca2+ uptake during K+ depolarization can be inactivated: Ca2+ uptake evoked by K+ depolarization is decreased in synaptosomes pre-depolarized in the presence of a high concentration of K+. In contrast, pre-depolarization does not change Ca2+ uptake evoked by veratridine. Ca2+ channel blockers, such as verapamil and diltiazem but not nifedipine, in concentrations of 10-100 microM, decrease the stimulation of Ca2+ uptake by high K+ concentration without influencing depolarization, whereas the effect of veratridine on Ca2+ uptake is only inhibited when its effect on Na+ channels is also prevented. It is concluded that Ca2+ uptake during K+ depolarization proceeds through voltage-dependent Ca2+ channels similar to those of squid axon, whereas veratridine activates an additional Ca2+ entry, possibly via influx through open Na+ channels. Different quantitative relationships are found between acetylcholine release of synaptosomes and the amount of Ca2+ taken up by different mechanisms: the same amount of Ca2+ uptake is accompanied by a greater increase of acetylcholine release if the uptake is induced by K+ depolarization rather than veratridine.

Ouabain-induced changes in extracellular aspartate, glutamate and GABA levels in the rabbit olfactory bulb in vivo

The effect of ouabain on extracellular amino acid levels was investigated in the rabbit olfactory bulb using brain dialysis. Extracellular field potentials, elicited by stimulation of the lateral olfactory tract (LOT), were recorded simultaneously. Ouabain (100 microM) induced a rapid increase in extracellular aspartate, glutamate and gamma-aminobutyric acid. LOT-evoked potentials changed concomitantly, suggesting a neuronal depolarization.

Effects of inhibitors of Na+,K+-ATPase on the membrane potentials and neurotransmitter efflux in rat brain slices

The potassium potential EK, of rat brain slices was estimated by determining the uptake of 86Rb+. The ERb was the same for slices prepared from five rostral brain regions, the average value being 66.4 mV. The ERb values in the presence of 20 microM ouabain were only slightly lower than the resting values; increasing concentrations of ouabain above 20 microM resulted in a graded depolarization in all five brain regions. High concentrations (1 mM) of two other inhibitors of Na+,K+-ATPase, dihydro-ouabain and strophanthidin, produced no more depolarization than did 20 microM ouabain. Competitive binding studies indicated that the differential effects were due to the relative binding to brain slices. Erythrosin B, an inhibitor of Na+,K+-ATPase, had no measurable effect on ERb. Intermediate concentrations of the Na+/H+ ionophore monensin slightly hyperpolarized striatal slices, whereas the same monensin concentrations plus 20 microM ouabain, 1 mM strophanthidin or 70 microM erythrosin B resulted in marked depolarization. Measurement of the membrane potential via uptake of methyltriphenylphosphonium cation indicated that ERb was indeed a valid estimation of the membrane potential. EK was measured directly by monitoring 42K+ uptake in striatal slices and was found to be essentially identical to ERb. Uptake of 22Na+ was consistent with the values for ERb or EK. Several conditions that resulted in little or no measurable depolarization of striatal slices did induce efflux of exogenously loaded GABA and dopamine; these conditions included 20 microM ouabain, 1 mM dihydro-ouabain or strophanthidin, and 70 microM erythrosin B. Neurotransmitter efflux in the absence of general cell depolarization was not accompanied by altered rates of respiration or decreased ATP levels.

Release of acetylcholine from rat brain synaptosomes by various agents in the absence of external calcium ions

The relationship between 86Rb+ distribution across synaptosomal membrane and [14C]acetylcholine (ACh) release have been studied in a rat brain cortex synaptosomal preparation using K+, ouabain and veratridine depolarization. Decrease in membrane potential, approximated from the 86Rb+ distribution, is accompanied by an increase in [14C]ACh release, but the extent of the increase at a certain depolarization is dependent on how the depolarization is induced. A substantial depolarization by K+ is necessary to enhance ACh release, as compared to ouabain and veratridine where only a slight depolarization is accompanied by an increase in ACh release. In Ca2+-free, EGTA-containing medium ouabain and veratridine can also increase [14C]ACh release. The relationship between membrane potential and ACh release is very similar in the presence of ouabain and veratridine both in Ca2+-containing and Ca2+-free medium. The effect of ouabain and veratridine on the Na-K exchange pump is different; ouabain can completely abolish Na-K-ATPase activity and 86Rb+ uptake of synaptosomes, whereas veratridine does not seem to influence the activity of the pump. m-Chloro-carbonylcianid phenyl hydrazon (50-500 nM) increases [14C]ACh release in a concentration-dependent manner without a considerable change of membrane potential or Na-K pump activity. The Ca2+ ionophore A 23187 induces a substantial increase in [14C]ACh release in the absence of external Ca2+. In this case neither Na-K pump activity nor membrane potential of synaptosomes is changed. A possible role of intracellular Ca2+ mobilization as a consequence of increased intracellular Na+ concentration in some depolarization-induced transmitter release is discussed.

Characterization of transmitter release as a response of vertebrate neural tissue to erythrosin B

A rat cerebral cortical slice preparation was used to study the response of transmitter release to the application of the food dye, Erythrosin B, a tetraiodinated derivative of fluorescein. Erythrosin B (100 microM) stimulated net release of previously taken up [3H]norepinephrine and [3H]gamma-aminobutyric acid (GABA). The Erythrosin-induced release of GABA (the only transmitter studied) occurred in the absence of added Ca2+, and in the presence of tetrodotoxin (TTX). Ultrastructural analysis of the vesicle content of frog neuromuscular junctions treated with Erythrosin B revealed a diminution in the number of synaptic vesicles present in the nerve terminal. By using fluorescein and some halogen-substituted derivatives including Erythrosin B, it was found that incubation with the unhalogenated compound caused no net release, whereas incubation with the iodine-, chlorine- or bromine-substituted compound did cause release. It was also found that somewhat greater release induced by Erythrosin B (at 100 microM) occurred in the light than in the dark. That Erythrosin B inhibits the Na+,K+,Mg2+-ATPase was confirmed in this preparation; it did so in both light and dark. The discrepancy between release and Na+,K+,Mg2+-ATPase blockade in the dark suggests that release either occurs by some other mechanism than by Na+,K+,Mg2+-ATPase blockade, or that an additional light-dependent process contributes to the release. We conclude that Erythrosin B can presumably induce net release of transmitters generally, that release does not occur via the TTX-sensitive Na+ channel, that release via vesicles does occur, and that light somewhat enhances the release.

Role of synaptosomal Na-accumulation in transmitter release

The mechanism whereby Na+, K+-ATPase inhibitors such as ouabain trigger transmitter release in a calcium-independent manner remains obscure. We have examined the possible role of intra-synaptosomal sodium ion accumulation in ouabain-induced acetylcholine (ACh) release by: 1) Measuring 22Na accumulation in cat cortical synaptosomes in the presence of ouabain, A23187, veratridine, or strophanthidin over the same time course in which we previously determined their effects on ACh release; and 2) measuring synaptosomal 22Na accumulation and ACh-release in the presence of ouabain plus tetrodotoxin in normal or calcium-free buffer. Our results indicate that tetrodotoxin-dependent 22Na accumulation is at least partially responsible for ouabain-induced ACh release in normal and calcium-free media, but that this ion-accumulation per se is not sufficient to elicit release with other secretogogues.

Effect of ouabain and furosemide on pepsinogen secretion by gastric glands in vitro

Gastric glands were isolated from rabbit stomach and pepsinogen secretion was measured after stimulation with isoproterenol, forskolin, 8-bromo cyclic adenosine monophosphate (8-bromo cAMP), cholecystokinin octapeptide (CCK-OP), carbachol, and hyperosmolar medium. The responses to these stimuli in medium containing 143 mM Na+ and 5.4 mM K+ (normal medium) were compared with responses to the same stimuli in media containing either 0 Na+ and 5.4 mM K+, or 143 mM Na+ and O K+. In addition, the effects of ouabain and furosemide on secretion elicited by these stimuli were determined. Medium containing 0 Na+ inhibited all stimuli. Medium containing 0 K+ inhibited the action of 8-bromo cAMP and stimuli postulated to be mediated by cAMP. Ouabain inhibited the same stimuli as O K+ medium, and, in addition, inhibited the response to hyperosmolar medium. However, ouabain enhanced the response to CCK-OP. Furosemide inhibited the response to hyperosmolar medium but had no effect on the action of any secretagogue employed. Intraglandular [Na+] increased and [K+] decreased after exposure to K+-free medium or ouabain. cAMP content of the glands was assayed after stimulation with both isoproterenol and hyperosmolar medium. Isoproterenol and hyperosmolar medium significantly increased cAMP levels. The results are discussed in relation to possible involvement of ion transport or intracellular ion concentration in the secretory process.

Measurement of intrasynaptosomal free calcium by using the fluorescent indicator quin-2

The recently synthesized calcium indicator quin -2 was incorporated into synaptosomes from guinea-pig cerebral cortex following uptake and internal hydrolysis of quin -2 tetra-acetoxymethyl ester. Incubation in physiological media containing 1 mM- or 2 mM-CaCl2 led to equilibrium cytosolic ionized calcium concentrations of 85 +/- 10 nM and 205 +/- 5 nM respectively (mean +/- S.E.M. from eight and eighteen preparations respectively). Cytosolic Ca2+ was elevated following increases in external Ca2+ concentration, plasma membrane depolarization, mitochondrial inhibition, calcium ionophore addition or replacement of external sodium by lithium. Preliminary experiments were performed to assess changes in cytosolic Ca2+ accompanying the release of the neurotransmitter acetylcholine.

Effect of potassium on the release of [3H]noradrenaline from rabbit and human pulmonary artery

The release of [3H]noradrenaline ( [3H]NA) from rabbit and human isolated pulmonary artery has been measured. Removal of external potassium ions enhanced both the resting and stimulated release of [3H]NA from the strips. On adding K+ to tissues which had been suspended in K+-free Krebs solution, the release of [3H]NA was reduced in both stimulated and unstimulated tissues. Selective inhibition of presynaptic alpha 2-adrenoceptors by yohimbine significantly potentiated the release of [3H]NA evoked by stimulation in K+-free solution. The presynaptic inhibitory effect of NA was much less pronounced when the release was enhanced by the removal of external K+. Since the activity of NA, K-ATPase may be affected by removing K+ or by adding it to tissue previously kept in K+-free solution, the results may indicate involvement of the sodium pump in NA release.

The synthesis and release of acetylcholine in normal and denervated rat diaphragms during incubation in vitro

1. Normal and denervated rat diaphragms and neural (central) and aneural (peripheral) parts of normal diaphragms were incubated under several different conditions likely to affect the metabolism of acetylcholine (ACh), with the aim of discovering specific features of the control of neural and aneural ACh in the muscle. The concentrations of ACh in the tissue and the medium were measured at the end of the incubations using a radioenzymatic assay, and the amount of ACh synthesized during the incubations was calculated by subtracting the initial amount of ACh present in the tissue from that found in the tissue plus the medium at the end of the incubations.2. Confirming earlier results obtained with bioassays, it was found that, in a medium with 5 mM-K(+) and 2.5 mM-Ca(2+), denervated diaphragms released ACh into the medium at a rate equal to 47% of that observed in normal diaphragms; the amount of ACh released from aneural parts of normal diaphragms was 51% of that released from their neural parts. The release from normal diaphragms was increased (83%) in a Ca(2+)-dependent manner by raising the concentration of K(+) to 30 mM. In the denervated diaphragms, 30 mM-K(+) brought about a Ca(2+)-independent increase (67%) in the rate of ACh release. The elevation of K(+) was without effect on the release of ACh from aneural parts of normal diaphragms.3. The results indicate that a Ca(2+)-dependent mechanism of ACh release, known to function in the nerve terminals, is not likely to participate in the efflux of ACh from the muscle fibres. The K(+)-induced but Ca(2+)-independent enhancement of ACh release from the denervated diaphragms probably occurs by diffusion of ACh along the altered electrochemical gradient. It is suggested that the surface membranes of the muscle fibres become more permeable to ACh after denervation.4. During incubations with 30 mM-K(+) and 10 muM-hemicholinium-3 (HC-3), an inhibitor of the carrier-mediated transport of choline, the rates of ACh release and synthesis in normal diaphragms were diminished to the levels found in the denervated diaphragms, in which the concentration, release and synthesis of ACh were not affected by HC-3. The synthesis of aneural ACh thus appears to be independent of the carrier-mediated supply of choline across cell membranes.5. The release of ACh from normal diaphragms incubated with 5 mM-K(+) was increased in the presence of 100 muM-ouabain, whereas the release from denervated diaphragms was not affected. This finding suggests that the mechanism of ACh release that is activated by ouabain in the nerve cells involves, in addition to the inhibition of Na(+)-K(+)-ATPase, some other steps which are not operative in the muscle fibres.6. The results corroborate earlier evidence indicating that aneural ACh is produced, stored and released in the diaphragms. They fit the view that the aneural ACh is located in the cytoplasm of the muscle fibres and that it leaves the muscle fibres by molecular ;leakage' rather than by a specialized release mechanism. The efflux of ACh from the muscle fibres is likely to constitute about 50% of the total resting efflux (release) of ACh from normal diaphragms.

Stimulation by vanadate of [3H]noradrenaline release from rabbit pulmonary artery and its inhibition by noradrenaline

Vanadate, the +5 oxidation state of vanadium, present in mammalian tissues, even in nerve tissue, and a competitive inhibitor of NaK-ATPase, significantly enhanced the release of [3H]noradrenaline evoked from rabbit isolated perfused pulmonary artery by electrical stimulation. Its effect proved to be concentration-dependent. Noradrenaline (10(-6) M) reduced the vanadate-potentiated release of [3H]noradrenaline. The effect of noradrenaline is mediated via alpha 2-adrenoceptors as evidenced by the finding that yohimbine 3 x 10(-7) M prevented its action. The effect of vanadate was dependent on external K ions. When the effect of vanadate on [3H]noradrenaline release was studied under conditions when the NaK-ATPase enzyme activity was inhibited by removal of external K for 45 min, vanadate was ineffective. This finding indicates that the effect is related to the inhibition of NaK-ATPase activity, a condition known to result in transmitter release.