Inhibitory action of Ruthenium red on neuromuscular transmission

The Polysite Pharmacology of TREK K2P Channels

K_2P (KCNK) potassium channels form "background" or "leak" currents that have critical roles in cell excitability control in the brain, cardiovascular system, and somatosensory neurons. Similar to many ion channel families, studies of K_2Ps have been limited by poor pharmacology. Of six K_2P subfamilies, the thermo- and mechanosensitive TREK subfamily comprising K_2P2.1 (TREK-1), K_2P4.1 (TRAAK), and K_2P10.1 (TREK-2) are the first to have structures determined for each subfamily member. These structural studies have revealed key architectural features that underlie K_2P function and have uncovered sites residing at every level of the channel structure with respect to the membrane where small molecules or lipids can control channel function. This polysite pharmacology within a relatively small (~70 kDa) ion channel comprises four structurally defined modulator binding sites that occur above (Keystone inhibitor site), at the level of (K_2P modulator pocket), and below (Fenestration and Modulatory lipid sites) the C-type selectivity filter gate that is at the heart of K_2P function. Uncovering this rich structural landscape provides the framework for understanding and developing subtype-selective modulators to probe K_2P function that may provide leads for drugs for anesthesia, pain, arrhythmia, ischemia, and migraine.

Ruthenium Complexes: An Alternative to Platinum Drugs in Colorectal Cancer Treatment

Colorectal cancer (CRC) is one of the intimidating causes of death around the world. CRC originated from mutations of tumor suppressor genes, proto-oncogenes and DNA repair genes. Though platinum (Pt)-based anticancer drugs have been widely used in the treatment of cancer, their toxicity and CRC cells' resistance to Pt drugs has piqued interest in the search for alternative metal-based drugs. Ruthenium (Ru)-based compounds displayed promising anticancer activity due to their unique chemical properties. Ru-complexes are reported to exert their anticancer activities in CRC cells by regulating different cell signaling pathways that are either directly or indirectly associated with cell growth, division, proliferation, and migration. Additionally, some Ru-based drug candidates showed higher potency compared to commercially available Pt-based anticancer drugs in CRC cell line models. Meanwhile Ru nanoparticles coupled with photosensitizers or anticancer agents have also shown theranostic potential towards CRC. Ru-nanoformulations improve drug efficacy, targeted drug delivery, immune activation, and biocompatibility, and therefore may be capable of overcoming some of the existing chemotherapeutic limitations. Among the potential Ru-based compounds, only Ru (III)-based drug NKP-1339 has undergone phase-Ib clinical trials in CRC treatment.

Polynuclear Ruthenium Amines Inhibit K2P Channels via a "Finger in the Dam" Mechanism

The trinuclear ruthenium amine ruthenium red (RuR) inhibits diverse ion channels, including K2P potassium channels, TRPs, the calcium uniporter, CALHMs, ryanodine receptors, and Piezos. Despite this extraordinary array, there is limited information for how RuR engages targets. Here, using X-ray crystallographic and electrophysiological studies of an RuR-sensitive K2P, K2P2.1 (TREK-1) I110D, we show that RuR acts by binding an acidic residue pair comprising the "Keystone inhibitor site" under the K2P CAP domain archway above the channel pore. We further establish that Ru360, a dinuclear ruthenium amine not known to affect K2Ps, inhibits RuR-sensitive K2Ps using the same mechanism. Structural knowledge enabled a generalizable design strategy for creating K2P RuR "super-responders" having nanomolar sensitivity. Together, the data define a "finger in the dam" inhibition mechanism acting at a novel K2P inhibitor binding site. These findings highlight the polysite nature of K2P pharmacology and provide a new framework for K2P inhibitor development.

Is hyperosmotic neurosecretion from motor nerve endings a calcium-dependent process?

Spontaneous liberation of neurotransmitter quanta is strongly affected by the osmotic pressure of the extracellular fluid. Elevation of the osmolarity by 20-30% increases the rate of release from motor nerve endings by more than one order of magnitude. In this respect the neuromuscular junction resembles some other secretory systems. The mechanism of this hyperosmotic neurosecretion is not yet understood; extracellular calcium ions are not directly responsible, since this effect can be produced in their absence. Recently, it has been suggested that the liberation of neurotransmitter is regulated by the intracellular concentration of free calcium ions. We have therefore examined the hypothesis that hyperosmotic neurosecretion originates from an increase in internal calcium concentration ([Ca]in). At the frog neuromuscular synapse however, it is impossible at present to estimate directly free [Ca]in; hence we used an indirect technique, which is based on two assumptions; first, the frequency of the miniature endplate potentials (m.e.p.p.s.) reflects free [Ca]in. Second, the movement of calcium ions across the presynaptic membrane is governed by the electrochemical gradient, and by the calcium conductance (g(Ca)). If hyperosmotic neurosecretion is caused by an increase in [Ca]in, then increasing g(Ca), under reversed electrochemical gradient for the calcium should cause a reduction in the effect of hyperosmotic stress on transmitter release. We report that hyperosmotic neurosecretion is dependent on [Ca]in.

Glycinergic mIPSCs in mouse and rat brainstem auditory nuclei: modulation by ruthenium red and the role of calcium stores

Spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in central neurons are usually highly variable in amplitude due to many factors such as intrinsic postsynaptic channel fluctuations at each release site, site-to-site variability between release sites, electrotonic attenuation due to variable dendritic locations of synapses, and the possibility of synchronous multivesicular release. A detailed knowledge of these factors is essential for the interpretation of mIPSC amplitude distributions and mean quantal size. We have studied glycinergic mIPSCs in two auditory brainstem nuclei, the rat anteroventral cochlear nucleus (AVCN) and the mouse medial nucleus of the trapezoid body (MNTB). Our previous results have demonstrated the location of glycinergic synapses on these neurons to be somatic, thus avoiding electrotonic complications. Spontaneous glycinergic mIPSCs were recorded from AVCN and MNTB neurons in brainstem slices, in the presence of TTX to block action potentials, and 6-cyano-7-nitroquinoxaline-2, 3-dione, (+/-)-2-amino-5-phosphonopentanoic acid and bicuculline to block glutamatergic and GABAergic synaptic currents. Ruthenium red (RuR), which was used to increase the frequency of mIPSCs, significantly changed the shape of most (90 %) mIPSC amplitude distributions by increasing the proportion of large-amplitude mIPSCs. The possibility was investigated (following previous evidence at GABAergic synapses) that large-amplitude glycinergic mIPSCs are due to synchronous multivesicular release initiated by presynaptic calcium sparks from ryanodine-sensitive calcium stores. Interval analysis of mIPSCs indicated that the number of potentially undetected (asynchrony < 0.5 ms) multivesicular mIPSCs was low in comparison with the number of large-amplitude mIPSCs. Ryanodine, thapsigargin and calcium-free perfusate did not reduce the frequency of large-amplitude mIPSCs (> 150 pA), arguing against a significant role for presynaptic calcium stores. Our results support previous evidence suggesting that RuR increases miniature postsynaptic current (mSC) frequency by a mechanism that does not involve presynaptic calcium stores. Our results also indicate that at glycinergic synapses in the AVCN and MNTB, site-to-site variability in mIPSC amplitude, rather than multivesicular release, is a major factor underlying the large range of amplitudes of glycinergic mIPSCs.

Spontaneous acetylcholine release in mammalian neuromuscular junctions

Spontaneous secretion of the neurotransmitter acetylcholine in mammalian neuromuscular synapsis depends on the Ca2+ content of nerve terminals. The Ca2+ electrochemical gradient favors the entry of this cation. We investigated the possible involvement of three voltage-dependent Ca2+ channels (VDCC) (L-, N-, and P/Q-types) on spontaneous transmitter, release at the rat neuromuscular junction. Miniature end-plate potential (MEPP) frequency was clearly reduced by 5 microM nifedipine, a blocker of the L-type VDCC, and to a lesser extent by the N-type VDCC blocker, omega-conotoxin GVIA (omega-CgTx, 5 microM). On the other hand, nifedipine and omega-CgTx had no effect on K(+)-induced transmitter secretion. omega-Agatoxin IVA (100 nM), a P/Q-type VDCC blocker, prevents acetylcholine release induced by K+ depolarization but failed to affect MEPP frequency in basal conditions. These results suggest that in the mammalian neuromuscular junction Ca2+ enters nerve terminals through at least three different channels, two of them (L- and N-types) mainly related to spontaneous acetylcholine release and the other (P/Q-type) mostly involved in depolarization-induced neurotransmitter release. Ca(2+)-binding molecule-related spontaneous release apparently binds Ca2+ very rapidly and would probably be located very close to Ca2+ channels, since the fast Ca2+ chelator (BAPTA-AM) significantly reduced MEPP frequency, whereas EGTA-AM, exhibiting slower kinetics, had a lower effect. The increase in MEPP frequency induced by exposing the preparation to hypertonic solutions was affected by neither external Ca2+ concentration nor L-, N-, and P/Q-type VDCC blockers, indicating that extracellular Ca2+ is not necessary to produce hyperosmotic neurosecretion. On the other hand, MEPP frequency was diminished by BAPTA-AM and EGTA-AM to the same extent, supporting the view that hypertonic response is promoted by "bulk" intracellular Ca2+ concentration increases.

Selective neurotoxicity of ruthenium red in primary cultures

The inorganic dye ruthenium red (RuR) has been shown to be neurotoxic in vivo when injected intracerebrally. In this work the toxicity of RuR was compared in primary cultures of rat cortical neurons, cerebellar granule neurons and cerebellar astroglia. Microscopic examination of the cultures revealed that RuR penetrates the somata of both types of neurons used and produces vacuolization and loss and fragmentation of neurites. In contrast, no RuR was seen inside cultured astrocytes and no morphological signs of damage were observed in these cells. RuR toxicity was also assessed by immunocytochemistry of alpha-tubulin and by biochemical measurement of the reduction of (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) by the cultured cells. The morphological alterations in the neurons were closely correlated with loss of tubulin immunoreactivity and particularly with a notable decrement in the ability to reduce MTT. Using the latter parameter, it was found that neuronal damage was independent of the age of the cultures, augmented progressively with time of incubation with RuR, from 8 to 24 h, and showed a clear dose-response curve from 20 to 100 microM RuR. Astrocytes showed only a slight decrease in MTT reduction after 24 h of incubation with 100 microM RuR. It is concluded that RuR seems to be toxic for neurons but not for astroglia, and that this selectivity is probably related to the ability of the neurons to internalize the dye. The possible mechanisms of RuR penetration and neuronal damage are discussed.

Effects of bipyridylium compounds on calcium release from triadic vesicles isolated from rabbit skeletal muscle

1. The effects of 1,1'-diheptyl-4,4'-bipyridinium dibromide (DHBP), a viologen for electrochromic memory display agent, on calcium release and ryanodine binding were studied with triad-rich sarcoplasmic reticulum (SR) vesicles isolated from rabbit skeletal muscle. 2. DHBP inhibited the calcium release induced by 2 mM caffeine and 2 micrograms ml-1 polylysine with an IC50 value of 5 micrograms ml-1 and 4 micrograms ml-1 respectively. 3. DHBP inhibited [3H]-ryanodine binding in a dose-dependent manner with an IC50 of 2.5 micrograms ml-1 and 90-100% inhibition at 20-30 micrograms ml-1. 4. Calcium uptake by SR was inhibited in the presence of caffeine and this inhibition was antagonized by concomitant addition of DHBP. 5. The effect of DHBP on muscle twitches was studied on the mouse diaphragm. Muscle twitches elicited by direct electrical muscle stimulation and contractions induced by either 10 mM caffeine or 1 microM ryanodine were blocked by pretreatment with DHBP. 6. Data from this study provided evidence that DHBP blocked the calcium release from SR by direct interaction with the calcium release channel, also known as the ryanodine receptor. A possible use of this agent as a specific inhibitor for calcium release and as a muscle relaxant was suggested.

Zymosan induced 45Ca uptake by retinal pigment epithelial cells

45Ca uptake was studied in isolated frog retinal pigment epithelial cells in response to the phagocytic stimuli, zymosan. 45Ca uptake was strongly stimulated immediately in the presence of zymosan particles. Calcium uptake was proportional to the zymosan concentrations. After 60 min in the presence of zymosan acid phosphatase and beta-glucuronidase activities showed a 25% and 50% increase, respectively. Rod outer segments induced a similar increase of these enzyme activities. The zymosan-induced lysosomal enzyme activities was inhibited by cytochalasin B and ruthenium red. The ionophore A23187 produced a remarkable increase in 45Ca uptake but did not affect the lysosomal enzyme activities. These results suggest that in vitro RPE cells are able to respond to zymosan as phagocytosable stimuli and that calcium mediate that response.

Ruthenium-red inhibits CGRP release by capsaicin and resiniferatoxin but not by ouabain, bradykinin or nicotine in guinea-pig heart: correlation with effects on cardiac contractility

1. The possible influence of ruthenium-red (RR) on contractility and outflow of calcitonin gene-related peptide (CGRP)-like and neuropeptide Y (NPY)-like immunoreactivity (LI) from the heart of the guinea-pig induced by capsaicin, resiniferatoxin, nicotine, ouabain or bradykinin was studied in vitro. 2. In the isolated right atrium, exposure to capsaicin evoked an increase in contractile rate and tension simultaneously with an enhanced outflow of CGRP-LI, indicating release from the atria. Repeated administration of capsaicin induced tachyphylaxis. Incubation with RR markedly attenuated the capsaicin-evoked release of CGRP-LI while no clear-cut effects were seen on contractile tension or rate. 3. In the isolated whole heart, perfusion with capsaicin induced an increased outflow of CGRP-LI and stimulated heart rate, while a negative inotropic effect was observed. A second administration of capsaicin to the same preparations failed to influence the CGRP-LI outflow and in these experiments the positive chronotropic effect was absent while the negative inotropic action remained unchanged. Capsaicin-perfusion in the presence of RR failed to induce any increased outflow of CGRP-LI from the hearts or changes in contractile activity. However, after 1 h of rinsing with Tyrode solution repeated capsaicin perfusion in the absence of RR caused a clear-cut (60% of control) release of CGRP-LI and contractile responses were restored. 4. Perfusion with resiniferatoxin evoked a RR-sensitive, clear-cut increased CGRP-LI output without any effects on contractile force or heart rate. Repeated administration of resiniferatoxin induced tachyphylaxis with respect to outflow. Capsaicin perfusion after resiniferatoxin did not influence cardiac rate, force or CGRP-LI outflow suggesting development of cross-tachyphylaxis. 5. Perfusion with RR did not influence the outflow of CGRP-LI or contractility changes evoked by perfusion with nicotine, ouabain or bradykinin. In addition, the release of NPY-LI by nicotine remained unchanged in the presence of RR. Furthermore, the positive chronotropic effect of human CGRP alpha remained intact in the presence of RR. 6. It is concluded that RR selectively inhibits capsaicin- and resiniferatoxin-induced excitation of cardiac sensory nerves as revealed by inhibition of both CGRP-LI release and the cardiostimulatory action of capsaicin. RR also seems to protect the cardiac capsaicin-sensitive fibres from the development of tachyphylaxis to capsaicin. Finally, RR prevents the capsaicin-evoked negative inotropic effect which is not related to activation of sensory nerves.

Calcitonin gene-related peptide and human epicardial coronary arteries: presence, release and vasodilator effects

1. In the present study, the levels of calcitonin gene-related peptide (CGRP)-like immunoreactivity (-LI) in human cardiopulmonary tissue were determined in combination with studies on CGRP-LI release from the left anterior descending coronary artery (LAD) and functional effects of CGRP on coronary arterial tone. 2. The highest levels of CGRP-LI were found in the LAD followed in declining order by the bronchus, right atrium, pulmonary artery, lung and left ventricle. 3. Exposure to capsaicin evoked a clear-cut increase in CGRP-LI outflow, suggesting release from isolated large specimen of the LAD. This release was Ca2(+)-dependent and was markedly attenuated by incubation with the mitochondrial Ca2(+)-inhibitor, ruthenium red. Exposure to potassium also released CGRP-LI in a Ca2(+)-dependent fashion from the LAD. 4. In functional experiments on human epicardial coronary arteries with an inner diameter of 0.4 to 0.8 mm, human CGRP alpha and beta relaxed the potassium-precontracted arteries equipotently. Substance P (SP) also relaxed these precontracted arteries but the relaxation could be prevented by incubation with methylene blue, an inhibitor of endothelium derived relaxing factor (EDRF)-mechanisms, which did not influence the effect of CGRP. 5. Capsaicin evoked a ruthenium red-sensitive relaxation of the potassium-precontracted arteries. However, ruthenium red did not affect the relaxations induced by CGRP or SP. Furthermore, the capsaicin effect was not influenced by methylene blue. 6. It is concluded that CGRP-LI is present in human cardiopulmonary tissue and can be released upon exposure to high concentrations of capsaicin as well as potassium. CGRP causes relaxation of arteries independently of EDRF activation and closely resembles the vasodilator effects of capsaicin. This supports the view that the coronary vasodilatation observed upon sensory nerve activation is mediated by CGRP. Ruthenium red inhibits capsaicin-induced CGRP-LI release and functional effects and may thus serve as an experimental tool in evaluating the function of capsaicin-evoked stimulation of peripheral nerve terminals.

Post-occlusive reactive hyperaemia in the heart, skeletal muscle and skin of control and capsaicin-pre-treated pigs

In the present study we have tried to characterize and quantify the post-occlusive hyperaemia in the pig heart, skeletal muscle and skin circulation. In addition, the possible involvement of caspaicin-sensitive sensory nerves in the reactive hyperaemia was investigated. Reperfusion after total stop-flow ischaemia for 1, 5 or 15 min elicited a marked hyperaemia in all vascular beds studied. The post-occlusive hyperaemia after 5 min ischaemia was 512 +/- 74%, 328 +/- 94% and 444 +/- 87% in the heart, femoral artery and saphenous artery respectively. Also, in the skin the blood flow following 5 min ischaemia was increased fivefold. Furthermore, the duration of the hyperaemia after 5 min ischaemia was significantly (P less than 0.01) longer in the heart (382 +/- 32 s) than that in the femoral artery (192 +/- 27 s), saphenous artery (182 +/- 48 s) and skin (95 +/- 14 s). Increasing the ischaemic time period prolonged the duration as well as elevated the peak increase of the hyperaemia. Capsaicin pre-treatment significantly reduced (by about 70%) the tissue levels of calcitonin gene-related peptide (CGRP)-like immunoreactivity (-LI), which is present in sensory nerves, but not neuropeptide Y-LI, which is of sympathetic origin, in the left ventricle of the heart, quadriceps muscle and skin in the pig. However, there were no differences in the post-occlusive hyperaemia in control and capsaicin-pre-treated pigs. Capsaicin administered intracutaneously caused a long-lasting (about 20 min) increase in skin blood flow in the pig. This enhanced blood flow was completely abolished after systemic capsaicin pre-treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological identification of retinal cells releasing taurine by light stimulation

The effect of drugs blocking synaptic activity at different retinal levels was examined in this study, in an attempt to identify the origin of the light-stimulated release of 3H-taurine from the chick retina. It was determined by autoradiography that the chick retina accumulates taurine in photoreceptors, in cells from the inner nuclear layer, and in processes of the inner plexiform layer. All these are possible sites for the release of taurine upon illumination. To discriminate among these possibilities, the effects of aspartate, tetrodotoxin, strychnine, picrotoxin, chlorpromazine, tubocurarine, atropine, glutamate diethyl esther, alpha-amino adipate and 2-amino-4-phosphonobutyrate were studied. Aspartate (10 mM), which is known to eliminate the light response of cells postsynaptic to photoreceptors, induced a marked increase of 150% in the resting efflux of 3H-taurine but did not decrease significantly the light-stimulated release. Tetrodotoxin, which blocks amacrine cell responses, decreased 3H-taurine release stimulated by light by less than 20%. The efflux of taurine was unaffected by strychnine, picrotoxin, tubocurarine, atropine, chlorpromazine, and 2-amino-4-phosphonobutyrate, whereas it was increased by glutamate diethyl esther and alpha-amino adipate. These results, all together, point to photoreceptors as the cells releasing 3H-taurine in response to light.

Ruthenium red: differential effects on excitation and excitation-contraction coupling in frog skeletal muscle

The effects of purified ruthenium red (RR6+) on action potentials, resting potentials, twitches, tetani and potassium contractures in fibre bundles dissected from frog leg muscles were compared to the effects of supranormal concentrations of Ca2+. The effects of RR6+ include a shift of action potential threshold and reduction in rates of depolarization, repolarization and conduction velocity. Resting potentials and action potential overshoots are unaffected by RR6+ in concentrations of 10 microM or less. Higher RR6+ concentrations cause membrane depolarization. Low concentrations of RR6+ cause twitch potentiation but do not affect tetanus tension. Unlike Ca2+, RR6+ has no effect on the mechanical activation curve. However, crude preparations of RR6+ contain an unidentified contaminant which does shift the mechanical activation curve to the right, i.e. it increases mechanical threshold. RR6+ itself appears to be largely excluded from the t-system.

The effects of the antibiotic, primycin, on spontaneous transmitter release at the neuromuscular junction

The effects of primycin, a potent ionophore in biological membranes, have been studied at the neuromuscular junction of the garter snake. Primycin in concentrations greater than 2 X 10(-7)M produced a time- and concentration-dependent depolarization of twitch muscle fibres. Primycin (10(-7)-5 X 10(-7)M) produced an increased rate of quantal release of acetylcholine, which was not maintained, and a slight reduction in quantal size. Time to onset and to peak effect of primycin were concentration-dependent whereas maximum frequency was not. Absence of extracellular Ca2+ produced a significant delay in the time to onset and to peak effect of primycin, but did not affect the peak miniature endplate potential (m.e.p.p.) frequency. Following 60 min exposure to primycin (5 X 10(-7)M), introduction of a high concentration of potassium (20 mM) produced no further increase in spontaneous release. In cut muscle preparations, exposure to primycin (10(-7)-5 X 10(-7)M) reduced peak endplate current (e.p.c.) amplitude until nerve stimulation resulted in failures or the release of one or two quanta. E.p.c. amplitude was not restored with prolonged washing. The effects of primycin on the nerve terminal are considered to be consistent with its ability to increase the permeability of membranes to calcium ions resulting in an influx of extracellular calcium, an efflux of mitochondrial calcium and eventual depletion of synaptic vesicles.

Calcium-activated ATPases in presynaptic nerve endings

We studied the properties of calcium-activated ATPases present in preparations of isolated presynaptic nerve ending (synaptosome) and its subfractions from mouse brain. ATPase activity in the preparation was stimulated by Ca2+ and by Mg2+, but not by Na+ and K+, when each was added alone. The substrate specificities were found to be similar. The ATPases hydrolyzed only the high-energy phosphate bond and similar activity was exhibited for all nucleoside triphosphates tested (ATP, CTP, GTP, UTP). Moreover, the enzymes were insensitive to mitochondrial markers and to ouabain, but were inhibited by La3+. La3+ produced uncompetitive inhibition of Ca2+-ATPase in intact synaptosomes. Inhibition by La3+ was greatly increased after lysis of the synaptosomes, suggesting that the active sites of the enzymes may be on the cytosolic face of the membranes. The Ca2+-ATPase activity in synaptosomes was increased by increasing concentrations of external K+, suggesting that Ca2+ influx may be involved The Ca2+-ATPase in synaptosomal plasma membranes and synaptic vesicles had higher specific activities than those of intact synaptosomes and were activated, both in the presence and the absence of Mg2+, by Ca2+ concentrations approximating the intracellular level (10(-7) M). It is concluded that the nonmitochondrial synaptosomal Ca2+-ATPase may play an important role in the regulation of intracellular Ca2+.

The effect of colchicine on neuromuscular transmission in the frog during repetitive stimulation

When colchicine 10(-4) mol . l-1 was applied at the beginning of repetitive stimulation of the nerve at 10 s-1, the facilitation was markedly inhibited. The quantal content showed a very slight increase and its maximum value was reached later. The maximum frequency of spontaneous release was also reached later in the presence of colchicine than in the absence of the drug. In addition, the synaptic delay was much more pronounced in the presence of colchicine than in the control experiment. The results suggest that a partial block by colchicine of the release process in the nerve terminal occurs. This effect may be due to the action of the drug on the nerve terminal membrane. The results cannot exclude the possibility that colchicine interferes with the transport of vesicles towards the sites of release located on the membrane.

The spontaneous and evoked release of spermine from rat brain in vitro

1 The efflux of previously accumulated [3H]-spermine from brain slices was measured using a continuous perfusion system. The spontaneous efflux was biphasic, consisting of an initial rapid efflux followed by a much slower release. 2 The slices were depolarized by the addition to the medium of high potassium concentrations, ouabain or veratrine. 3 At concentrations greater than 30 mM, potassium evoked a striking increase in the release of [3H]-spermine. Following uptake in the presence of 5.7 x 10(-9)M [3H]-spermine, K+-evoked release was dependent on the presence of calcium ions. Release of spermine after uptake at 5.6 x 10(-8)M or 5.0 x 10(-7)M was not calcium-dependent. 4 The calcium-dependent, K+-stimulated release of spermine was inhibited in the presence of diphenylhydantoin (5 x 10(-5)M) or ruthenium red (10(-5)M). 5 Following uptake of 5.7 x 10(-9)M [3H]-spermine in a sodium-free medium, the calcium-dependent, K+-stimulated release was significantly inhibited. 5 Ouabain (10(-4)M) caused a large but calcium-independent increase in the efflux of [3H]-spermine. 7 Veratrine-induced release was less substantial but was increased in a calcium-free medium. Release evoked by veratrine was abolished in the absence of sodium. 8 These results are discussed with respect to a possible 'neurotransmitter' or 'neuromodulator' role for spermine.

Toxicology and pharmacology of some ruthenium compounds: Vascular smooth muscle relaxation by nitrosyl derivatives of ruthenium and iridium

A series of compounds were synthesized from ruthenium trichloride, and their ip LD50s were determined in mice: pentamminenitrosylruthenium(II) chloride, 8.9; chloronitrobis(2,2'-dipyridyl)ruthenium(II), 55;dichlorobis(2,2'-dipyridyl)ruthenium(II), 63; ruthenium trichloride, 108; and potassium pentachloronitrosylruthenate(II), 127 mg/kg. The two bis-bipyridyl complexes produced death in convulsions within minutes, whereas the remaining compounds resulted in long, debilitating courses with death occurring in 4-7d. When given in massive overdoses, however, the compounds with inorganic ligands also produced rapid convulsive death in mice, and when given iv to anesthetized cats, they produced respiratory arrest. The major toxic effects of all the complexes appeared to be due to the metal and not to its associated ligands. Only complexes having nitrosyl ligand specifically relaxed vascular smooth muscle. Potassium pentabromoiridate(III) also relaxed rabbit aortic strips that had been contracted by adrenergic agonists, but potassium pentachloroiridate(III) did not. None of the complexes was as active as nitroprusside in relaxing aortic strips or in decreasing arterial blood pressure in cats. No compound tested was as potent as cisplatin in antitumor activity. The pentamminenitrosylruthenium(II) complex also relaxed guinea pig ileum and frog rectus abdominus when these isolated muscles had been contracted by acetylcho line. It appears that these organoruthenium compounds may produce death in central respiratory arrest, as do the inorganic complexes when given iv or ip in massive overdoses. In minimally lethal doses, the complexes with inorganic ligands may affect a variety of contractile tissues, perhaps by a general mechanism involving Ca. These complexes are apt to be generally cytotoxic as well.

Regulation of calcium fluxes in pancreatic islets: glucose-induced calcium-calcium exchange

Glucose provokes an initial fall followed by a secondary rise in 45Ca efflux from prelabeled pancreatic islets. Prior exposure of the islets to calcium-depleted media does not suppress the secondary rise, provided that the extracellular calcium concentration is normalized 2 min before addition of glucose. Such a treatment significantly reduces insulin release. The secondary rise, as distinct from the initial fall, is inhibited under conditions known to interfere with calcium entry into the beta-cell, e.g., in the presence of ruthenium red or cobalt. Calcium itself in high concentrations provokes a dramatic increase in 45Ca efflux. The magnitude of this calcium-induced efflux is enhanced by prior exposure of the islets to calcium-depleted media. Imidazole and the theophylline do not modify the effect of glucose on 45Ca efflux. It is proposed that the glucose-induced secondary rise in 45Ca efflux corresponds to a calcium-calcium exchange process in which influent 40Ca displaces 45Ca from intracellular sites.

Effect of repetitive stimulation on the frog neuromuscular transmission

Presynaptic and postsynaptic effects on the neuromuscular transmission were studied during 20 min of indirect stimulation at 10/s. During the 'facilitation' period, there was an increase in the quantal content, in the frequency of miniature endplate potentials and in their amplitude. All these parameters were decreased during the 'depression' period. Besides, the end-plate current (e.p.c.), recorded during this high rate of stimulation, increasingly lengthened. The falling phase of the e.p.c. was exponential during facilitation, while marked deviations from the exponential time course were observed during depression. The experiments showed that a possible change in the kinetics between the receptors and the mediator was not responsible for the lengthened time course of the e.p.c. Therefore, it is assumed that either the delayed diffusion of the transmitter from the synaptic cleft or an altered mechanism of the release of acetylcholine may be involved. The latter possibility is supported by a progressive prolongation of the synaptic delay, which was observed during a prolonged repetitive stimulation.

Transmitter release: ruthenium red used to demonstrate a possible role of sialic acid containing substrates

1. The possible function of sialic acid-containing substrates (SACS) in synaptic terminals of Aplysia was studied by intracellular injection of ruthenium red and of neuraminidase. 2. Ruthenium red, a dye known to have sialic acid as a molecular target, blocked transmission irreversibly in both cholinergic (buccal ganglion) and non-cholinergic (cerebral ganglion) synapses. 3. An intracellular site of action is likely because much less ruthenium red was necessary to block transmission when it was injected intracellularly than when it was presented by bath perfusion. 4. Ca2+ spikes recorded in the presence of tetrodotoxin or in Na+-free solution were not modified by ruthenium red or neuraminidase injections or perfusions. It is therefore improbable that these substances blocked transmission by blocking voltage-dependent Ca2+ influx. 5. Strong electrotonic depolarization of a pre-synaptic interneurone in the presence of 10(-4) M-tetrodotoxin caused a sustained post-synaptic response, which was abolished by ruthenium red. This result eliminates axonal conduction block as the principal mechanism of ruthenium red action. 6. Post-synaptic responses to ionophoretically applied acetylcholine (ACh) were not modified by bath perfusion of 2 x 10(-2) M-ruthenium red. 7. Biochemical analysis of pools of [3H]ACh was performed after injection of a precursor, [3H]acetate, into an identified interneurone. Ruthenium red appeared to increase significantly the 'free' (cytoplasmic) ACh pool without any change of 'bound' (vesicular) [3H]ACh-pool. 8. A model is proposed in which SACS act as intracellular Ca2+ receptors involved in transmitter release.

Changes in transmitter release induced by ion-containing liposomes

The changes in quantal transmitter release induced by egg phosphatidylcholine liposomes with different internal ionic composition were examined at the frog neuromuscular junction by using conventional electrophysiological techniques. It was found that liposomes containing calcium or sodium ions increase both evoked and spontaneous transmitter release, while liposomes containing potassium do not. The results suggest that phosphatidylcholine liposomes are able to transfer their aqueous medium into the presynaptic nerve terminal.

Triton detergents and the frog neuromuscular end-plate: an electrophysiological and ultrastructural study

The effects of the nonionic detergents Triton X-45 and Triton X-100 were studied in the frog muscle end-plate, by intracellular recordings of spontaneous miniature end-plate potentials (m.e.p.p.'s) and the potential changes produced by iontophoretic application of acetylcholine (ACh-potentials). In addition, the ultrastructural changes produced by Triton X-100 were studied by transmission electron microscopic and freeze-fracture techniques. It was found that Triton X-45 and Triton X-100 caused a rapidly developing reduction of the amplitude of the m.e.p.p.'s. The response to iontophoretic application of acetylcholine was reduced by Triton X-100. Following return to normal Ringer solution the ACh-potentials recovered, although not completely. The dissociation constant calculated from the rate constants for onset and offset of the reaction (Kp=k2/k1) was 5--50 micron depending on the type of stoichiometric reaction presumed to occur between Triton X-100 and the cholinergic receptor. The ultrastructural changes observed indicate that the nerve terminal plasma membrane and mitochondria are affected by Triton X-100. Leakage of Ca2+ from the latter may therefore be the cause of the increase in m.e.p.p. frequency. It is concluded that the influence on the amplitude of the m.e.p.p.'s and the ACh-potentials can be attributed to a direct effect of the detergent upon the acetylcholine receptor protein.

Quelling of spontaneous transmitter release by nerve impulses in low extracellular calcium solutions

1. The effect of nerve stimulation on spontaneous transmitter release was studied at the frog neuromuscular synapse which was bathed in a solution containing very low extracellular calcium concentration. Conventional methods for intracellular and extracellular recording were used and the pattern of quantal liberation following the nerve stimulus was determined. 2. Stimulation of the motor nerve (at rates between 0.09 and 2Hz) caused a reduction in the frequency of the miniature e.p.p.s in comparison to the prestimulation values. 3. The mean distribution of the time of occurrence of the miniature e.p.p.s during the interstimulus period showed periodic oscillations. 4. The quelling effect of nerve stimulation on transmitter release is explained by the hypothesis that a low [Ca]o a reversed electrochemical gradient for calcium occurs and nerve stimulation causes an increased calcium conductance leading to calcium efflux which in turn temporarily reduces [Ca]i and transmitter release.

Intracellular and extracellular calcium ions in transmitter release at the neuromuscular synapse

The theme of this presentation has been to show that the control of transmitter release at the neuromuscular synapse is achieved by extracellular and intracellular calcium. For the fast information transfer represented by the end-plate potential, the electrochemical gradient for calcium across the presynaptic membrane and the associated calcium conductance seem to play the primary role. For slower processes such as tetanic and posttetanic potentiation, the combined effect of both sources for calcium determine the amount of transmitter liberated.

Release of noradrenaline by the ionophore X537A from normal and reserpinized guinea-pig atrium

The effects of ionophore X537A on the release of 3H-noradrenaline and its metabolites from the superfused guinea-pig left atrium were investigated. Concentrations of ionophore of 10 and 30 mocrometer greatly increased the release of tritium. Of the total increase in radioactivity elicited by X537A 44% was accounted for as noradrenaline and 50% was due to deaminated metabolites. The ionophore-evoked release of tritium was independent of the extracellular calcium ions and was not affected by agents which modify calcium movements such as verapamil, ryanodine, ruthenium red and tetracaine. X537A released 3H-noradrenaline from extragranular sites in MAO-inhibited atria from reserpine-treated animals and this release was also calcium independent. It is concluded that the ability of X537A to release noradrenaline from vesicular or cytoplasmic sites is not related to its ability to couple with and transport calcium ions through membranes. The ionophore might modify the ionic distribution outside and inside the neuronal membrane which would lead to leakage of the transmitter.

Depression of synaptic transmission by diphenylhydantoin

Diphenylhydantoin (phenytoin, DPH) depresses synaptic transmission at the frog neuromuscular synapse by presynaptic and postsynaptic mechanisms. In normal Ringer's solution the amplitude of the neurally evoked end-plate potentials and their quantal content are reduced. Somewhat paradoxically, miniature end-plate potential (mepp) frequency is increased by the drug. These effects could result if DPH blocked both calcium transport at the axonal membrane and intracellular calcium sequestration. Mepp amplitude is reduced, and DPH also induces nerve conduction block at high rates of stimulation. The relevance of these effects to the anticonvulsive activity of DPH is discussed.

Calcium-binding of synaptosomes isolated from rat brain cortex. IV. Effects of ruthenium red on the Co-operative nature of calcium-binding

Ruthenium red combines with isolated synaptosomes, resulting in strong inhibition of their Ca2+-binding. In isotonic saline media, however, the dye-induced inhibition of Ca2+-binding is significantly greater than that expected for the amount of bound dye and Hill's exponent of the Ca2+-binding decreases to 1 with an increase in the amount of the dye bound. On the other hand in isotonic mannitol-sucrose solution, inhibition of synaptosmal Ca2+-binding brought about by the dye is proportional to the amount of dye bound. Based on these results, the effects of the dye on the co-operative nature of synaptosomal Ca2+-binding is discussed.

The interaction of pH and divalent cations at the neuromuscular junction

1. 1. The effects of acidic pH on transmitter release were studied at the frog neuromuscular junction, using intracellular recording techniques. 2. Acid pH reduced the amplitude of the end-plate potentials (e.p.p.s) and accelerated the frequency of the miniature e.p.p.s(m.e.p.p.s). 3. At pH 6-0 the m.e.p.p. frequency was on the average 2-5 times greater than at pH 7-4. This multiplication was independent of the divalent ion concentration of the medium over a large range. 4. Reduction of the e.p.p. amplitude at low pH was the result of a decrease in m, the number of quanta of transmitter liberated by the nerve impulse. 5. The effect of low pH on m was blocked by high concentrations of Mg2+ and by lower concentrations of Mn2+ ions. This occlusion was found even when the total concentration of divalents in the bathing solution was kept constant. 6. These results indicated that H+ and Mn2+ ions bind to an acidic site which regulates Ca-mediated release of acetylcholine (ACh). The acid dissociation constant (KH) was determined using both a kinetic and a surface charge model. The pKa of the site calculated from the kinetic model was 5-7, while a pKa of 3-6 was obtained from the surface charge model. 7. It is suggested that protonation of the acidic site mentioned above reduces evoked transmitter release by blocking the influx of Ca into the nerve terminal following the nerve action potential.

Tonic release of transmitter at the neuromuscular junction of the crab

1. Synaptic transmission was studied at the neuromuscular junction of the crab Ocypoda cursor, using conventional electrophysiological technique. 2. It was found that fibres of the extensor muscle and those composing the internal layer of the closer muscle have only post-synaptic inhibition (S fibres) while the fibres at the external layer of the closer muscle have in addition presynaptic inhibition (R fibres). 3. In S fibres, addition of GABA reduces input membrane resistance (Rm) and e.p.s.p. amplitude approximately to the same degree. The effect shows desensitization. In R type fibres, GABA reduces the e.p.s.p. much more than expected from changes in Rm. The post-synaptic effect of GABA on Rm shows desensitization, while the presynaptic effect does not show desensitization. 4. In about 50 percent of the cases, after desensitization occurred, Rm increased by about 10-30 percent above the control. Similar increase in Rm occurred after application of picrotoxin. These results suggest that initially the membrane resistance was lower due to tonic release of inhibitory transmitter. 5. The Q10 of Rm was found to vary between 2 and 3. In Ca2+ free media, Cl- free media, or in picrotoxin the Q10 is about 1-3. 6. In R fibres, addition of picrotoxin increased the amplitude of the e.p.s.p. by 30-60 percent above the expected increase due to changes in Rm. 7. In S fibres the mean slope of log e.p.s.p. vs. log [Ca2+] was found to be 1-63, while in R fibres the slope was 0-93. These results suggest the presence of tonic release of the inhibitory transmitter which acts both post-synaptically and presynaptically.

The effects of tetraphenylboron on spontaneous transmitter release at the frog neuromuscular junction

1 The effects of tetraphenylboron (TPB) on spontaneous transmitter release were studied in the frog sartorius muscle preparation. 2 TPB (0.001-1 mM) produced a time-dependent increase in miniature endplate potential (m.e.p.p.) activity that was not sustained. TPB (0.1 mM) produced similar effects on m.e.p.p. frequency in normal Ringer solutions, in the absence of Ca2+ or Cl- and in the presence of excess Ca2+ and of tetrodotoxin. The effect of TPB (0.01 mM) was reduced but not abolished in the absence of Ca2+. 3 As m.e.p.p. frequency fell from its peak level in TPB (0.04 mM) m.e.p.p. amplitude was reduced. The reduction of m.e.p.p. amplitude was not prevented by choline (30-300 muM). 4 When m.e.p.p. activity fell below the noise level in the presence of TPB (0.1 mM), lanthanum (0.5 mM) was ineffective in promoting measurable m.e.p.p. activity. 5 The effects of TPB were slowly reversible by washing. 6 The results indicate that TPB acts to reduce the nerve terminal stores of acetylcholine, probably by a combination of rapid release and concomitant inhibition of transmitter storage.

On the role of mitochondria in transmitter release from motor nerve terminals

1. The changes in transmitter release produced by mitochondrial inhibitors has been studied at the frog neuromuscular junction using conventional electrophysiological techniques for stimulation and intracellular recording. 2. Inhibitors of the electron transport chain and inhibitors of oxidative phosphorylation produce an increase in the frequency of appearance of the miniature end-plate potentials. This increase in frequency is observed also in calcium-free media. Mitochondrial inhibitors also augment the amount of transmitter liberated by a nerve impulse. 3. Ruthenium red, which is an inhibitor of calcium uptake by mitochondria, increases the spontaneous transmitter release but decreases the quantal content. The latter effect of Ruthenium red is antagonized by calcium. 4. The mitochondrial content of the motor nerve terminals is, on the average, 6.59%. 5. The experimental results are explained on the hypothesis that spontaneous release of transmitter reflects the resting level of intracellular free calcium and the evoked release reflects the sum of the resting calcium and the calcium brought in by the action potential. The mitochondria play a role in transmitter release by participating in the regulation of the intracellular free Ca.