Actions of neomycin on neuronal L-, N-, and non-L/non-N-type voltage-sensitive calcium channel responses

Inadvertent relaxation of the ductus arteriosus by pharmacologic agents that are commonly used in the neonatal period

Premature birth and disruption of the normal maturation process leave the immature ductus arteriosus unable to respond to postnatal cues for closure. Strategies that advocate conservative management of the patent ductus arteriosus (PDA) in premature infants are dependent on identification of the symptomatic PDA and understanding the risk factors that predispose to PDA. Exposure of premature infants to unintended vasodilatory stimuli may be one of the risk factors for PDA that is under recognized. In this article, we summarize the clinical factors that are associated with PDA and review commonly used neonatal drugs for their vasodilatory properties. Data demonstrating relaxation of the ductus arteriosus by gentamicin and other aminoglycoside antibiotics, by cimetidine and other H2 receptor antagonists, and by heparin are provided as examples of neonatal therapies that have unanticipated effects that may promote PDA.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Therapeutically relevant concentrations of neomycin selectively inhibit P-type Ca2+ channels in rat striatum

The effects of neomycin on voltage-activated Ca(2+) channels (VACCs) were studied by Ca(2+)-dependent K(+)- and veratridine-evoked [3H]dopamine release from rat striatal slices. Neomycin (0.01-1 mM) concentration dependently reduced K(+)-evoked [3H]dopamine release (IC(50) approximately 25 microM), producing approximately 98% inhibition at 1 mM. Contribution of N-, P- and Q-type Ca(2+) channels to this neomycin-sensitive [3H]dopamine release was tested by the combined application of 100 microM neomycin and selective Ca(2+) channel blockers. The effects of neomycin combined with 1 microM of omega-conotoxin GVIA (N-type Ca(2+) channels) or with 100 nM of omega-conotoxin MVIIC (Q-type Ca(2+) channels) were additive, excluding involvement of N- and Q-type Ca(2+) channels. However, the combined effects of neomycin with 30 nM of omega-agatoxin-IVA (P-type Ca(2+) channels) were not additive, suggesting involvement of P-type Ca(2+) channels in neomycin-induced inhibition of [3H]dopamine release. On the other hand, veratridine-evoked [3H]dopamine release was shown to be mediated by Q-type Ca(2+) channels only. In addition, neither the inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase thapsigargin (500 nM) nor the blocker of sarcoplasmic reticulum ryanodine Ca(2+) channels ryanodine (30 microM) modulate veratridine-evoked [3H]dopamine release, suggesting no contribution of intracellular Ca(2+) stores. Neomycin (up to 100 microM) did not affect veratridine-evoked [3H]dopamine release, suggesting that intracellular Ca(2+) stores are not a prerequisite for the action of neomycin. Lack of inhibitory effect of neomycin is taken as additional indirect evidence for the involvement of P-type Ca(2+) channels. In conclusion, therapeutically relevant concentrations of neomycin preferentially block P-type Ca(2+) channels which regulate dopamine release in rat striatum. This block could be responsible for aminoglycoside-induced toxicity.

Simultaneous monitoring of slow cell motility and calcium signals of the guinea pig outer hair cells

'Slow' motility (shape changes over seconds to minutes) of the mammalian cochlear outer hair cell (OHC) could play a protection role from intense sound pressure and is associated with elevation of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). In the present work, a new approach was elaborated using fluorescent imaging for continuous monitoring of both [Ca(2+)](i) changes and slow motility of OHCs employing the Ca(2+) fluorescent indicator Fura-2. Whole OHC fluorescence and that of cell segments were analyzed to discriminate between fluorescence changes caused by [Ca(2+)](i) rise and those related to change of the cell shape. The reliability of the method was examined by simultaneous monitoring of [Ca(2+)](i) and OHC length changes induced by change of buffer osmolarity or by increase of KCl concentration. The method revealed that the time course of [Ca(2+)](i) increase and rate of cell shortening often do not coincide. It was also observed that [Ca(2+)](i) increased in 70 mM KCl more slowly than the rate of KCl delivery to OHCs. The comparison of the time courses of [Ca(2+)](i) elevation, induced by increase of K(+)/Na(+) ratio and by substitution of Na(+) with N-methyl-D-glucamine(+), indicated that the relatively slow kinetics of [Ca(2+)](i) increase in the OHC is partially attributed to regulation of Ca(2+) homeostasis by the Na(+)/Ca(2+) exchanger.

The mechanism by which aminoglycoside antibiotics cause vasodilation of canine cerebral arteries

The effects of aminoglycoside antibiotics were examined in canine cerebral arteries and in cultured cerebrovascular smooth muscle cells stimulated with oxyhemoglobin (OxyHb), a blood constituent which has been implicated in the pathogenesis of cerebrovascular spasm. In cerebral arterial rings precontracted with OxyHb (10 microM), the aminoglycosides caused a concentration-dependent decrease in isometric tension. The EC50s for the relaxation were 0.46+/-0.1 mM (n=6), 0.53+/-0.08 mM (n=12), 1.6+/-0.3 mM (n=7) and 3.9+/-0.5 mM (n=5) for neomycin, gentamicin, streptomycin and kanamycin, respectively. This order of potency corresponds approximately to the number of positive charges in the molecules. The aminoglycosides also inhibited the contractions to prostaglandin F2alpha (1 microM) and depolarizing concentrations of potassium chloride (60 mM). The order of potency was neomycin > gentamicin > streptomycin > kanamycin. The relaxation was maintained in vascular preparations denuded of endothelium. Neomycin (5 mM) abolished the Ca2+-independent contraction to PGF2alpha. In Fura-2-loaded cerebrovascular smooth muscle cells, OxyHb (1 microM) significantly enhanced the concentration of intracellular calcium ([Ca2+]i) by 330%. The administration of neomycin, gentamicin, kanamycin and streptomycin in concentrations corresponding to the EC50 from contractility studies, reduced the effects of OxyHb on [Ca2+]i by about 50% to 221+/-35 nM (n=7), 270+/-31 nM (n=7), 229+/-33 nM (n = 6) and 240+/-6 nM (n = 5), respectively. These results suggests that the effects of the aminoglycosides on the OxyHb-induced contraction and the long-term increase in [Ca2+]i, may arise from several effects, including inhibition of PLC, protection of calcium extrusion mechanisms, and interference with the process of [Ca2+]i accumulation.

Involvement of N- and P/Q- but not L- or T-type voltage-gated calcium channels in ischaemia-induced striatal dopamine release in vitro

Calcium influx and transmitter efflux are central events in the neuropathological cascade that occurs during and following cerebral ischaemia. This study explored the role of voltage-gated calcium channels (VGCCs) in ischaemia-induced striatal dopamine (DA) release in vitro. Slices (350 microm thickness) of rat neostriatum were superfused (400 ml/h) with an artificial cerebrospinal fluid (aCSF) at 34 degrees C and subjected to episodes of 'ischaemia' by reduction of the glucose concentration from 4 to 2 mM and gassing with 95% N2/5% CO2. DA release was monitored with fast cyclic voltammetry at implanted carbon fibre microelectrodes. The time to onset, time to peak, rate and magnitude of DA release were measured. Non-selective blockade of VGCCs with a high concentration of Ni2+ (2.5 mM), markedly delayed (P < 0.01) and slowed (P < 0.05) DA release but preferential blockade of T-type VGCCs with a lower concentration (200 microM) had no effect. DA release was also unaffected by selective antagonism of L-type VGCCs with nimodipine and nicardipine (10 microM each). Selective blockade of N-type VGCCs with omega-conotoxin GVIA (100 nM) delayed DA release (P < 0.05) but did not affect its rate or magnitude. Blockade of P- and possibly Q-type VGCCs with omega-agatoxin IVA (up to 200 nM) both delayed (P < 0.05) and slowed (P < 0.05) DA release. Preferential blockade of P- type VGCCs with neomycin (500 microM) also delayed (P < 0.05) and slowed (P < 0.05) DA release. These findings suggest that N-, P- and possibly Q- but not L- or T-type VGCCs mediate ischaemia-induced DA release. Although it is not possible to say, on the basis of these results, that the effects are directly upon the dopamine terminals, these calcium channels nevertheless constitute promising targets for therapeutic intervention.

Evoked GABA release is not mediated by N-type VDCC in the frontal cortex of awake rats: effects of neomycin

The purpose of the present study was to analyze the Ca2+ channel involved in GABA release under resting and K(+)-evoked conditions in vivo. We used microdialysis to investigate the effects of the voltage-dependent calcium channel (VDCC) blockers neomycin, kanamycin, and omega-conotoxin GVIA, and the voltage-dependent Na+ channel blocker tetrodotoxin, in the frontal cortex of awake rats. The GABA content in frontal dialysates was analyzed by high performance liquid chromatography coupled to fluorescence detection. Basal GABA release was kanamycin, omega-conotoxin, and tetrodotoxin resistant, whereas neomycin induced a significant increase from the basal level. The K(+)-evoked release of GABA was kanamycin and omega-conotoxin resistant, but tetrodotoxin sensitive. The effects of neomycin were masked by the action of this drug on basal release. These results suggest that neomycin may affect GABA release in the frontal cortex through a mechanism independent of VDCC. In addition, the K(+)-evoked release of GABA in this cortical area was not mediated by the N-type voltage-dependent calcium channels, but was dependent on neural activity or TTX sensitive.

Voltage-sensitive Ca2+ channels in rat striatal synaptosomes: role on the [Ca2+]i responses to membrane depolarization

The fluorescent Ca2+ indicator Indo-1 was used to study the effect of depolarization evoked by KCl or 4-aminopyridine (4-AP) on the intracellular free calcium concentration responses (delta[Ca2+]i) in rat striatal synaptosomes. Depolarization of the synaptosomes with [KCl] > 7.5 mM induced a rapid increase of the [Ca2+]i followed by a decay towards a plateau. The size of the [Ca2+]i response varied sigmoidally with the synaptosomal membrane potential, with a transition potential of -27.3 mV. Depolarization with 4-AP evoked a dose-dependent sustained increase of the [Ca2+]i. Nitrendipine, omega-Conotoxin GVIA (omega-CgTx) and omega-Agatoxin IVA (omega-Aga IVA) were used to evaluate the relative role of L-, N-, P- and possibly Q-type voltage-sensitive Ca2+ channels (VSCCs) on the [Ca2+]i changes evoked by each of the two depolarizing agents. Nitrendipine caused only about 10% inhibition of the effect of either agent on the [Ca2+]i, suggesting that the L-type VSCCs have a modest contribution. The omega-CgTx decreased the response to KCl and 4-AP by 15 and 30%, respectively, but the latter effect may be partially due to a non-specific effect on Na+ channels. The omega-Aga IVA reduced the response to 4-AP by 26.5%, and this effect was additive to that of omega-CgTx, further suggesting that the striatal nerve terminals possess P- and/or Q-type, in addition to N-type Ca2+ channels. Neomycin (0.35 mM), tentatively used as an antagonist of the P-type channels, had a potent effect, decreasing the response to K(+)-depolarization and to 4-AP by, respectively, 32.5 and 48.5%. It is suggested that at the concentration used the antibiotic also partially blocks VSCCs which do not belong to the L-, N-, P- or Q-type VSCCs. We conclude that striatal nerve endings are equipped with at least four to five pharmacologically distinct classes of VSCCs, which are sensitive to well known antagonists of the L-, N-, P-, and Q-type VSCCs.

Comparative actions of synthetic omega-grammotoxin SIA and synthetic omega-Aga-IVA on neuronal calcium entry and evoked release of neurotransmitters in vitro and in vivo

The effects of synthetic omega-grammotoxin SIA (omega-GsTxSIA) and synthetic omega-Aga-IVA were tested in in vitro and in vivo neurochemical assays that are reflective of voltage-sensitive calcium channel function. Synthetic omega-GsTx SIA inhibited K(+)-evoked rat and chick synaptosomal 45Ca2+ flux, K(+)-evoked release of [3H]D-aspartate and [3H]norepinephrine from rat hippocampal brain slices and K(+)-evoked release of [3H]norepinephrine from chick cortical brain slices with potency values that were comparable to those found previously with omega-GsTx SIA purified from the venom of the tarantula spider Grammostola spatulata. These results indicate that trace contaminants do not account for the pharmacology of purified omega-GsTx SIA. omega-GsTx SIA caused a complete inhibition of rat synaptosomal 45Ca2+ flux and hippocampal slice [3H]D-aspartate release, whereas omega-Aga-IVA caused a maximal inhibition of approx 75%. omega-GsTx SIA and omega-Aga-IVA caused an identical partial inhibition of K(+)-evoked increases of intracellular calcium in cortical neurons in primary culture. The addition of nitrendipine to either omega-GsTx SIA or omega-Aga-IVA resulted in an additive and virtually complete inhibition of the cortical neuron intracellular calcium response. In in vivo microdialysis studies, the K(+)-evoked release of glutamate from hippocampus of awake freely moving rats was inhibited with the following rank order of potency: omega-conotoxin GVIA > omega-GsTx SIA > omega-Aga-IVA. Complete inhibition of K(+)-evoked hippocampal glutamate release was observed with 300 nM omega-conotoxin GVIA and 3 microM omega-GsTx SIA. In urethane anesthetized rats, omega-CgTx GVIA caused a partial inhibition, whereas omega-GsTx SIA caused a concentration-dependent and complete inhibition, of basal serotonin release in the hippocampus. Therefore, omega-GsTx SIA was shown to inhibit responses that are sensitive to omega-conotoxin GVIA, omega-Aga-IVA and omega-conotoxin MVIIC, consistent with the notion that omega-GsTx SIA inhibits N-, P- and Q-type high threshold voltage-sensitive calcium channels.

Relation of [Ca2+]i to dopamine release in striatal synaptosomes: role of Ca2+ channels

We compared the effects of KCl and 4-aminopyridine (4-AP) stimulation on the coupling of Ca2+ channel activation to [3H]dopamine ([3H]DA) release in rat striatal synaptosomes and used specific Ca2+ channel blockers to discriminate between the different VSCC's activated by the two stimulatory agents. We found that whereas [3H]DA release is strictly Ca(2+)-dependent in the case of KCl depolarization, 4-AP, at concentrations above 100 microM, progressively causes a large Ca(2+)-independent release of [3H]DA. Thus, at 1 to 3 mM 4-AP, as much as 80-95% of the [3H]DA release is Ca(2+)-independent and can be partially blocked by nomifensine, indicating that some [3H]DA release is occurring through reversal of the DA carrier. Therefore, in the studies relating [Ca2+]i to [3H]DA release we selected 4-AP concentrations lower than 100 microM and corrected for the Ca(2+)-independent release. Under these conditions, we determined that: (1) Ca2+ entry through N-type VSCC's is involved in [3H]DA release both in the case of KCl depolarization (35% inhibition by omega-CgTx) and in 4-AP stimulation (23% inhibition by omega-CgTx); (2) Ca2+ entering through P-type and/or Q-type VSCC's is also involved in [3H]DA release due to 4-AP stimulation (26% inhibition by 200 nM omega-Aga IVA); (3) Neomycin (0.35 mM) inhibited the [3H]DA release due to 4-AP stimulation by about 20% and decreased the KCl induced [3H]DA release by 55%; the effects of neomycin (0.35 mM) and omega-CgTx were additive in both cases, indicating that, at this concentration, the antibiotic does not affect significantly N-type Ca2+ channels; (4) When applied together, omega-CgTx and omega-Aga IVA inhibited the 4-AP stimulated [3H]DA release by about 40-50%, suggesting that the remaining large fraction of the VSCC's activated by 4-AP stimulation are non-N, non-P VSCC's and are coupled to Ca(2+)-dependent [3H]DA release; (5) The contribution of L-type VSCC's is uncertain, since there seemed to be a small contribution in the case of KCl depolarization, but not in the case of 4-AP stimulation. On the whole, the results suggest that the release of [3H]DA in the rat striatal nerve terminals depends on Ca2+ entry through N-, P-, possibly Q-, and other non-N-, non-P-type VSCC's when either KCl or 4-AP stimulation is utilized.

Inhibition of phospholipase C with neomycin improves metabolic and neurologic outcome following traumatic brain injury

Activation of phospholipase C has been implicated as a factor in the development of irreversible tissue damage following injury to the central nervous system. We have used phosphorus magnetic resonance spectroscopy and a battery of postinjury motor function tests to characterize the role that phospholipase C activity may play in determining biochemical and neurologic outcome following traumatic brain injury in rats. Moderate (2.7 atmospheres) fluid percussion induced lateral brain injury caused a decline in free magnesium concentration, phosphorylation potential, and increased mitochondrial rate of oxidative phosphorylation. Neurologic motor score at 24 h and 1 week posttrauma in these animals was consistent with moderate injury. In contrast, treatment with the phospholipase C inhibitor neomycin B (15 mg/kg i.v.) immediately prior to injury significantly improved free magnesium status, bioenergetic state and neurological outcome (P < 0.01) after injury. We propose that phospholipase C activated second messenger pathways affecting magnesium homeostasis are involved in determining outcome after brain injury.

Differential inhibition of neuronal calcium entry and [3H]-D-aspartate release by the quaternary derivatives of verapamil and emopamil

1. Verapamil and emopamil are structurally related phenylalkylamine calcium channel/5-HT2 receptor antagonists that differ in their anti-ischaemic properties in experimental studies. The quaternary ammonium derivatives of these compounds were prepared and tested in assays of neuronal voltage-sensitive calcium channel (VSCC) function to determine whether the compounds act at intra- or extracellular sites. 2. The compounds were tested in K(+)-evoked: (1) rat brain synaptosomal 45Ca2+ influx, (2) release of [3H]-D-aspartate from rat hippocampal brain slices and (3) increase of intracellular calcium in rat cortical neurones in primary culture. 3. Verapamil, emopamil and the emopamil quaternary derivative caused concentration-dependent and comparable (IC50 values approximately 30 microM) inhibition of synaptosomal 45Ca2+ influx and [3H]-D-aspartate release. The verapamil quaternary derivative was considerably less active in these assays (IC50 > 300 microM). 4. The evoked increase of intracellular calcium in cortical neurones was inhibited with the following rank order of potency (IC50 value, microM): emopamil (3.6) > verapamil (17) > emopamil quaternary derivative (38) > verapamil quaternary derivative (200). 5. The results suggest that verapamil and emopamil inhibit nerve terminal VSCC function (synaptosomal 45Ca2+ influx and [3H]-D-aspartate release) by acting at distinct intracellular and extracellular sites, respectively. Verapamil and emopamil may inhibit cell body VSCC function (evoked increase of intracellular calcium in neocortical neurones) by acting at both intracellular and extracellular sites. 6. The different 'sidedness' of action of emopamil and verapamil on nerve terminal VSCC function and/or the preferential inhibition of cell body VSCC function by emopamil may at least partially explain the relatively greater neuroprotective efficacy of emopamil in experimental models of ischaemia.

Electron probe microanalysis of gentamicin-induced changes on ionic composition of the vestibular gelatinous membrane

Gentamicin-induced changes in ionic composition in the otolithic membrane of adult OF1 mice were evaluated in the gelatinous layers of the saccule and utricle by quantitative electron probe X-ray microanalysis. The otolithic membranes were plunge-frozen and freeze-dried to prevent the redistribution of elements. Quantitative analysis was carried out with an energy dispersive detector using the peak-to-background (P/B) ratio method and different salts dissolved in dextran as standards to calibrate the P/B ratio against the concentration of the elements P, S and K in the microprobe. Gentamicin selectively decreased the concentrations of P (P < 0.001) and S (P < 0.01) in the gelatinous membrane of the saccule, and had no effect in the utricle. The concentration of K also increased in the utricular gelatinous membrane (P < 0.05). The mechanism of ototoxicity in the gelatinous membrane is unknown, but the ability of aminoglycosides to block calcium channels may induce disturbances in the ionic equilibrium of the endolymphatic fluid, and thus affect the biochemical composition of the gelatinous membrane. This technique can be useful to evaluate the distribution of ions in the process of drug-induced ototoxicity.

Effects of ion channel blockers and phorbol ester treatments on [3H]dopamine release and neurotensin facilitation of [3H]dopamine release from rat mesencephalic cells in primary culture

In this work, we tested the effect of ion channel blockers and of phorbol ester treatments on [3H]dopamine ([3H]DA) release and neurotensin (NT)-induced facilitation of [3H]DA release from cultures of rat fetal mesencephalic cells. The potassium channel blockers tetraethylammonium and 4-aminopyridine increased basal [3H]DA release and decreased K(+)-evoked [3H]DA release, whereas apamin was without effect. K(+)-evoked [3H]DA release was decreased by omega-conotoxin and nifedipine, totally suppressed by cadmium, and unaffected by amiloride. These results show the differential sensitivity of [3H]DA release to blockade of various ion channels and suggest the involvement of N-type, L-type, and non-L-non-N-type, but not T-type, voltage-sensitive calcium channels in K(+)-evoked release. Phorbol 12-myristate 13-acetate increased both spontaneous and K(+)-evoked [3H]DA release, suggesting a modulatory action of protein kinase C on DA release in this system. Unexpectedly, however, the effects of the phorbol ester were not counteracted by the protein kinase C inhibitors H7, staurosporine, or polymyxin B. NT-induced facilitation of K(+)-evoked [3H]DA release was insensitive to most of the ion channel blockers, except cadmium (64% decrease in NT effect), suggesting that the corresponding potassium and calcium channels were not involved in the effect of NT on [3H]DA release in this system. The NT effect was totally suppressed by phorbol ester treatments, indicating a possible desensitization of the corresponding transduction mechanisms after protein kinase C activation.

Characteristics of [125I]omega-conotoxin MVIIA binding to rat neocortical membranes

[125I]omega-Conotoxin MVIIA (omega-CTM) binding to N-type voltage-sensitive calcium channels (VSCCs) was characterized using rat neocortical membranes. [125I]omega-CTM bound rapidly and with high affinity; these parameters were similar to binding using omega-conotoxin GVIA ([125I]omega-CTG). Unlike [125I]omega-CTG, however, [125I]omega-CTM readily dissociated from its binding site. Monovalent and divalent cations, polyamines, and aminoglycosides inhibited [125I]omega-CTM binding. Since [125I]omega-CTM appears to bind to the same site as [125I]omega-CTG in mammalian neurons, the reversibility of [125I]omega-CTM binding makes this ligand preferable for equilibrium binding analyses.

Profile of prostaglandins induced by endothelin-1 in human brain capillary endothelium

The vasoactive peptide, endothelin-1 (ET-1) has been implicated in the pathophysiology of various diseases. Recently, we have shown that human brain endothelial cells both secrete and express immunoreactive ET-1 high-affinity ETA receptors coupled to activation of phospholipase C (PLC). The present study demonstrates concentration-dependent stimulation of prostanoids [thromboxane B2 (TxB2), prostaglandin F2 alpha (PGF2 alpha), 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha) prostaglandin E2 (PGE2), and prostaglandin D2 (PGD2)] production by ET-1 in capillary endothelial cells derived from human brain (HBCEC). The increase in the vasoconstrictive prostanoids TxA2 and PGF2 alpha temporally preceded that of the vasodilatory PGI2, PGE2 and PGD2, and was seen after 15 min of incubation with ET-1 (10 nM). Increased production of vasodilatory prostanoids was observed between 4-8 h of incubation, whereas normalization of both vasoconstrictive and vasodilatory prostaglandins occurred 24 h after addition of ET-1. Both ET-1-stimulated prostanoid and IP3 production were inhibited by BQ123, a specific antagonist of ETA receptors. ET-1-induced prostanoid secretion by HBCEC was also inhibited by dexamethasone (50 microM) and diminished by neomycin (50 microM) and verapamil (10 microM) but not by nifedipine. Phorbol myristate ester potentiated ET-1-stimulated prostanoid secretion, whereas it inhibited IP3 production. Data indicate that ET-1 activates phospholipase A2 (PLA2) and PLC in HBCEC by different intracellular mechanisms. The subsequently induced secretion of vasoactive prostanoids by HBCEC may contribute both qualitatively and temporally to the vasoactive actions of ET-1.

Effects of site-specific acetylation on omega-conotoxin GVIA binding and function

Chemical modification of omega-conotoxin GVIA (omega-CgTXGVIA) was performed using nonsaturating concentrations of acetic anhydride to generate seven distinct derivatives. Following separation of these peptides using reverse-phase HPLC (RP-HPLC), their individual molecular weights were determined using fast bombardment mass spectrometry (FAB-MS). Three peptides contained a single acetylated amino group, three possessed two acetylated amino groups, and the last contained three acetylations. For each peptide, the specific site of acetylation was confirmed using a scheme of tryptic digestion, under nonreducing conditions, followed by RP-HPLC and FAB-MS. Biological profiles for each peptide were obtained by analyzing their capacity to displace native 125I-omega-CgTx GVIA binding to rat hippocampal membranes and to block K(+)-stimulated 45Ca2+ influx into chick brain synaptosomes. The data indicate that successive additions of acetyl moieties to omega-CgTx GVIA lead to a loss of both binding affinity and Ca2+ influx inhibitory potency. Within the monoacetylated series, acetylation of the amino terminal of Cys-1, as compared to the epsilon-amino group of either Lys-2 or Lys-24, leads to the greatest shift in potency. In summary, these results indicate that basic (i.e., primary amino) groups, which are brought into close proximity as a result of disulfide bridging, are important in the functional blockade of neuronal Ca2+ channels by omega-CgTx GVIA.

Effects of stimulus intensity on the inhibition by omega-conotoxin GVIA and neomycin of K(+_-evoked [3H]norepinephrine release from hippocampal brain slices and synaptosomal calcium influx

The effects of various K+ concentrations on the inhibition of [3H]norepinephrine release from rat hippocampal brain slices and evoked synaptosomal 45Ca2+ influx by omega-conotoxin GVIA (omega-CgTx) and neomycin were examined. K+ (15-75 mM) caused a concentration-dependent release of [3H]norepinephrine that was greater than 90% dependent on extracellular calcium. The ability of omega-CgTx to inhibit [3H]norepinephrine release was optimal at 25 mM K+ and was reduced substantially at higher concentrations of K+. omega-CgTx maximally inhibited [3H]norepinephrine release by 49% (15 mM K+), 58% (25 mM K+), 22% (50 mM K+), and 12% (75 mM K+). In contrast, neomycin caused a concentration-dependent and virtually complete inhibition of [3H]norepinephrine release at all concentrations of K+, with IC50 values of 210 microM (15 mM K+), 150 microM (25 mM K+), 450 microM (50 mM K+), and 1500 microM (75 mM K+). omega-CgTx (1 microM) had little effect (10% or less inhibition) on hippocampal synaptosomal 45Ca2+ influx at any concentration of K+, whereas 3 mM neomycin caused at least 75% inhibition of 45Ca2+ influx, with the largest inhibition (96%) occurring at 25 mM K+. The results suggest that increasing stimulus intensity decreases the contribution of N-type voltage-sensitive calcium channels (VSCC) in mediating K(+)-evoked release of [3H]norepinephrine. The comparative absence of omega-CgTx-sensitive synaptosomal 45Ca(2+)-influx sites suggests that N-type calcium channels are a small subset of channels in rat hippocampal synaptosomes. The demonstration that neomycin can inhibit omega-CgTx-sensitive and -insensitive neurotransmitter release and calcium influx suggests that neomycin may block N-type VSCC as well as non-N-type VSCC.