The calmodulin antagonists, trifluoperazine and R24571, depolarize the mitochondria within guinea pig cerebral cortical synaptosomes

Interference of calmidazolium with measurement of mitochondrial membrane potential using the tetraphenylphosphonium electrode or the fluorescent probe rhodamine 123

Calmidazolium (CMZ) is a positively charged, hydrophobic compound used as a calmodulin antagonist. It may cause unspecific effects in mitochondria, e.g., a decrease in membrane potential (deltapsi), swelling, and uncoupling. Several groups have advised against use of CMZ in studying signal transduction in mitochondria. We report here that it interferes with measurement of deltapsi in rat liver mitochondria (RLM) when using the tetraphenyl phosphonium (TPP+) electrode. We also found that CMZ reduces the signal, indicating an apparent drop in deltapsi. CMZ itself gave a signal with the TPP+ electrode in the absence of RLM. At high concentrations, > 10 microM, it also reduced the fluorescence quenching of the probe rhodamine 123. This may be due to an interference with mitochondrial uptake and binding of this positively charged probe or to an uncoupling effect. It is concluded that CMZ and similar positively charged calmodulin antagonists such as trifluoperazine may be used in mitochondria if these interferences are controlled and calibration is carried out under the experimental conditions used.

Ca/calmodulin-dependent kinase II inhibitor KN62 attenuates glutamate release by inhibiting voltage-dependent Ca(2+)-channels

The effect of KN62 (1-[N,O-bis(5-isoquinolinesulphonyl)-N -methyl-L-tyrosyl]-4-phenylpiperazine), a putative inhibitor of Ca/calmodulin-dependent kinase II (Ca/CaM-K II), on glutamate release from isolated nerve-terminals (synaptosomes) was examined. The drug caused a potent inhibition of KCl- and 4-aminopyridine-evoked glutamate release from isolated nerve-terminals (synaptosomes). Examination of the effect of the inhibitor on Ca(2+)-influx revealed that the diminution of glutamate release could be attributed to a decrease in cytosolic Ca. A direct effect of KN62 on synaptosomal Ca(2+)-channels was confirmed in experiments where Ba, which does not support CaM-dependent processes, was used in place of Ca. Additionally, whole-cell patch-clamping of cerebellar granule neurones directly demonstrated inhibition of Ca-currents by KN62. We therefore suggest that, in cellular systems, conclusions based on the use of KN62 as a Ca/CaM-K II blocker may be ambiguous and should be viewed with caution unless the effect of the drug on Ca-influx has also been quantified. The effect of KN62 on Ca(2+)-influx appears to be specific to slowly-or non-inactivating conductances, and therefore presents KN62 as a potentially useful tool in this context.

Trifluoperazine increases fatty acid turnover in phospholipids in cultured human fibroblasts

A 24-hr pretreatment of cultured human fibroblasts with trifluoperazine induced a marked increase in incorporation of saturated (stearic, palmitic) and unsaturated (oleic, arachidonic) fatty acids into phospholipids (1.5- to 2-fold for 5.10(-5) M trifluoperazine). Concomitantly, incorporation into cholesteryl esters was strongly inhibited (20% of control for 5.10(-5) M trifluoperazine). The drug did not change the phospholipid composition of treated cells. The effect of trifluoperazine on oleic acid incorporation into phospholipids was time-dependent and reached a maximum after a six-hr preincubation with the drug. Trifluoperazine also induced an increase in the rate of chase of oleic acid from the different phospholipid classes. In vitro preincubation of cell-free extracts with trifluoperazine resulted in activation of phospholipid acyltransferases, whereas cholesterol acyltransferase activity was decreased. The rapid effect of trifluoperazine together with its effect on a cell-free system suggests a direct action of this amphiphilic drug on the acyltransferase activities, probably by modification of the structural organization of cellular membranes.

Effects of trifluoperazine on the cholinergic function of the hippocampus of the rat

The changes induced in the level and turnover rate (TRACh) of acetylcholine (ACh) in the hippocampus and in the in vitro release of [3H]ACh by intracerebroventricular injection of trifluoperazine (TFP) were studied. Rats were killed by microwave irradiation at various times after treatment with drug or vehicle and the levels of ACh were measured by gas chromatography. After the administration of 100 micrograms trifluoperazine, the content of ACh of the whole hippocampus increased from 24.0 +/- 2.9 to 35.2 +/- 4.2 nmols/g wet weight over a 60 min period, while the turnover rate was markedly decreased (from 0.86 to 0.53 nmols/min/g wet tissue weight). The potassium-evoked release of [3H]ACh from slices of hippocampus under in vitro conditions was decreased in a concentration-dependent manner by preincubation with 50 and 100 microM of trifluoperazine for 60 min. The results support the assumption that trifluoperazine can alter neurotransmission, possibly by inhibiting the calcium-binding protein calmodulin, and/or by interfering with cell metabolism.

Leptinotoxin-h action in synaptosomes and neurosecretory cells: stimulation of neurotransmitter release

Guinea pig brain cortex synaptosomes and neurosecretory PC12 cells were loaded with [3H]3,4-dihydroxyphenylethylamine ([3H]DA, [3H]dopamine) and then exposed to leptinotoxin-h (LPTx) (purified and partially purified preparations, obtained from the hemolymph of Leptinotarsa haldemani). In a Ca2+-containing Ringer medium the toxin induced prompt and massive release of the neurotransmitter. Half-maximal effects were obtained at concentrations estimated of approximately 3 X 10(-11) M for synaptosomes, and 1.5 X 10(-10) M for PC12 cells. Release responses in the two experimental systems investigated were dependent to different extents on the Ca2+ concentration in the medium. In synaptosomes clear, although slow, release of [3H]DA was elicited by the toxin even in Ca2+-free, EGTA-containing medium, provided that high (in the 10(-10) M range) concentrations were used; near-maximal responses were observed at 10(-5)M Ca2+. In contrast, the toxin-induced release from PC12 cells was appreciable only at 3 X 10(-5) M Ca2+, and was maximal at 2 X 10(-4) M and above. In both synaptosomes and PC12 cells Sr2+ and Ba2+ could substitute for Ca2+; Co2+ was inhibitory, whereas Mn2+ failed to modify the release induced by the toxin in Ca2+-containing medium. Organic blockers of the voltage-dependent Ca2+ channel (verapamil and nitrendipine) and calmodulin blocking drugs (trifluoperazine and calmidazolium) failed to inhibit the toxin-induced release of [3H]DA. LPTx induced profound morphological effects. Synaptosomes treated in the Ca2+-containing medium exhibited fusion of synaptic vesicles, formation of numerous infoldings and large cisternae, and alterations of mitochondria. In the Ca2+-free medium the effects were similar, except that their appearance was delayed, and mitochondria were well preserved. Swelling was observed in PC12 cells, accompanied by enlargement of the Golgi area, accumulation of multivesicular bodies, mitochondrial alterations, and decreased number of secretion granules (Ca2+-containing medium). Morphometric analyses revealed a good correlation between the decrease of both synaptic vesicles (synaptosomes) and neurosecretory granules (PC12 cells), and the release of [3H]DA measured biochemically. This is a good indication that the release effect of the toxin is due to stimulation of exocytosis. Taken as a whole, these results confirm the similarity of the effects of LPTx with alpha-latrotoxin of the black widow spider venom, mentioned in the companion article. However, differences in effect and target specificity suggest that the two toxins are specific to separate binding sites.(ABSTRACT TRUNCATED AT 400 WORDS)

Bioenergetic actions of beta-bungarotoxin, dendrotoxin and bee-venom phospholipase A2 on guinea-pig synaptosomes

Low concentrations of beta-bungarotoxin or bee-venom phospholipase A2 cause a progressive Ca2+-dependent increase in the proton permeability of the mitochondria within the synaptosomal cytosol, manifested as an increase in oligomycin-insensitive respiration and a partial depolarization of the mitochondrial membrane potential. This uncoupling appears to be a consequence of fatty acids liberated by phospholipase A2 activity at the plasma membrane, since it can be mimicked by the addition of oleate-albumin complexes, in which case there is no requirement for external Ca2+. Dendrotoxin does not affect the mitochondrial proton permeability in situ, but protects partially against the uncoupling action of beta-bungarotoxin. In contrast, this effect of bee-venom phospholipase A2 is unaffected by dendrotoxin. beta-Bungarotoxin, but not bee-venom phospholipase A2, induces a slow progressive depolarization of the plasma membrane. The action of beta-bungarotoxin at the plasma membrane appears not to be related to fatty acid production, since it is augmented rather than inhibited by raising albumin concentrations in the medium. It is concluded that beta-bungarotoxin has at least two actions on intact synaptosomes, both of which may involve interaction at the plasma membrane with a site common to dendrotoxin: first, a mitochondrial uncoupling mediated by fatty acids and, secondly, a depolarization at the plasma membrane.