Ca2+ stores in the chick embryo retina cells

The Ca2+ stores of digitonin permeabilized chick embryo retina cells in culture were characterized, by using the fluorescence of Fluo-3 potassium salt to follow continuously the free [Ca2+] in the medium. After ATP dependent Ca2+ accumulation, the Ca2+ release was induced by several agents; 10 microM cyclic-ADP-ribose (cADPR), 40 microM Ins (1,4,5)P3 10 microM thapsigargin (Th), 25 microM ionomycin (Ion), 15 microM CCCP together with 4.5 micrograms/ml oligomycin (CCCP/Olig), 50 microM arachidonic acid (AA). Neither Ins(1,4,5)P3 nor cADPR were able to mobilize Ca2+ from internal stores in these cells, but Th and AA were effective in releasing Ca2+. Four major Ca2+ stores in chick embryo retina cells were distinguished: i) the thapsigargin sensitive Ca2+ store, most likely the ER; ii) the Ca2+ store sensitive to oligomycin and CCCP, most likely the mitochondrial Ca2+ store, iii) an AA sensitive Ca2+ store, which is distinct from the previous two; and, iv) the Ca2+ store only sensitive to ionomycin. The capacities of these different Ca2+ stores of the chick embryo retina cells, relative to the total intracellular stores, are: 63.3%, 14.1%, 8.2%, for the ER, the mitochondrial and for the AA sensitive Ca2+ stores, respectively.

Modulation of Ca2+ channels by activation of adenosine A1 receptors in rat striatal glutamatergic nerve terminals

We determined that activation of adenosine A1 receptors in striatal synaptosomes with 100 nM N6-cyclopentyladenosine (CPA) inhibited both the release of endogenous glutamate and the increase of intracellular free Ca2+ concentration ([Ca2+]i), due to 4-aminopyridine (4-AP) stimulation, by 28 and 19%, respectively. Furthermore, CPA enhanced the inhibition of endogenous glutamate release due to omega-conotoxin GVIA (omega-Cgtx GVIA), omega-Cgtx MVIIC or omega-Cgtx GVIA plus omega-Cgtx MVIIC. Similar effects were observed in the [Ca2+]i signal. The inhibitory effects of CPA and omega-Cgtx GVIA were additive, but the effects of CPA and omega-Cgtx MVIIC were only partially additive. These results suggest that P/Q-type Ca2+ channels and other type(s) of Ca2+ channel(s), coupled to glutamate release, are inhibited subsequently to activation of adenosine A1 receptors.

Domoic acid induces the release of glutamate in the rat hippocampal CA3 subregion

We investigated the role of kainate receptor activation in modulating the influx of Ca2+ coupled to the exocytotic release of glutamate in rat hippocampal synaptosomal fractions (P2). In whole hippocampus synaptosomes stimulation with domoic acid increased the intracellular free Ca2+ concentration ([Ca2+]i) in a dose-dependent manner with an EC50 of 0.16 microM, whereas the EC50 for kainate was 0.86 microM and for (+/-)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) was 43.04 microM. Stimulation of the synaptosomes with 10 microM domoic acid induced Ca(2+)-dependent release of endogenous glutamate. Also, in synaptosomes isolated from the CA3 sub-region of the hippocampus the domoic acid-induced release of glutamate was higher than that from the dentate gyrus (221.3%), from the CA1 (188.1%) or from the whole hippocampal synaptosomes (131.5%). These results support the existence of a presynaptic kainate receptor which may control the exocytotic release of glutamate in the CA3 sub-region of the rat hippocampus.

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.

Involvement of class A calcium channels in the KCl induced Ca2+ influx in hippocampal synaptosomes

Using Ca2+ channel toxins, we determined the types of voltage-sensitive calcium channels activated by two levels of KCl depolarization in hippocampal synaptosomes. The increase in the intracellular Ca2+ concentration ([Ca2+]i) induced by 30 mM KCl was equally sensitive to either omega-agatoxin IVA (omega-Aga IVA) or to omega-conotoxin MVIIC (omega-CgTx MVIIC), and the inhibition produced by these two peptides was not additive. The present results indicate that omega-Aga IVA and omega-CgTx MVIIC do not distinguish between two different VSCC in hippocampal synaptosomes and that they both inhibit a channel with the alpha 1A subunit which is present in the rat hippocampus.

Regulation of intracellular [Ca2+] and GABA release by presynaptic GABAB receptors in rat cerebrocortical synaptosomes

In this study we determined the changes in the intracellular free Ca2+ concentration, associated with the inhibitory modulation of the exocytotic release of GABA by GABAB receptor activation in rat cerebrocortical synaptosomes. We observed that SK&F 97541 and (-)baclofen both act as agonists of the presynaptic GABAB receptors in modulating GABA release and Ca2+ influx due to KCl (10 mM) depolarization, but SK&F 97541 is more potent than (-)baclofen in modulating both Ca2+ influx and GABA release. Thus, activation of GABAB receptors by either SK&F97541 (10 microM) or by (-)baclofen (100 microM) caused about 18% inhibition of the increase in [Ca2+]i, due to KCl depolarization, and inhibited the [3H]GABA release by about 30%. The pharmacological similarities of the GABAB receptor activation in producing inhibition of both calcium channel mediated influx of Ca2+ and transmitter release suggest that presynaptic inhibition of GABA release by GABAB receptor activation may result, at least in part, from inhibition of Ca2+ influx through P-type (or possibly Q-type) Ca2+ channels, sensitive to omega-Agatoxin IVA (200 nM). Furthermore, modulation of GABA release of GABAB receptors was abolished by preincubation with pertussis toxin, suggesting that a pertussis toxin sensitive G protein may be the coupling factor between GABAB receptors and the voltage-dependent Ca2+ channels associated with the exocytotic release of GABA in rat cerebrocortical nerve terminals.

A functionally active presynaptic high-affinity kainate receptor in the rat hippocampal CA3 subregion

We studied the modulation of the intracellular free calcium concentrations ([Ca2+]i) by kainate/AMPA receptor activation in synaptosomes isolated from whole rat hippocampus, or from its CA1, CA3 or dentate gyrus subregions. The receptor was activated either by 100 microM S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolopropionic acid (AMPA) (EC50 = 26.6 +/- 4.9 microM) or by 100 microM kainate (EC50 = 0.81 +/- 0.1 microM), but the effects of these agonists were not additive. The response to either AMPA or kainate was competitively inhibited by 10 microM 6-cyano-7-nitroquinoxaline-2,3-dioxine. Higher [Ca2+]i responses to 100 microM AMPA or to 100 microM kainate were observed in the CA3 subregion (43.2 +/- 2.5 nM or 42.8 +/- 2.3 nM, respectively) than in the whole hippocampus (22.4 +/- 1.1 nM or 22.4 +/- 1.6, respectively), in the CA1 subregion (26.4 +/- 1.1 nM or 26.6 +/- 2.6 nM, respectively) or in dentate gyrus (24.6 +/- 1.4 nM or 21.5 +/- 1.0 nM, respectively). These results indicate that the CA3 subregion of the hippocampus is enriched in a presynaptic high-affinity kainate receptor which modulates the [Ca2+]i in nerve terminals.

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.

Modulation of dopamine and noradrenaline release and of intracellular Ca2+ concentration by presynaptic glutamate receptors in hippocampus

1. We studied the release of [3H]-dopamine and [3H]-noradrenaline (NA) from hippocampal synaptosomes induced by glutamate receptors and the associated Ca2+ influx through Ca2+ channels. The release of tritiated neurotransmitters was studied by use of superfusion system and the intracellular free Ca2+ concentration ([Ca2+]i) was determined by a fluorimetric assay with Indo-1 as a probe for Ca2+. 2. Presynaptic glutamate receptor activation induced Ca(2+)-dependent release of [3H]-dopamine and [3H]-NA from rat hippocampal synaptosomes. Thus, L-glutamate induced the release of both neurotransmitters in a dose-dependent manner (EC50 = 5.62 microM), and the effect of 100 microM L-glutamate was inhibited by 83.8% in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dioxine (CNQX), but was not affected by 1 microM (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801). 3. Other glutamate receptor agonists also stimulated the Ca(2+)-dependent release of [3H]-dopamine and [3H]-NA as follows: N-methyl-D-aspartate (NMDA), at 200 microM, released 3.65 +/- 0.23% of the total 3H catecholamines, and this effect was inhibited by 81.2% in the presence of 1 microM MK-801; quisqualate (50 microM), S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolopropionic acid (AMPA) (100 microM) or kainate (100 microM) released 1.57 +/- 0.26%, 1.93 +/- 0.17% and 2.09 +/- 0.22%, of the total 3H catecholamines, respectively. 4. The ionotropic glutamate receptor agonist, AMPA, induced an increase in the [Ca2+]i which was inhibited by 58.6% in the presence of 10 microM CNQX. In contrast, the increase in [Ca2+]i due to stimulation by glutamate was not sensitive to CNQX or MK-801.5. Nitrendipine, at I JAM, did not inhibit the neurotransmitter release induced by AMPA, but, both 0.5 micro M -conotoxin GVIA (w-CgTx) and 100 nM w-Aga IVA reduced catecholamine release to 49.03 +/- 3.79% and 46.06 +/- 10.51% of the control, respectively. In the presence of both toxins the release was reduced to 12.58 +/- 4.64% of the control.6. The results indicate that activation of presynaptic glutamate receptors of the NMDA and non-NMDA type induces the release of [3H]-dopamine and [H]-NA from rat hippocampal synaptosomes and that the release induced by AMPA involves the activation of N- and P-type Ca2" channels which allow the influx of Ca2" that triggers the 3H catecholamines release.

Release of [3H]GABA evoked by glutamate receptor agonists in cultured chick retina cells: effect of Ca2+

The effect of glutamate receptor agonists (NMDA, kainate, quisqualate and AMPA) on the [Ca2+]i and on [3H]GABA release was studied in cultured chick embryonic retinal cells. The release of [3H]GABA evoked by NMDA, in the absence of Ca2+, was prevented by the NMDA receptor antagonist (+)-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801), and that produced by kainate and quisqualate was prevented by 6-cyano-7-nitroquinoxaline-2,3-dioxine (CNQX). All the agonists tested increased the [Ca2+]i, and when the GABA transporter was blocked by 1-(2-(((diphenyl-methylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3- pyridine-carboxylic acid (NNC-711), NMDA, AMPA or quisqualate, but not kainate, did not induce [3H]GABA release unless Ca2+ (1 mM) was present, showing that exocytotic release of [3H]GABA occurs in retinal cells under these conditions.

Calcium channel blockers inhibit the (Ca2+ + Mg2+)-ATPase activity and the 125I-calmodulin binding in brain membranes

Ca2+ channel blockers belonging to three distinct chemical groups (dihydropyridines, phenylalkylamines and diphenylalkylamines) differentially inhibit the (Ca2+ + Mg2+)-ATPase activity of synaptic plasma membranes (Santos et al., J. Neurochem. 52, S49D, 1989). We now report that (-)-desmethoxyverapamil and flunarizine are the most potent inhibitors of the Ca(2+)-activated ATPase activity of synaptic plasma membranes, decreasing the Vmax by 41% and 37%, respectively, with no significant effects on the Km for Ca2+ (162.7 +/- 14.9 nM free [Ca2+]), while nitrendipine did not affect these parameters. Trifluoperazine was the most potent inhibitor of the Ca(2+)-activated ATPase of synaptic plasma membranes with an IC50 of 8-10 microM. To clarify whether the inhibitory effects of Ca2+ channel blockers and of trifluoperazine on the (Ca2+ + Mg2+)-ATPase occur through the inhibition of the interaction of calmodulin with the enzyme, we studied their effects on the binding of 125I-calmodulin to the membrane proteins separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by electrotransfer to nitrocellulose and autoradiography. The autoradiograms revealed Ca(2+)-dependent CaM binding proteins of about 140, 70 and 55 kDa. Trifluoperazine (30-40 microM) inhibited by 50-60% the binding of 125I-calmodulin to the 140 kDa band, which probably includes the (Ca2+ + Mg2+)-ATPase protein. Flunarizine and (-)-desmethoxyverapamil (100 microM) inhibited the 125I-calmodulin binding to the 140 kDa peptides by 100 and 90%, respectively, and they inhibited by 55 and 40%, respectively, the binding of 125I-calmodulin to the peptides in the 70-55 kDa range, whereas nitrendipine did not show any effect. The results suggest that the inhibitory effects of (-)-desmethoxyverapamil and flunarizine, as well as trifluoperazine, on the (Ca2+ + Mg2+)-ATPase activity of synaptic plasma membranes are mediated by inhibition of the calmodulin interaction with the enzyme.

Relation of exocytotic release of gamma-aminobutyric acid to Ca2+ entry through Ca2+ channels or by reversal of the Na+/Ca2+ exchanger in synaptosomes

The specific inhibitor of the gamma-aminobutyric acid (GABA) carrier, NNC-711, (1-[(2-diphenylmethylene)amino]oxyethyl)- 1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride, blocks the Ca(2+)-independent release of [3H]GABA from rat brain synaptosomes induced by 50 mM K+ depolarization. Thus, in the presence of this inhibitor, it was possible to study the Ca(2+)-dependent release of [3H]GABA in the total absence of carrier-mediated release. Reversal of the Na+/Ca2+ exchanger was used to increase the intracellular free Ca2+ concentration ([Ca2+]i) to test whether an increase in [Ca2+]i alone is sufficient to induce exocytosis in the absence of depolarization. We found that the [Ca2+]i may rise to values above 400 nM, as a result of Na+/Ca2+ exchange, without inducing release of [3H]GABA, but subsequent K+ depolarization immediately induced [3H]GABA release. Thus, a rise of only a few nanomolar Ca2+ in the cytoplasm induced by 50 mM K+ depolarization, after loading the synaptosomes with Ca2+ by Na+/Ca2+ exchange, induced exocytotic [3H]GABA release, whereas the rise in cytoplasmic [Ca2+] caused by reversal of the Na+/Ca2+ exchanger was insufficient to induce exocytosis, although the value for [Ca2+]i attained was higher than that required for exocytosis induced by K+ depolarization. The voltage-dependent Ca2+ entry due to K+ depolarization, after maximal Ca2+ loading of the synaptosomes by Na+/Ca2+ exchange, and the consequent [3H]GABA release could be blocked by 50 microM verapamil. Although preloading the synaptosomes with Ca2+ by Na+/Ca2+ exchange did not cause [3H]GABA release under any conditions studied, the rise in cytoplasmic [Ca2+] due to Na+/Ca2+ exchange increased the sensitivity to external Ca2+ of the exocytotic release of [3H]GABA induced by subsequent K+ depolarization. Thus, our results show that the vesicular release of [3H]GABA is rather insensitive to bulk cytoplasmic [Ca2+] and are compatible with the view that GABA exocytosis is triggered very effectively by Ca2+ entry through Ca2+ channels near the active zones.

Regulation of carrier-mediated and exocytotic release of [3H]GABA in rat brain synaptosomes

In this study we investigated the role of external monovalent cations, and of intracellular Ca2+ concentration ([Ca2+]i) in polarized and depolarized rat cerebral cortex synaptosomes on the release of [3H]-gamma-aminobutyric acid (3H-GABA). We found that potassium-depolarization, in the absence of Ca2+, of synaptosomes loaded with 3H-GABA releases 7.4 +/- 2.1% of the accumulated neurotransmitter, provided that the external medium contains Na+, and an additional 19.0 +/- 2.5% is released upon adding 1.0 mM CaCl2 to the exterior. The Ca(2+)-independent release component does not occur in a choline medium and it is only 3.4 +/- 0.8% of the 3H-GABA accumulated in a Li+ medium, but both ions support the Ca(2+)-dependent release of 3H-GABA (13.4 +/- 0.6% in choline and 15.4 +/- 1.5% in Li+), which suggests that the exocytotic release is independent of the external monovalent cation present, whereas the carrier-mediated release specifically requires Na+ outside. Furthermore, previous release of the cytosolic 3H-GABA due to predepolarization in the absence of Ca2+ does not influence the amount of 3H-GABA subsequently released by exocytosis due to Ca2+ addition (19.1 +/- 2.5% or 19.1 +/- 1.1%, respectively). In choline or Li+ medium, the value of the [Ca2+]i is raised by Na+/Ca2+ exchange to 663 +/- 75 nM or 782 +/- 54 nM, respectively, within three minutes after adding 1.0 mM Ca2+, in the absence of depolarization, and parallel release experiments show no release of 3H-GABA in the choline medium, but a substantial release (7.1 +/- 2.1%) of 3H-GABA occurs in the Li+ medium without depolarization. Subsequent K(+)-depolarization shows normal Ca(2+)-dependent release of 3H-GABA in the choline medium (14.1 +/- 2.0%) but only 8.6 +/- 1.1% release in the Li+ medium, which suggests that raising the [Ca2+]i by Na+/Ca2+ exchange, without depolarization, supports some exocytotic release in Li+, but not in choline media. The role of [Ca2+]i and of membrane depolarization in the release process is discussed on the basis of the results obtained and other relevant observations which suggest that both Ca2+ and depolarization are essential for optimal exocytotic release of GABA.

Calcium stores in electropermeabilized HL-60 cells before and after differentiation

Non-induced HL-60 cells (N-IND) and HL-60 cells induced to differentiate with 2 microM retinoic acid (IND) were electropermeabilized with electrical discharges, and the intracellular Ca2+ stores were measured in each type of cell. Both N-IND and IND cells accumulate Ca2+ in the presence of ATP after electropermeabilization. The Ca2+ is stored in at least two different compartments; accumulation in one of the compartments is inhibited by oligomycin and CCCP, and it is not releasable by Ins(1,4,5)P3. The maximal accumulation of Ca2+ by the Ins(1,4,5)P3 sensitive pool is about 0.3 nmol/10(6) cells and 0.9 nmol/10(6) cells for the N-IND and for the IND cells, respectively, and the half-maximal value occurs at a free Ca2+ concentration of 0.23 microM and 0.63 microM, respectively. The oligomycin + CCCP sensitive pool hardly accumulates any Ca2+ at this level of free Ca2+, but at higher free [Ca2+] (greater than microM) its maximal capacity is 80-100-fold higher than the Ins(1,4,5)P3-sensitive pool (about 17-18 nmol/10(6) cells). It is concluded that at physiological free Ca2+ concentrations, the non-mitochondrial Ca2+ pool is regulating the intracellular free Ca2+ in N-IND and IND HL-60 cells, and that this Ca2+ pool can be mobilized by Ins(1,4,5)P3. Furthermore, the capacity of this pool increases about 3-fold when the cells are induced to differentiate with retinoic acid.

Partition of Ca2+ antagonists in brain plasma membranes

The partition coefficients (Kp) of three prototype Ca2+ antagonists, nitrendipine, (-)-desmethoxyverapamil and flunarizine were determined in native synaptic plasma membranes (SPM) isolated from sheep brain cortex and in liposomes prepared with the total lipids extracted from the membranes. We found that at 25 degrees and at 5 x 10(-6) M drug concentration the Kp values of the drugs for native SPM are higher than those obtained for liposomes, and are of the order of 334 +/- 53, 257 +/- 36 and 23 X 10(3) for nitrendipine, (-)desmethoxyverapamil and flunarizine, respectively, whereas the Kp values in liposomes are 190 +/- 41, 118 +/- 10 and 6 x 10(3) for the same drugs. The results suggest that the presence of membrane proteins favors the incorporation of the drugs in the membranes. Furthermore, the Kp values of the three Ca2+ antagonists studied increase with temperature in native membranes, but not in liposomes. It is concluded that the physical partitioning in membranes of drugs which act on Ca2+ channels may play some role in the mechanism of interaction of these drugs with the Ca2+ channel proteins.

gamma-Aminobutyric acid release from synaptosomes as influenced by Ca2+ and Ca2+ channel blockers

We studied the correlation between the high affinity binding of Ca2+ channel blockers to purified synaptic plasma membranes (SPM) and the effect of these drugs in blocking the 45Ca2+ uptake and the release of [3H]gamma-aminobutyric acid [( 3H]GABA) by preloaded synaptosomes. The Ca2+ channel blocker binding sites were characterized by studying the binding of the dihydropyridine, [3H]nimodipine, and of the phenylalkylamine, (-)-[3H]desmethoxyverapamil, to purified SPM isolated from sheep brain cortex synaptosomes. The purified SPM had high affinity binding sites for both Ca2+ channel blockers. The binding parameters were similar to those previously reported for whole brain homogenates: KD = 0.64 nM and Bmax = 160 fmol/mg of protein for [3H]nimodipine, and KD = 7.9 nM and Bmax = 1,500 fmol/mg of protein for (-)-[3H]desmethoxyverapamil. The Ca2+ channel blockers inhibited the release of [3H]GABA induced by K+ depolarization in the presence or in the absence of Ca2+. The Ca2+-dependent component of [3H]GABA release was inhibited by verapamil, (-)-D 600, d-cis-diltiazem, nifedipine and PY 108-86 with IC50 values of 2.2 X 10(-5) M, 6.3 X 10(-5) M, 3 X 10(-4) M, greater than 10(-4) M and 3 X 10(-5) M, respectively. Furthermore, the Ca2+ channel blockers also inhibited the Ca2+-independent [3H]GABA release which occurred in the presence, but not in the absence, of external Na+. The Ca2+ channel blockers at concentrations which inhibited [3H]GABA release inhibited the entry of Ca2+ through the Ca2+ channels and also the entry of Ca2+ by Na+/Ca2+ exchange. We conclude that the concentrations of Ca2+ blockers necessary to block Ca2+ uptake through the Ca2+ channels and by Na+/Ca2+ exchange coincide with the concentrations at which they inhibit [3H]GABA release, but that their effect on the relationship between Ca2+ uptake and [3H]GABA release is different for the various blockers. The effects of the drugs on Ca2+ movements and [3H]GABA release are not specifically mediated through the high affinity binding of the drugs since relatively high concentrations were necessary (greater than 10(-5) M) for the effects reported here.

Occurrence and experimental infection of toads (Bufo marinus and B. granulosus) with Mycobacterium chelonei subsp. abscessus

In a survey of 234 Amazonian toads and frogs, six strains of Mycobacterium chelonei subsp. abscessus were isolated from the liver or spleen of four of 66 Bufo marinus (6.1%) and from the kidney or peritoneal fluid of two of 86 B. granulosus (2.3%). There were no histopathological lesions in the viscera of the infected animals. Experimental infection of 29 captive B. marinus and B. granulosus, by the intraperitoneal route, with a pooled inoculum of M. chelonei subsp. abscessus caused five deaths near the end of a 2-month observation period. M. chelonei subsp. abscessus was isolated from the liver, spleen, kidney, gonad, heart and lung of toads killed at various intervals after inoculation, and intracellular acid-fast bacilli were seen in these organs. Histological evidence of invasion of tissues by mycobacteria became apparent from the 45th day after infection. The susceptibility to infection of B. marinus and B. granulosus suggests that these toads may serve as a fortuitous animal host for M. chelonei subsp. abscessus.