Effects of calcium antagonists on KCl-evoked calcium uptake by rat cortical synaptosomes

Evidence for two populations of bitter responsive taste cells in mice

Taste receptor cells use multiple signaling mechanisms to detect different taste stimuli in the oral cavity. Ionic stimuli (sour, salty) interact directly with ion channels to elicit responses, whereas bitter, sweet, and umami tastants activate G protein-coupled receptors to initiate phospholipase C (PLC)-dependent release of calcium from intracellular stores. However, the precise role for PLC in taste responses remains unclear. One study reported that bitter, sweet, and umami detection is abolished in PLCbeta2 knock-out animals, indicating that the perception of these stimuli depends solely on PLCbeta2. In contrast, another study found that PLCbeta2 knock-out mice have a reduced, but not abolished, capacity to detect these taste qualities, suggesting a PLCbeta2-independent signaling pathway may be involved in the detection of taste stimuli. Since PLCbeta2-expressing taste cells do not have conventional synapses or express voltage-gated calcium channels (VGCCs), we sought to determine if any taste cells responding to bitter express VGCCs. We characterized calcium responses generated by bitter stimuli to activate the PLC pathway and 50 mM KCl to activate VGCCs. Comparisons of evoked calcium responses found that these two stimuli generated significantly different responses. Surprisingly, although most responsive taste cells responded to bitter or 50 mM KCl, some taste cells responded to both. Analysis of dual responsive cells found that bitter responses were inhibited by the PLC inhibitor U73122. Immunocytochemical analysis detected PLCbeta3 and IP(3)R1, indicating the presence of multiple PLC signaling pathways in taste cells.

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.

Effect of flunarizine and methylprednisolone on functional recovery after experimental spinal injury

The effect of flunarizine and methylprednisolone on the recovery of somatosensory evoked potentials (SEPs) was evaluated in an experimental model of spinal cord impact injury in anesthetized cats. In addition, the effect of flunarizine on posttraumatic spinal cord blood flow (SCBF) (using the hydrogen clearance technique) and interstitial calcium and potassium activity (ion-selective electrodes) was investigated. After the injury (600 g.cm), SEPs disappeared, followed by a spontaneous recovery to 17% of the preinjury amplitude at the end of the 4 h observation period. Flunarizine treatment (0.1 mg/kg IV, given 5 and 120 min after injury) resulted in a significantly improved recovery of SEPs, reaching 52% of the preinjury amplitude. Methylprednisolone treatment (30 mg/kg IV, given 5 min after injury) resulted in a 30% recovery level, significantly better than in untreated animals but significantly inferior to flunarizine treatment. Combination of both treatments resulted in a 62% recovery level, significantly better than after methylprednisolone treatment alone. Flunarizine treatment had no significant effect on the postinjury evolution of SCBF and interstitial potassium activity; it did, however, significantly accelerate the recovery of interstitial calcium activity, which sharply decreased immediately after injury. It is concluded that intravenous administration of the calcium entry blocker flunarizine improves the functional recovery of the spinal cord in the acute phase after experimental spinal impact injury. The observed improvement is not achieved by an effect on local blood flow but is possibly related to an inhibitory effect of the drug on cellular calcium entry.

Inhibition of K(+)-evoked [3H]D-aspartate release and neuronal calcium influx by verapamil, diltiazem and dextromethorphan: evidence for non-L/non-N voltage-sensitive calcium channels

The effects of inhibitors of voltage-sensitive calcium channels (VSCC) on K(+)-evoked [3H]D-aspartate release from rat hippocampal slices and the K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture were examined. K+ caused a concentration-dependent release of [3H]D-aspartate that was approximately 85% dependent on the presence of extracellular calcium. Neither the marine snail toxin, omega-conotoxin GVIA, nor the dihydropyridine VSCC antagonist, nitrendipine, had any effect on K(+)-evoked release of [3H]D-aspartate. omega-Conotoxin GVIA and nitrendipine caused a relatively small (20-30%) inhibition of K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture. This suggests that K(+)-evoked [3H]D-aspartate release is not dependent on L- or N-type VSCC, whereas K(+)-evoked neuronal calcium influx was only partially dependent on L- and N-type VSCC. Verapamil, dextromethorphan and diltiazem caused a concentration-dependent inhibition of K(+)-evoked release of [3H]D-aspartate with IC50 values of 30, 100 and 120 microM, respectively. The K(+)-evoked increase in intracellular calcium was inhibited with essentially the same rank order of potency, but with slightly greater potencies (IC50 values for verapamil, diltiazem and dextromethorphan were 20, 50 and 50 microM, respectively). At 300 microM, neither verapamil, diltiazem nor dextromethorphan inhibited [3H]D-aspartate release evoked by the calcium ionophore ionomycin, suggesting that these compounds are not acting intracellularly to inhibit the ability of free cytosolic calcium to evoke release.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcium antagonists on (Na+ + K+)-ATPase, Mg2+-ATPase and Ca2+-ATPase activities of rat cortical synaptosomes

1. The effects of 11 calcium antagonists on (Na+ + K+)-ATPase, Mg2+-ATPase and Ca2+-ATPase activities of rat cortical synaptosomes were studied. 2. All the calcium antagonists studied had inhibitory effects on ouabain-sensitive (Na+ + K+)-ATPase, Mg2+-ATPase and Ca2+-ATPase activities in synaptosomes at high concentrations (10 or 100 microM). 3. Calcium antagonists such as trifluoperazine, flunarizine and cinnarizine had inhibitory effects on Ca2+-ATPase activity at low concentrations (1-10 microM). 4. Trifluoperazine and La3+ had inhibitory effects on Mg2+-ATPase activity at low concentration (1 microM). 5. Our results suggest that most of the calcium antagonists studied have little effects on neuronal (Na+ + K+)-ATPase, Mg2+-ATPase and Ca2+-ATPase activities at therapeutic dose ranges (1 microM or lower).

Effects of calcium ionophore A23187 and calcium antagonists on 32Pi incorporation into polyphosphoinositides of rat cortical synaptosomes

The role of Ca2+ on 32Pi incorporation into polyphosphoinositides (PPI) of rat cortical synaptosomes was studied. Stimulation of muscarinic receptor by carbachol (1 mM) resulted in a decrease in 32Pi incorporation into phosphatidylinositol-4,5-bisphophaphate (TPI) and phosphatidylinositol-4-phosphate (DPI), and an increase in 32Pi incorporation into phosphatidylinositol (PI) and phosphatidic acid (PA), whereas no significant effect on other membrane phospholipids was found. This response could be blocked by atropine (1 microM). The stimulatory effect of carbachol required Ca2+ in the medium; the presence of 0.5 mM EGTA blocked the effect of carbachol on PPI turnover completely. Calcium ionophore A23187, at 1 microM, had a similar effect on PPI turnover by carbachol (1 mM). At higher concentrations (10-100 microM) of A23187, the PPI turnover rate was much enhanced. Depolarization of the membrane by high potassium (60 mM) in the presence of calcium resulted in an enhanced PPI turnover, which was similar to the results of the carbachol (1 mM) effect but to a lesser extent. Calcium antagonists, diltiazem and trifluoperazine, at 10 microM could block the carbachol effect on 32Pi incorporation into PPI in this preparation. Our results suggest that the enhancement of PPI turnover in rat cortical synaptosomes by carbachol, calcium ionophore or high potassium requires Ca2+, and it can be blocked by compounds which interfere with the availability of this ion, such as EGTA or calcium antagonists.