Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.4.24.64 (MPP)
 1,876 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

1. The calcium dependence of spontaneous transmitter release from nerve terminals of different lengths was examined at neuromuscular junctions in frog muscle. Miniature endplate potential (MEPP) frequency was positively correlated with the endplate potential (EPP) quantal content and was dependent on external Ca2+. The higher the resting MEPP frequency in a 0.25 mM-Ca2+ Ringer solution, the greater the dependence on external Ca2+. MEPP frequency in all terminals dropped to approximately the same low level in a Ca2(+)-free Ringer solution containing EGTA. This suggests that terminals with higher release levels have a larger Ca2+ influx at rest. 2. Several tests were done to try to characterize the mode of Ca2+ entry into resting terminals. omega-Conotoxin (omega-CgTx) blocked evoked release and reduced MEPP frequency, but not as effectively as zero Ca2(+)-EGTA Ringer solution. Some component of Ca2+ influx thus appears to enter through channels insensitive to omega-CgTx. Tetrodotoxin (TTX) did not affect MEPP frequency, indicating that the Ca2+ did not enter through TTX-sensitive Na+ channels that might be opening spontaneously at rest. Hyperpolarization of the terminal by reducing the K+ in the Ringer solution caused no consistent differences in MEPP frequency, suggesting that the Ca2+ influx is relatively insensitive to small changes in membrane potential around the resting level. Strong buffering of the Ringer solution with citrate, to overwhelm any differences in Ca2+ buffering within different junctional clefts, had no significant effect on the MEPP frequency. 3. Evidence that the Na(+)-Ca2+ exchanger helps set the internal Ca2+ level was obtained. Reduction of the Na+ concentration in the Ringer solution caused increases in MEPP frequency ranging from 6 to 440%. However, these changes were not correlated with resting MEPP frequency, hence differences in MEPP frequency probably are not the result of differences in Na(+)-Ca2+ exchanger function in terminals having a uniform Ca2+ leak. 4. Although MEPP frequency was generally correlated with quantal content, in subsets of junctions grouped according to their similar quantal contents, there was a positive correlation between MEPP frequency and terminal length. 5. In zero Ca2(+)-EGTA Ringer solution, the low residual MEPP frequency is independent of terminal length, even when MPP frequency is sharply increased by tetanic stimulation.
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PMID:Dependence of spontaneous release at frog junctions on synaptic strength, external calcium and terminal length. 257 68

The effects of N-methyl-4-phenylpyridinium cation (MPP+) and of an endogenously formed analog, 2,9-di-methyl-norharmanium cation (2,9-Me2NH+), on extracellular dopamine were studied in the striatum of freely moving rats. Perfusion of either 2,9-Me2NH+ or MPP+ through a microdialysis probe evoked a marked and dose-dependent increase of dopamine levels. Tetrodotoxin and Ca(2+)-free medium prevented the increase in dopamine levels induced by 2,9-Me2NH+, but not that induced by MPP+. Cocaine, 3 microM, intensified the 2,9-Me2 NH(+)-induced increase in extracellular dopamine and slightly attenuated the MPP(+)-induced efflux. S(-)-3-(3-Hydroxy-phenyl)-N-propylpiperidine, that acts as an antagonist of dopamine autoreceptors in the presence of a dopamine reuptake inhibitor, markedly enhanced the increase in extracellular dopamine elicited by 2,9-Me2NH+, but not that by MPP+. These results suggested that 2,9-Me2NH+ was a potent dopamine reuptake blocker, whereas MPP+ acts as an amphetamine-like dopamine releaser rather than a reuptake inhibitor on the membrane transporter.
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PMID:Differences in dopamine efflux induced by MPP+ and beta-carbolinium in the striatum of conscious rats. 896 Aug 77