Ouabain-sensitive K(+)-dependent outward current caused by threo-beta-hydroxy-L-glutamic acid on a snail neuron.
1. An analog of L-glutamic acid, threo-beta-hydroxy-L-glutamic acid (threo-L-BHGA), was applied locally to the giant neuron of an Achatina snail by pneumatic brief pressure ejection and induced an outward current (Iout) on the ventral-left cerebral distinct neurone (v-LCDN). The present study aimed to elucidate the ionic mechanisms of the Iout caused by threo-L-BHGA (ItL-BHGA) of v-LCDN and the effects of ouabain on this current under voltage clamp. 2. The reversal potentials of ItL-BHGA (EtL-BHGA) of v-LCDN in varied K+o were fitted to the Nernst equation as ItL-BHGA = IK (K+ current) and were almost unchanged in Cl-o-free and Na+o-reduced (20% of normal) states. The ItL-BHGA is due to the increase in permeability of the neuromembrane to K+(K(+)-dependent) and is neither Na(+)- nor Cl-(-)-dependent. K(+)-channel blockers, a mixture of tetraethyl-ammonium (TEA) and 4-amino-pyridine (4-AP), blocked ItL-BHGA mainly in a noncompetitive and partly in an uncompetitive manner. 3. Unexpectedly, ItL-BHGA of v-LCDN was almost abolished in the Na+o-free state and significantly reduced in the Cl-o-free state. However, an Na(+)-channel blocker, tetrodotoxin, showed a tendency to enhance ItL-BHGA. On the other hand, ItL-BHGA was enhanced in K+o-free state. 4. Ouabain markedly inhibited ItL-BHGA in both noncompetitive and uncompetitive manners. Benzamil, an inhibitor of the Na(+)-Ca2+ exchange applied simultaneously with ouabain could not prevent ouabain inhibition on ItL-BHGA. The currents induced by other putative neurotransmitters, including a K(+)-dependent Iout caused by dopamine on v-LCDN, were not affected by ouabain. 5. According to our previous study, the threo-L-BHGA receptors are not linked with protein kinases or calmodulin. Then, ItL-BHGA could be produced by the receptor K+ channel complex or the receptor-G-protein-K+ channel combination. The present results indicate that the ATPase activity inhibited by ouabain and the presence of extracellular Na+ and Cl- are needed for threo-L-BHGA to activate the K(+)-dependent structure. Furthermore, the K+o-free state, which inactivates the Na(+)-K+ pump, and tetrodotoxin, which suppresses the Na+ channel at least partly, did not affect the structure to be activated.
Zhang
W
W
Department of Neurophysiology, Gifu University School of Medicine, Japan.
Takeuchi
H
H
Kurono
M
M
Emaduddin
M
M
D015761:4-Aminopyridine / Q000494:pharmacology; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D004298:Dopamine / Q000494:pharmacology; D005971:Glutamates / Q000494:pharmacology*; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects*; D009479:Neuropeptides / Q000494:pharmacology; D018377:Neurotransmitter Agents / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology*; D026902:Potassium Channel Blockers; D012908:Snails; D009435:Synaptic Transmission / Q000187:drug effects*; D019789:Tetraethylammonium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D005971:Glutamates; D009479:Neuropeptides; D018377:Neurotransmitter Agents; D026902:Potassium Channel Blockers; C013407:benzamil; C059927:achatin I; D013779:Tetrodotoxin; C011859:hydroxyglutamic acid; D010042:Ouabain; D019789:Tetraethylammonium; D000584:Amiloride; D015761:4-Aminopyridine; D011188:Potassium; D004298:Dopamine