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 (t hreo-L-BHGA), was applied locally to the giant neuron of an Achatina s nail by pneumatic brief pressure ejection and induced an outward curre nt (I-out) on the ventral-left cerebral distinct neurone (v-LCDN). The present study aimed to elucidate the ionic mechanisms of the I-out ca used by threo-L-BHGA (ItL-BHGA) of v-LCDN and the effects of ouabain o n this current under voltage clamp. 2. The reversal potentials of ItL- BHGA (EtL-BHGA) Of v-LCDN ire varied K-0(+) were fitted to the Nernst equation as ItL-BHGA=I-K (K+ current) and were almost unchanged in Cl- 0(-)-free and Na-0(+)-reduced (20% of normal) states. The ItL-BHGA is due to the increase in permeability of the neuromembrane to K+(K+ depe ndent) and is neither Na+- nor Cl--dependent. K+-channel blockers, a m ixture of tetraethyl-ammonium (TEA) and 4-amino-pyridine (4-AP), block ed ItL-BHGA mainly in a noncompetitive and partly in an uncompetitive manner. 3. Unexpectedly, ItL-BHGA of v-LCDN was almost abolished in th e Na-0(+)-free state and significantly reduced in the Cl-0(-)-free sta te. However, an Na+- channel blocker, tetrodotoxin, showed a tendency to enhance ItL-BHGA. On the other hand, ItL-BHGA was enhanced in K-0()-free state. 4 Ouabain markedly inhibited ItL-BHGA in both noncompeti tive and uncompetitive manners. Benzamil, an inhibitor of the Na+-Ca2 exchange, applied simultaneously with ouabain could not prevent ouaba in inhibition on ItL-BHGA. The currents induced by other putative neur otransmitters, including a K+ dependent I-out 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 ca lmodulin. 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-0(+)-free stat e, which inactivates the Na+-K+ pump, and tetrodotoxin, which suppress es the Na+ channel at least partly, did not affect the structure to be activated. (C) 1997 Elsevier Science Inc.