Inositol derivatives modulate spontaneous transmitter release at the frog neuromuscular junction.
One of the consequences of G-protein-coupled receptor activation is stimulation of phosphoinositol metabolism, leading to the generation of IP3 and its metabolites 1,3,4,5-tetrakisphosphate (IP4) and inositol 1,2,3,4,5,6-hexakisphosphate (IP6). Previous reports indicate that high inositol polyphosphates (IP4 and IP6) are involved in clathrin-coated vesicular recycling. In this study, we examined the effects of IP4 and IP6 on spontaneous transmitter release in the form of miniature endplate potentials (MEPP) and on enhanced vesicular recycling by high K+ at frog motor nerve endings. In resting conditions, IP4 and IP6 delivered intracellularly via liposomes, caused concentration-dependent increases in MEPP frequency and amplitude. Pretreatment with the protein kinase A (PKA) inhibitor H-89 or KT 5720 reduced the IP4-mediated MEPP frequency increase by 60% and abolished the IP6-mediated MEPP frequency increases as well as the enhancement in MEPP amplitude. Pretreatment with antibodies against phosphatidylinositol 3-kinase (PI 3-K), enzyme also associated with clathrin-coated vesicular recycling, did not alter the IP4 and IP6-mediated MEPP frequency increases, but reduced the MEPP amplitude increase by 50%. In our previous reports, IP3, but not other second messengers releasing Ca2+ from internal Ca2+ stores, is able to enhance the MEPP amplitude. In order to dissociate the effect of Ca2+ release vs. metabolism to IP4 and IP6, we evaluated the effects of 3-deoxy-3-fluoro-inositol 1,4,5-trisphosphate (3F-IP3), which is not converted to IP4 or IP6. 3F-IP3 produced an increase then decrease in MEPP frequency and a decrease in MEPP amplitude. In elevated vesicle recycling induced by high K+-Ringer solution, IP4 and IP6 have similar effects, except decreasing MEPP frequency at a higher concentration (10(-4) M). We conclude that (1) high inositol polyphosphates may represent a link between IP3 and cAMP pathways; (2) the IP3-induced increase of MEPP amplitude is likely to be due to its high inositol metabolites; (3) PI 3-K is not involved in the IP4 and IP6-mediated MEPP frequency increases, but may be involved in MEPP size.
Brailoiu
Eugen
E
East Tennessee State University, Department of Pharmacology, James H. Quillen College of Medicine, P.O. Box 70577, Johnson City, TN 37614, USA.
Miyamoto
Michael D
MD
Dun
Nae J
NJ
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D000906:Antibodies / Q000494:pharmacology; D002227:Carbazoles / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D007294:Inositol / Q000031:analogs & derivatives / Q000494:pharmacology*; D000072181:Inositol Polyphosphate 5-Phosphatases; D007546:Isoquinolines / Q000494:pharmacology; D008081:Liposomes / Q000378:metabolism; D009469:Neuromuscular Junction / Q000187:drug effects* / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000276:immunology / Q000494:pharmacology; D010744:Phosphoric Monoester Hydrolases / Q000494:pharmacology; D011758:Pyrroles / Q000494:pharmacology; D011894:Rana pipiens; D013449:Sulfonamides
D000906:Antibodies; D002227:Carbazoles; D004791:Enzyme Inhibitors; D007211:Indoles; D007546:Isoquinolines; D008081:Liposomes; D018377:Neurotransmitter Agents; D011758:Pyrroles; D013449:Sulfonamides; D007294:Inositol; C057416:KT 5720; D019869:Phosphatidylinositol 3-Kinases; D010744:Phosphoric Monoester Hydrolases; D000072181:Inositol Polyphosphate 5-Phosphatases; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide