Regulation of inositol trisphosphate-induced membrane currents in Xenopus oocytes by a Jurkat cell calcium influx factor.
The functional interactions of a Jurkat cell-derived calcium influx factor (CIF) with Ins(1,4,5)P3 were examined by microinjection and voltage-clamp recording of current responses in Xenopus oocytes. CIF, which stimulates Ca2+ entry directly on microinjection, was active at dilutions at which it had no direct effect by augmenting both initial rapid Ins(1,4,5)P3-mediated Ca2+ discharge-activated currents and later sustained Ca2+ entry-activated currents. Augmented initial membrane currents were 3-5-fold greater in peak amplitude than currents evoked by injection of the same dose of Ins(1,4,5)P3 alone. The augmented initial response was not decreased by removal of extracellular Ca2+, suggesting that there is potentiation of Ins(1,4,5)P3-mediated discharge from intracellular Ca2+ stores. However, the augmentation of Ins(1,4,5)P3-mediated discharge cannot be due to an enhanced production of endogenous Ins(1,4,5)P3 because maximal Ins(1,4,5)P3-activated currents saturate (approx. 500 nA) with supramaximal levels of Ins(1,4,5)P3 (10-50 microM). Depletion of Ca2+ stores, by pretreatment with thapsigargin or by prior injection with the Ins(1,4,5)P3 receptor antagonist heparin, abolished membrane currents elicited by Ins(1,4,5)P3/CIF co-injection, further suggesting that the Ins(1,4,5)P3 receptor was the target for the initial-current potentiating actions of CIF. In this regard, CIF also induced augmented initial currents with co-injection of either Ins(2,4,5)P3 or Ins(1,3,4,5)P4. The augmentation of Ins(1,4,5)P3-mediated currents by CIF was bell-shaped with regard to Ins(1,4,5)P3 concentration, reminiscent of the regulatory influence of Ca2+ on Ins(1,4,5)P3 responses. Co-injection of Ins(1,4,5)P3 and CIF also augmented (2-3-fold) later current responses arising from sustained Ca2+ entry. The augmented late-current responses were not due to enhanced Ca2+ store depletion because supramaximal levels of Ins(1,4,5)P3 (50 microM) or injection of the poorly metabolized Ins(1,4,5)P3 analogue, Ins(2,4,5)P3, cannot activate the same magnitude of Ca(2+)-entry-dependent currents. These results suggest that CIF at low levels interacts with Ins(1,4,5)P3 to sensitize two pathways of Ca2+ signalling: initial discharge and later Ca2+ entry. Thus under physiological conditions CIF might be more potent as a co-messenger than as a direct Ca2+ entry signal and might provide a novel type of direct feedback regulation between the stores-activated influx pathway and the Ins(1,4,5)P3 receptor. Moreover these results suggest that CIF modulation of the receptor for Ins(1,4,5)P3 may underlie control of both augmentation of discharge and Ca2+ entry, as has been predicted from the conformational coupling model of Ca2+ entry.
Thomas
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Department of Biological Chemistry, University of California, Davis School of Medicine 95616-8635, USA.
Kim
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Hanley
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D000818:Animals; D001685:Biological Factors / Q000302:isolation & purification / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002462:Cell Membrane / Q000502:physiology; D004357:Drug Synergism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D019169:Jurkat Cells; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008954:Models, Biological; D009865:Oocytes / Q000502:physiology*; D018408:Patch-Clamp Techniques; D015398:Signal Transduction; D019284:Thapsigargin / Q000494:pharmacology; D013997:Time Factors; D014982:Xenopus laevis
D001685:Biological Factors; D004791:Enzyme Inhibitors; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium