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 µM). 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 µM) or injection of the poorly metabolized Ins(1,4,5)P₃ analogue, Ins(2,4,5)P₃, cannot activate the same magnitude of Ca²⁺-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.