The functional interactions of a Jurkat cell-derived calcium influx factor (CIF) with Ins(1,4,5)P-3 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)P-3-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)P-3 alone. The augmented initial response was not decreased by removal of extracellular Ca2+, suggesting that there is potentiation of Ins(1,4,5)P-3-mediated discharge from intracellular Ca2+ stores. However, the augmentation of Ins(1,4,5)P-3-mediated discharge cannot be due to an enhanced production of endogenous Ins(1,4,5)P-3 because maximal Ins(1,4,5)P-3-activated currents saturate (approx. 500 nA) with supramaximal levels of Ins(1,4,5)P-3 (10-50 mu M). Depletion of Ca2+ stores, by pretreatment with thapsigargin or by prior injection with the Ins(1,4,5)P-3 receptor antagonist heparin, abolished membrane currents elicited by Ins(1,4,5)P-3/CIF co-injection, further suggesting that the Ins(1,4,5)P-3 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)P-3 or Ins(1,3,4,5)P-4. The augmentation of Ins(1,4,5)P-3-mediated currents by CIF was bell-shaped with regard to Ins(1,4,5)P-3 concentration, reminiscent of the regulatory influence of Ca2+ on Ins(1,4,5)P-3 responses. Co-injection of Ins(1,4,5)P-3 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)P-3 (50 mu M) or injection of the poorly metabolized Ins(1,4,5)P-3 analogue, Ins(2,4,5)P-3, cannot activate the same magnitude of Ca2+-entry-dependent currents. These results suggest that CIF at low levels interacts with Ins(1,4,5)P-3 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)P-3 receptor. Moreover these results suggest that CIF modulation of the receptor for Ins(1,4,5)P-3 may underlie control of both augmentation of discharge and Ca2+ entry, as has been predicted from the conformational coupling model of Ca2+ entry.
