Potentiation of Fc∈ Receptor I–Activated Ca(2+) Current (I(CRAC)) by Cholera Toxin: Possible Mediation by Adp Ribosylation Factor

Antigen-evoked influx of extracellular Ca(2+) into mast cells may occur via store-operated Ca(2+) channels called calcium release–activated calcium (CRAC) channels. In mast cells of the rat basophilic leukemia cell line (RBL-2H3), cholera toxin (CT) potentiates antigen-driven uptake of (45)Ca(2+) through cAMP-independent means. Here, we have used perforated patch clamp recording at physiological temperature to test whether cholera toxin or its substrate, Gs, directly modulates the activity of CRAC channels. Cholera toxin dramatically amplified (two- to fourfold) the Ca(2)+ release–activated Ca(2+) current (I(CRAC)) elicited by suboptimal concentrations of antigen, without itself inducing I(CRAC), and this enhancement was not mimicked by cAMP elevation. In contrast, cholera toxin did not affect the induction of I(CRAC) by thapsigargin, an inhibitor of organelle Ca(2+) pumps, or by intracellular dialysis with low Ca(2+) pipette solutions. Thus, the activity of CRAC channels is not directly controlled by cholera toxin or Gsα. Nor was the potentiation of I(CRAC) due to enhancement of phosphoinositide hydrolysis or calcium release. Because Gs and the A subunit of cholera toxin bind to ADP ribosylation factor (ARF) and could modulate its activity, we tested the sensitivity of antigen-evoked I(CRAC) to brefeldin A, an inhibitor of ARF-dependent functions, including vesicle transport. Brefeldin A blocked the enhancement of antigen-evoked I(CRAC) without inhibiting ADP ribosylation of Gsα, but it did not affect I(CRAC) induced by suboptimal antigen or by thapsigargin. These data provide new evidence that CRAC channels are a major route for Fc∈ receptor I–triggered Ca(2+) influx, and they suggest that ARF may modulate the induction of I(CRAC) by antigen.