Emptying of Ca²⁺ stores elicits changes in cytosolic pH

Ca²⁺ release from cellular stores activates vascular smooth muscle (VSM) contraction in response to many agonists. Emptying of the stores often activates Ca²⁺ influx across the plasmalemma. The mechanism by which store emptying activates Ca²⁺ influx is unclear, though evidence exists for a messenger that diffuses from the stores to activate a Ca²⁺ current. Ca²⁺ itself is not the messenger. Some Ca²⁺ storage compartments in VSM are acidic. We co-loaded A7R5 VSM cells with Fura-2 and Snarf-1, to simultaneously monitor cell Ca²⁺ and pH. Inhibition of SERCA Ca²⁺-ATPase by thapsigargin (TG) initiated a sustained increase in cell Ca²⁺ and H⁺. In the absence of extracellular Ca²⁺, TG also elicited an increase in Ca²⁺ and H⁺, but cell Ca²⁺ declined to basal levels within minutes. Adding Ca²⁺ back elicited a rapid increase in [Ca²⁺]i, and the increase was not inhibited by pretreatment with L-type Ca²⁺ channel blockers. Release of Ca²⁺ by elevating IP₃ levels using arginine vasopressin (AVP) also induced Ca²⁺ release, and subsequent influx of extracellular Ca²⁺, however [H⁺]i decreased, and the magnitude of the Ca²⁺ influx was significantly greater if AVP was used to empty the Ca²⁺ stores. Addition of TG after AVP elicits cell acidification and a small release of Ca²⁺indicating that an AVP insensitive pool is the source of protons. We conclude that changes in H⁺ levels influence the magnitude of the store operated Ca²⁺ influx, but are not likely to be the physiologically relevant initiating diffusable messenger.