Activation of endoplasmic reticulum calcium leak by 2-APB depends on the luminal calcium concentration

It has been shown that 2-APB is a nonspecific modulator of ion channel activity, while most of the channels are inhibited by this compound, there are few examples of channels that are activated by 2-APB. Additionally, it has been shown that, 2-APB leads to a reduction in the luminal endoplasmic reticulum Ca²⁺ level ([Ca²⁺]_ER) and we have carried out simultaneous recordings of both [Ca²⁺]_i and the [Ca²⁺]_ER in HeLa cell suspensions to assess the mechanism involved in this effect. This approach allowed us to determine that 2-APB induces a reduction in the [Ca²⁺]_ER by activating an ER-resident Ca²⁺ permeable channel more than by inhibiting the activity of SERCA pumps. Interestingly, this effect of 2-APB of reducing the [Ca²⁺]_ER is auto-limited because depends on a replete ER Ca²⁺ store; a condition that thapsigargin does not require to decrease the [Ca²⁺]_ER. Additionally, our data indicate that the ER Ca²⁺ permeable channel activated by 2-APB does not seem to participate in the ER Ca²⁺ leak revealed by inhibiting SERCA pump with thapsigargin. This work suggests that, prolonged incubations with even low concentrations of 2-APB (5 μM) would lead to the reduction in the [Ca²⁺]_ER that might explain the inhibitory effect of this compound on those signals that require Ca²⁺ release from the ER store.