Identifying the Ca super(++) signalling sources activating chloride currents in Xenopus oocytes using ionomycin and thapsigargin

The calcium ionophore, ionomycin (IM), and the sarcoplasmic/endoplasmic reticulum (SER) calcium pump inhibitor, thapsigargin (TG), were used to study the roles of Ca super(++) from different sources in regulating Ca super(++)-dependent Cl super(-) currents in Xenopus oocytes. The Ca super(++)-dependent Cl super(-) currents, Ic, were measured in voltage-clamped oocytes (Vc = -60 mV). In the presence of extracellular Ca super(++), both TG (0.1 to 10 mu M) and IM (0.1 to 10 mu M) induce release of Ca super(++) from SER and activated capacitative Ca super(++) entry (CCE) across the plasma membrane leading to activation of both "fast" and "slow" Cl super(-) currents. The fast Ic was produced by Ca super(++) release from SER while Ca super(++) entry across the plasma membrane activated the slow Ic. Intracellular application of the calcium buffer, BAPTA, blocked activation of the slow Ic due to Ca super(++) entry via CCE pathways, but not via IM-mediated movement across the plasma membrane. It is concluded that predominantly Ca super(++) release from stores regulates a fast Ic while Ca super(++) entry through CCE pathways regulates a slow Ic. Further, the CCE and slow Ic pathways must be located in spatially separated compartments since BAPTA can effectively abolish the effects of Ca super(++) entry via the CCE pathway, but not by the IM-mediated entry pathway.