Speaker
Description
$Ca^{2+}$ diffusion within cells and penetration of $Ca^{2+}$ through their membrane engages a wide field of theoretical and experimental research. Therefore, the monitoring of rapid changes of the $Ca^{2+}$ concentration beneath the cell membrane is of great interest. Here, we make use of BK-type $Ca^{2+}$-activated $K^+$ channels to determine the $Ca^{2+}$ activity of PMCA, which transport $Ca^{2+}$ ions out of cells. Due to their large conductance and their particular gating kinetics the BK channels may be used as fast and reliable sensors for intracellular $Ca^{2+}$ - concentration beneath the plasma membrane. Experimentally we monitor the PMCA-mediated $Ca^{2+}$ clearance (or transport) by the decay of BK-currents following their activation by a short (0.8 ms) period of $Ca^{2+}$ -influx through Cav2.2 channels. To relate the experimentally observed temporal evolution of the $K^+$ current to the underlying temporal evolution of the $Ca^{2+}$ concentration we implement a theoretical model for the $Ca^{2+}$-dependence of the BK-current and of the PMCA pump strength. Next to the transport in and out of a cell and the diffusion of $Ca^{2+}$ ions within the cell, we expand our model by the reaction of the $Ca^{2+}$ concentration with a buffer solution, as well defined EGTA concentration is present in all experimental measurements. We fit the PMCA pump strength by the best match of the predicted time course of the $K^+$ current with the experimental data. It turns out that this pump strength is at least 2 orders of magnitude larger than what has been assumed so far.
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