The two ionophores SQI-Et (Figure 8.1) and SQI-Pr (Figure 8.2) are electrogenic sodium (Na+) ionophores developed by Drs. Tsien and Minta. These ionophores facilitate the transport of Na+ ions through lipid membranes, with the net effect of allowing Na+ to flow from a compartment with higher Na+ concentration, through the membrane, into a compartment with lower Na+ concentrations. For example, these ionophores can be used to equilibrate intracellular and extracellular Na+ concentrations.
The biophysical studies on erythrocyte “ghosts” performed by Dr. Mario Moronne at the Department of Physiology, University of California, Berkeley showed that SQI-Et has a selectivity for Na+ over K+ of 100:1. The ionophores carry Na+ ions across the cell membrane and consequently raise the cell membrane potential. (Figures 8.3 and 8.4).
SQI-Et and SQI-Pr have an intrinsic fluorescence excitable by UV light. Therefore, they are not well-suited for use with SBFI, which is a UV-excitable Na+ indicator. These ionophores are ideally suited for use in conjunction with long-wavelength Na+ indicators like ANG-1 and ANG-2. The properties of SQI-Et and SQI-Pr are listed in Table 8.1.
Table 8.1 Sodium Ionophores
Figure 8.1 SQI-Et
Figure 8.2 SQI-Pr
Figure 8.3 Effect of SQI-Pr on membrane polorization (Em).
Figure 8.4: Increase in the fluorescence of membrane potential indicator DiS-C3 due to depolarization by SQI-Et (top) and SQI-Pr (bottom).