Ca2+ transport properties of ionophores A23187, ionomycin, and 4-BrA23187 in a well defined model system.

Models for the electroneutral transport of Ca2+ by ionophores A23187, ionomycin, and 4-BrA23187 have been tested in a defined system comprised of 1-palmitoyl-2-oleoyl-sn-glycerophosphatidylcholine vesicles prepared by freeze-thaw extrusion. Quin-2-loaded and CaCl2-loaded vesicles were employed to allow the investigation of transport in both directions. Simultaneous or parallel measurements of H+ transport and membrane potential, respectively, indicate that for any of these ionophores, electrogenic transport events do not exceed 1 in 10,000 when there is no preexisting transmembrane potential. When a potential of approximately 150 mV is imposed across the membrane, transport catalyzed by A23187 remains electroneutral; however, for ionomycin and 4-BrA23187, approximately 10% of transport events may be electrogenic. The defined vesicle system has also been utilized to determine how the rate of Ca2+ transport varies as a function of ionophore and Ca2+ concentration and with the direction of transport. Some aspects of the results are unexpected and should be considered by investigators using ionophores in biological systems. These include the apparent failure of these compounds to fully equilibrate Ca2+ with a high affinity Ca2+ indicator when these species are separated by a membrane, rates of transport that vary markedly with the direction of transport, and extents of transport that are a function of ionophore concentration. At least some of these unexpected behaviors can be explained by a strong influence of delta pH on forward and reverse transport kinetics. In the case of A23187, the data also give some initial insights into the relationship between formation of the transporting species and the entry of this species into the membrane hydrophobic region.