An attempt to quantify K+ fluxes in rat liver mitochondria.

K+ homeostasis in mitochondria is maintained by concerted action of the electrophoretic K+ uniport, driven by the membrane potential, and the electroneutral K+/H+ exchange, driven by [K+] and [H+] gradients. Knowing the driving forces of both pathways and the magnitude of net K+ fluxes under various conditions in rat liver mitochondria suspended in isotonic sucrose medium it was possible to evaluate corresponding rate constants. The rate constant for the K+ uniport under energized conditions was calculated as 0.11 nmol x min-1 x mV-1 x mg protein-1 and that of the antiport was about ten times lower. Activation of the exchanger by depletion of mitochondrial Mg2+ with the ionophore A23187 resulted in an almost tenfold increase of its rate constant, approaching that of the uniport. Quinine and Mg2+ inhibited both K+ transport pathways, whereas glibenclamide, blocker of the ATP-sensitive K+ channel, inhibited the uniport only. The uniport was activated by K+ channel opener P1060.