Cd2+-induced swelling-contraction dynamics in isolated kidney cortex mitochondria: role of Ca2+ uniporter, K+ cycling, and protonmotive force

The nephrotoxic metal Cd²⁺ causes mitochondrial damage and apoptosis of kidney proximal tubule cells. A K⁺ cycle involving a K⁺ uniporter and a K⁺/H⁺ exchanger in the inner mitochondrial membrane (IMM) is thought to contribute to the maintenance of the structural and functional integrity of mitochondria. In the present study, we have investigated the effect of Cd²⁺ on K⁺ cycling in rat kidney cortex mitochondria. Cd²⁺ (EC₅₀ 19 µM) induced swelling of nonenergized mitochondria suspended in isotonic salt solutions according to the sequence KCl = NaCl > LiCl >> choline chloride. Cd²⁺-induced swelling of energized mitochondria had a similar EC₅₀ value and showed the same cation dependence but was followed by a spontaneous contraction. Mitochondrial Ca²⁺ uniporter (MCU) blockers, but not permeability transition pore inhibitors, abolished swelling, suggesting the need for Cd²⁺ influx through the MCU for swelling to occur. Complete loss of mitochondrial membrane potential (_m) induced by K⁺ influx did not prevent contraction, but addition of the K⁺/H⁺ exchanger blocker, quinine (1 mM), or the electroneutral protonophore nigericin (0.4 µM), abolished contraction, suggesting the mitochondrial pH gradient (pH_m) driving contraction. Accordingly, a quinine-sensitive partial dissipation of pH_m was coincident with the swelling-contraction phase. The data indicate that Cd²⁺ enters the matrix through the MCU to activate a K⁺ cycle. Initial K⁺ load via a Cd²⁺-activated K⁺ uniporter in the IMM causes osmotic swelling and breakdown of _m and triggers quinine-sensitive K⁺/H⁺ exchange and contraction. Thus Cd²⁺-induced activation of a K⁺ cycle contributes to the dissipation of the mitochondrial protonmotive force. bongkrekic acid; cyclosporin A; lanthanum; Ru360; ruthenium red