Magnesium and ATP dependence of K‐Cl co‐transport in low K+ sheep red blood cells.

1. In low K+ (LK) sheep red blood cells, depletion of adenosine triphosphate (ATP) by glycolysis inhibition induced specific effects on ouabain‐resistant Cl(‐)‐dependent K+ transport (K‐Cl co‐transport), depending on the osmolarity: stimulation in isosmotic while inhibition in hyposmotic solutions. However, these effects depended upon the presence of internal Mg2+. 2. In LK sheep red blood cells, ATP constituted nearly 90% of the Mg2+ buffering capacity. As no significant reduction of total Mg2+ was observed after ATP depletion, the overall internal Mg2+ in ATP‐depleted cells exists in the free form. 3. The dependence of K+ efflux on internal Mg2+ was also directly related to the presence of ATP. In control cells, Mg2+ constituted an endogenous inhibitor, inducing a 70% inhibition of K‐Cl fluxes but only 30% in ATP‐depleted cells. The Cl(‐)‐insensitive component of K+ efflux was unaffected by the divalent cation. 4. After Mg2+ removal, the rate of K+ efflux was significantly increased at all osmolarities, between 240 mosM (swollen cells) and 440 mosM (shrunken cells). Hence, Mg(2+)‐depleted LK sheep red cells lose volume sensitivity of K‐Cl co‐transport. 5. Internal K+ or Cl‐ were not required for the Mg2+ inhibition, and Mg2+ did not interfere with the internal binding sites for Cl‐ or K+. Hence, the sites for Mg2+ or MgATP, and for K+ and Cl‐ are independent of each other.