Elevating intracellular free Mg2+ preserves sensitivity of Na+-K+ pump to ATP at reduced temperatures in guinea pig red blood cells

Red cells of hibernating species have a higher relative rate of Na+-K+ pump activity at low temperature than the red cells of a mammal with a typical sensitivity to cold. The kinetics of ATP stimulation of the Na+-K+ pump were determined in guinea pig and ground squirrel red cells at different temperatures between 5 and 37 degrees C by measuring ouabain-sensitive K+ influx at different levels of ATP. In guinea pig cells, elevation of intracellular free Mg2+ to 2 mmol . l(-1) by use of the divalent cation ionophore A23187 caused the apparent affinity of the pump for ATP to increase with cooling to 20 degrees C, rather than to decrease, as occurs in cells not loaded with Mg2+. In ground squirrel cells raising intracellular free Mg2+ had little effect on apparent affinity of the pump for ATP at 20 degrees C. ATP affinity rose slightly with cooling both in Mg2+-enriched and in control ground squirrel cells. Increased intracellular free Mg2+ in guinea pig cells stimulated Na+-K+ pump activity so that at 20 degrees C the pump rate was the same in the Mg2+-enriched guinea pig and control ground squirrel cells. Pump activity in Mg2+- enriched guinea pig cells at 5 degrees C was significantly improved but still lower than pump activity in control cells from ground squirrel. Thus, loss of affinity of the Na+-K+ pump for ATP that occurs with cooling in cold-sensitive guinea pig red cells can be, at least partially, prevented by elevating cytoplasmic free Mg2+. Conversely, in ground squirrel red cells natural rise of free Mg2+ may in part account for the preservation of the ATP affinity of their Na+-K+ pump with cooling.