ELEVATING INTRACELLULAR FREE MG2-K+ PUMP TO ATP AT REDUCED TEMPERATURES IN GUINEA-PIG RED-BLOOD-CELLS( PRESERVES SENSITIVITY OF NA+)

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 cell s at different temperatures between 5 and 37 degrees C by measuring ou abain-sensitive K+ influx at different levels of ATP. In guinea pig ce lls, elevation of intracellular free Mg2+ to 2 mmol . l(-1) by use of the divalent cation ionophore A23187 caused the apparent affinity of t he 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 squirr el cells raising intracellular free Mg2+ had little effect on apparent affinity of the pump for ATP at 20 degrees C. ATP affinity rose sligh tly with cooling both in Mg2+-enriched and in control ground squirrel cells. Increased intracellular free Mg2+ in guinea pig cells stimulate d Na+-K+ pump activity so that at 20 degrees C the pump rate was the s ame in the Mg2+-enriched guinea pig and control ground squirrel cells. Pump activity in Mg2+- enriched guinea pig cells at 5 degrees C was s ignificantly improved but still lower than pump activity in control ce lls from ground squirrel. Thus, loss of affinity of the Na+-K+ pump fo r ATP that occurs with cooling in cold-sensitive guinea pig red cells can be, at least partially, prevented by elevating cytoplasmic free Mg 2+. 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.