Metabolic Functions of Myo-inositol

The roles of K+ and the Na+- and K+-activated adenosine triphosphatase (Na+-K+ pump) in the transport of α-aminoisobutyric acid (AIB) in KB cells were evaluated. It was shown that 0.33 µm ouabain causes half-maximal inhibition of the Na+-K+ pump. Similar sensitivity to the inhibitor was shown earlier for AIB transport. However, whereas the pump is inhibited maximally within 90 sec, inhibition of AIB transport is time-dependent and is maximal after 30 min of the cells' exposure to the inhibitor. Selective inactivation of the Na+-K+ pump by ouabain or by omission of extracellular K+ is without effect on the rate of AIB influx, provided that the concentration of intracellular K+ remains above 30 mm. Depletion of intracellular K+ leads to a loss of concentrative AIB transport. This inactivation is not immediate; rather, it is time-dependent. Subsequent addition of KCl fails to reverse this inactivation, even though the cells can concentrate K+ 11-fold to a level greater than 55 mm. The intracellular K+ concentration which protects against inactivation of the AIB transport system is above 30 mm. Extracellular K+, other than being the source of intracellular K+, is not required for active AIB transport. It is concluded that the role of the Na+-K+ pump in AIB transport is to maintain the necessary concentration of intracellular K+ for maintaining the functional integrity of the AIB transport system.