MECHANISM OF THE STIMULATION OF SERINE AND ALANINE TRANSPORT INTO ISOLATED RAT-LIVER CELLS BY BICARBONATE IONS

Bicarbonate ions stimulate the transport of serine and alanine into isolated hepatocytes. The effect of bicarbonate is to increase the V(max). Of the transport process without changing the apparent K(m). The intracellular pH was estimated from the distribution of the weak base methylamine and the weak acid 5,5'-dimethyloxazolidine-2,4-dione (DMO) across the plasma membrane. The addition of bicarbonate to a cell suspension caused the internal pH to become more acid. The initial rate of serine, alanine and glycine transport was a linear function of the function of the initial difference in pH across the membrane. It is concluded that bicarbonate activates the transport of these amino acids primarily by increasing the pH difference across the plasma membrane. It is suggested that the uptake of serine together with Na+ ions occur in exchange for H+ ions, which are translocated outwards on the same carrier system. Some preliminary evidence consistent with this model is presented.