Both processes are dependent on cellular energy since they are suppressed by 2,4 dinitrophenol or anoxia, indicating that they derive their energy from the hydrolysis of ATP. Similar mechanisms have been identified and characterized in isolated guinea pig small intestinal cells and everted rat jejunum . The enterocyte regulates its Na content by two pumps located in the basolateral plasma membrane. One exchanges Na for K , is inhibited by ouabain, and insensitive to ethacrynic acid and furosemide. The second transports Na with Cl? and water, is insensitive to ouabain, but is inhibited by ethacrynic acid and furosemide. These results confirmed the evidence from experiments with inside out basolateral plasma membrane vesicles from guinea pig small intestinal epithelial cells , rat jejunum and rat proximal tubule , where two distinct mechanisms capable of accumulating sodium in the intravesicular space were demonstrated when ATP was added to the incubation medium. One transports sodium actively in the absence of potassium, whereas the other requires potassium to be present within the vesicles.
The two mechanisms can also be differentiated Kinase Inhibitor Library by their affinities for sodium, their optimal pH, and their behavior towards different inhibitors. Thus, the active mechanism that transports sodium in the absence of potassium is refractory to ouabain but is inhibited by ethacrynic acid and furosemide, while the mechanism that causes sodium accumulation in the vesicles in the presence of internal potassium is strongly inhibited by ouabain, weakly inhibited by ethacrynic acid, and insensitive to furosemide. ATP is a specific stimulator of both processes and the requirement for magnesium is absolute in both cases. These two active Na transport mechanisms, identified in epithelial cells of the small intestine and proximal tubule, are associated with ATPase activities located in the basolateral plasma membranes of such cells. The two Mg2 dependent, sodium stimulated ATPase activities have been identified in microsomal fractions and crude basolateral plasma membrane fractions of the renal proximal tubule and purified basolateral plasma membranes of small intestinal cells .
In these preparations, the Na ATPase is stimulated by sodium alone or to a lesser extent by Li , whereas the Na K ATPase requires both sodium and potassium for activation. These facts link the enzymes to the sodium transport systems. The Na ATPase specifically hydrolyzes ATP, as does the Na K ATPase, though the latter has some effect on GTP and ITP. This property defines Hematoxylin the two enzymes as ATPases. The fact that the enzyme is stimulated indifferently by different sodium salts essentially excludes the possibility that the Na ATPase is an anion stimulated ATPase, whose existence has been postulated .