Kinetic properties of the KCl transport at the secreting apical membrane of the oxyntic cell

Abstract

We have previously reported marked structural and functional differencces between (H+/K+)-ATPase-rich vesicles obtained from resting fundic gastric mucosa and those obtained from secreting tissue. The functional differences seem to arise from activation of a permeability pathway for K+ salts in the vesicles originating from stimulated tissue. By providing the intravesicular K+ needed for the ATPase-mediated H+/K+ exchange, this pathway facilitates formation of large pH gradients (J.M. Wolosin & J.G. Forte.FEBS Lett.125:208–212, 1981). We have now expanded our observations on the characteristics of the K-salt transport. The dependence of intravesicular acidification on the concentration of various anions or of K+ was monitored using the rate of fluorescence quenching of the pH probe, acridine orange. Rates of acid accumulation followed the series I−>Br−>Cl−>NO3−>CH3SO3−>isethionate>gluconate. The effects of exogenous ionophores on pH gradients suggested that K+-salt transport is electroneutral. On the basis of the known properties of the H+/K+ pump, we devised a method by which the rates of unidirectional K+ salt influx can be monitored via the resulting rates of H+ accumulation. Accordingly, the dependence of these rates on the concentration of K+ and Cl− was studied and subjected to a kinetic analysis. It was found that the dependence follows that expected for an electroneutral K+ and Cl− transport system with apparent Michaelis constants equal to 15±2mm for K+ and 46±5mm for Cl−. It is concluded that intravesicular acid accumulation, believed to represent HCl secretion in whole tissue, results from the combined action of a KCl symport and the well known (H++K+)-ATPase. Though the calculated maximal transport capacity of the symport exceeds by several times the maximal transport capacity of the pump, at physiological concentrations of the ions both transport systems are expected to transport K+ in opposite directions at similar rates. Implications of these observations to HCl secretion by intact tissue are discussed.