Evaluation of ion gradient-dependent H+ transport systems in isolated enterocytes from the chick.

The experiments reported here evaluate the capability of isolated intestinal epithelial cells to accomplish net H+ transport in response to imposed ion gradients. In most cases, the membrane potential was kept constant by means of a K+ plus valinomycin "voltage clamp" in order to prevent electrical coupling of ion fluxes. Net H+ flux across the cellular membrane was examined at pH 6.0 (the physiological lumenal pH) and at pH 7.4 using methylamine distribution or recordings of changes in media pH. Results from both techniques suggest that the cells have an Na+/H+ exchange system in the plasma membrane that is capable of rapid and sustained changes in intracellular pH in response to an imposed Na+ gradient. The kinetics of the Na+/H+ exchange reaction at pH 6.0 [Kt for Na+ = 57 mM, Vmax = 42 mmol H+/liter 30MG (3-O-methylglucose) space/min] are dramatically different from those at pH 7.4 (Kt for Na+ = 15 mM, Vmax = 1.7 mmol H+/liter 30MG space/min). Experiments involving imposed K+ gradients suggest that these cells have negligible K+/H+ exchange capability. They exhibit limited but measurable H+ conductance. Anion exchange for base equivalents was not detected in experiments performed in media nominally free of bicarbonate.