ACTIVATION AND PHYSIOLOGICAL ROLE OF Na+/H+ EXCHANGE IN LAMPREY (LAMPETRA FLUVIATILIS) ERYTHROCYTES

The effects of intracellular acidification, osmotic shrinkage and ss-adrenergic stimulation on sodium transport across the membrane of lamprey (Lampetra fluviatilis) erythrocytes were investigated. Unidirectional ouabain-insensitive sodium flux, measured using radioactive 22Na, was increased markedly by intracellular acidification, to a lesser extent by osmotic shrinkage and only modestly by ss-adrenergic stimulation. Na+/H+ exchange was activated in all of these cases. However, net sodium influx (and cell swelling caused by the influx of osmotically obliged water) was seen only in cells subjected to intracellular acidification. In contrast, practically no changes in red cell pH or in water or ion (Na+, K+ and Cl-) contents were seen after osmotic shrinkage or ss-adrenergic stimulation. Calculations of the [Na+]o/[Na+]i and [H+]o/[H+]i ratios across the erythrocyte membrane suggest that the virtual lack of net sodium movements in osmotically shrunken erythrocytes is due to the absence of a driving force for net transport of these ions via the Na+/H+ exchange pathway. It also appears that, in physiological conditions, the increase in the activity of the Na+/H+ exchanger by ss-adrenergic stimulation is too small to mediate detectable net sodium transport.

The Distribution of Ammonia and H+ Between Tissue Compartments in Lemon Sole (Parophrys Vetulus) at Rest, During Hypercapnia and Following Exercise

The distribution of ammonia and [14C]DMO was compared in white muscle, heart, brain, red cells and plasma of lemon sole (Parophrys vetulus Girard) at rest, during hypercapnia and following strenuous exercise. In red cells at rest, measured intracellular ammonia levels were equal to those predicted by the plasma to red cell pH gradient. Red cells are unusual in that hydrogen ions are passively distributed according to membrane potential (EM), whereas in other tissues this is not the case. In white muscle, heart and brain under all experimental conditions, intracellular ammonia levels far exceed those predicted by transmembrane pH gradients. Calculated ENHNH4+ values in these tissues are very close to published resting values of EM. We conclude that, in lemon sole, NH4+ permeates cell membranes and that intracellular ammonia stores are not determined by transmembrane pH gradients.