title: Ca2+-activated Na+ fluxes in human red cells. Amiloride sensitivity.
abstract: The effect of Ca2+ on the ouabain- and bumetanide-resistant Na+ fluxes in intact red cells was studied at relatively constant internal Ca2+, membrane potential, and cell volume. The red cell calcium concentration was modified using the ionophore A23187. In fresh red cells, the Na+ influx and efflux (1.2 +/- 0.13 and 0.26 +/- 0.07 mmol/liter cells x h, respectively) were not affected by amiloride (1 mM). When external Ca2+ was raised from 0 to 150 microM, in the presence of A23187, both the Na+ influx and efflux were stimulated (about 3.5-fold). The Ca2+-activated Na+ efflux and influx had an apparent Km for activation by Ca2+o of about 25 microM. The Ca2+-dependent Na+ transport was inhibited 30-60% by amiloride (ID50 = 17.3 +/- 8 microM). Amiloride, however, had no effect on the Ca2+-dependent K+ influx. The amiloride-sensitive (AS) transport pathway was a linear function of the Na+o concentration in the range from 0 to 75 mM. The Ca2+i activation seems to depend on the metabolic integrity of red cells. 1) It does not take place in ATP-depleted red cells; 2) ATP-repletion of ATP-depleted red cells fully restored AS Na influx; and 3) ATP-enrichment (ATP-red cells) enhanced the AS Na influx by about 100%. The Ca2+-activated AS Na+ influx was not affected by either DIDS or trifluoperazine. The present results indicate that in human erythrocytes an increase in internal Ca2+ activates on otherwise silent AS Na+-transport system, which is dependent on the metabolic integrity of the red cells.
mesh_terms: D000255:Adenosine Triphosphate / Q000097:blood; D000584:Amiloride / Q000494:pharmacology*; D001693:Biological Transport, Active / Q000187:drug effects; D002034:Bumetanide / Q000494:pharmacology; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000097:blood / Q000494:pharmacology*; D004910:Erythrocyte Membrane / Q000502:physiology; D004912:Erythrocytes / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008564:Membrane Potentials; D010042:Ouabain / Q000494:pharmacology; D012964:Sodium / Q000097:blood*
chemical_list: D002034:Bumetanide; D000001:Calcimycin; D010042:Ouabain; D000584:Amiloride; D000255:Adenosine Triphosphate; D012964:Sodium; D002118:Calcium