The effect of acute expansion of the extracellular fluid volume (ECV) with isotonic (0.9%) saline on the activity of the lymphocyte Na+/H+ antiport (NHE) was studied in a total of 18 healthy volunteers. Saline was infused at a constant rate so that 4 mmol kg-1 b.w. was administered over 2 h. NHE activity was measured by quantifying cytosolic pH (pHi) recovery following acidification of the cells with propionic acid and by pH clamping at various pHi values between 7.2 and 5.8 using nigericin. Both methods demonstrate NHE activation associated with intravenous saline infusion, the kinetic difference being a marked decrease in the Hill coefficient n from 3.28 +/- 0.21 (SEM) to 2.22 +/- 0.11 in the absence of changes in baseline pHi (7.14 +/- 0.02 vs. 7.08 +/- 0.02; P = 0.15), Vmax (42.8 +/- 2.7 vs. 48.1 +/- 2.8 mmol L-1 min-1; P = 0.08) and pK (6.32 +/- 0.04 vs. 6.35 +/- 0.02). NHE activation was associated with significant decreases in serum chloride (P = 0.016), calcium (P = 0.008), total cholesterol (P = 0.008), low-density lipoproteins (P = 0.016) and high-density lipoproteins (P = 0.008). Moreover, saline infusion induced extracellular acidification with a decrease in pH from 7.39 +/- 0.01 to 7.37 +/- 0.01 (P = 0.016), HCO3- from 23.3 +/- 0.43 mmol L-1 to 21.3 +/- 0.25 mmol L-1 (P = 0.008) and base excess from -1.03 +/- 0.38 mmol L-1 to -3.00 +/- 0.31 mmol L-1 (P = 0.008). Our results show for the first time that acute ECV expansion with isotonic saline is followed by an activation of the lymphocyte NHE. The underlying mechanism(s) remain to be investigated. However, the demonstration in our study of marked changes in acid-base balance induced by acute saline points to a possible inter-relationship of antiporter activation and extracellular acidification.

The Na+/H+ antiport is a membrane transport protein that extrudes intracellular protons in exchange for extracellular sodium. Some details of its physiological and pathophysiological role remain poorly defined. Experimental evidence suggests that the antiporter is involved in the regulation of cell volume. In the present study, we therefore investigated the activity of the lymphocyte Na+/H+ antiport in nine healthy volunteers following acute hypertonic (2.5%) saline infusion (4 mmol NaCl/kg over 120 min). Antiport activity was measured after acidifying the cells with Na+ propionate (5-40 mM) using the fluorescent dye bis-carboxyethyl carboxyfluorescein. Hypertonic saline induced significant increases in plasma osmolality (308.4 +/- 2.3 vs. 293.5 +/- 2.7 mOsm/kg; P < 0.01), serum Na+ (150.8 +/- 3.7 vs. 138.9 +/- 0.5 mmol/kg; P < 0.01), and Cl- concentrations (118.0 +/- 3.9 vs. 101.1 +/- 1.0 mmol/kg; P < 0.01). Extracellular hypertonicity was followed by a stimulated activity of the lymphocyte Na+/H+ antiport with an increase in the apparent Vmax values from 2.44 +/- 0.16 to 3.27 +/- 0.34 10(-3) s-1 (P < 0.01) and a slight rise in pK, from 6.81 +/- 0.03 to 6.87 +/- 0.03 (P < 0.05) after hypertonic saline. In addition to antiport activation, cytosolic alkalinization was observed with cytosolic pH values averaging 6.90 +/- 0.02 before and 6.99 +/- 0.02 (P < 0.01) after hypertonic saline. Our results show for the first time that acute extracellular hypertonicity in man due to hypertonic NaCl loading is associated with a stimulated lymphocyte Na+/H+ antiport activity and cytosolic alkalinization.