Hypertonic saline infusion induces activation of the lymphocyte Na⁺/H⁺ antiport and cytosolic alkalinization in healthy human subjects

Abstract

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 V _max 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.