Insulin activation of red blood cell Na+/H+ exchange decreases the affinity of sodium sites

Abstract

We have previously reported increased activity of Na+/H+ and Na+/Li+ exchanges in red blood cells (RBC) of patients with hypertension and diabetic nephropathy. The presence in human red blood cells (RBC) of insulin receptors has led us to examine the effects of this hormone on the kinetic parameters of Na+/H+ exchange as a first approach to define its mechanism of action. The antiporter activity was measured as net Na+ influx driven by an outward H+ gradient in acid-loaded, Na-depleted RBCs preincubated with or without (w/wo) insulin (0 to 100 μU/ml) for different time periods. The effects of insulin on the H+ and Na+ activation kinetics of Na+/H+ exchange were examined in RBCs of normal subjects fasted for 12 hours. Insulin (50 μU/ml for 1 hr) increased the V(max) from 28 ± 6 to 49 ± 8 mmol/liter cell x hr (N = 10, P < 0.0005) and the K(m) for Na+ from 72 ± 10 to 142 ± 19 mM (N = 4, P < 0.05) but did not change the K(m) for intracellular H+. Insulin also increased the V(max) of Na+/Li+ exchange at pH(i) 7.4 (0.34 ± 0.03 to 0.45 ± 0.04 mmol/liter cell x hr, N = 9, P < 0.005) as well as the K(m) for Na+ (31 ± 3 to 76 ± 10 mM, P < 0.0003). Therefore, insulin can modulate Na+ sites of Na+/Li+ or Na+/H+ exchanges independent of the occupancy of H+ sites to favor the release of bound Na+ into the cytoplasm. Insulin stimulation of Na+/H+ exchange required endogenous cytosolic Ca2+ levels. The kinetic effects of insulin on Na+/H+ and Na+/Li+ exchanges were imitated by okadaic acid (300 μM), an inhibitor of protein phosphatases which dephosphorylate serine-threonine residues. Okadaic acid increased the V(max) of Na+/H+ and Na+/Li+ exchanges and the K(m) for Na+ as insulin did. In conclusion, insulin stimulation of the Na+/H+ antiporter occurs by a novel kinetic mechanism leading to a decreased affinity for external Na+ without changes in the affinity for H(i). On the basis that insulin effects were imitated by okadaic acid, we hypothesize that this hormone may increase the phosphorylated state of serine-threonine residues of this antiporter protein.