Sodium dependence of sodium-proton exchange in platelets from patients with essential hypertension

The sodium dependence of recovery of cytoplasmic pH (pHi) after an acid load was studied in platelets from 15 patients with untreated essential hypertension (mean arterial pressure 117 +/- 2.3 mmHg, mean +/- SE) and in 15 normotensive controls (mean arterial pressure 90 +/- 2.2 mmHg). Proton flux was measured in gel-filtered platelets loaded with pH sensitive fluorescent indicator. Sodium dependent proton efflux was prevented by 5 (N,N hexamethylene) amiloride, a potent inhibitor of sodium-proton exchange. The relationship between initial rate of proton efflux and extracellular sodium concentration obeyed Michaelis-Menten kinetics. The maximum initial rate of proton efflux was similar in hypertensive and normotensive subjects (1.93 +/- 0.35 mmol H+1/sec, mean +/- SE, and 1.88 +/- 0.26 mmol H+l/sec respectively). The Michaelis-Menten constant (the sodium concentration at which the initial rate of proton efflux is half maximal) was also similar in the two groups: 75.7 +/- 13.9 mmol/l and 8.15 +/- 10.6 mmol/l respectively. There was no significant correlation between either of these parameters and arterial blood pressure.