Na-nitroprusside and HgCl 2 modify the water permeability and volume of human erythrocytes

Abstract

The passage of water through the aquaporin-1 (AQP1) transmembrane channel protein of the human erythrocyte is known to be inhibited by organic mercurials such as p-chloromercuribenzoate (pCMB), which react with the free SH-group of the critical cysteine (Cys189) located near the constriction of the AQP1 water-specific channel. Sodium nitroprusside (SNP), which is known as a nitric oxide (NO) donor in interactions with SH-containing molecules, is shown here to suppress the diffusional water permeability ( P d) of the erythrocyte membrane, presumably as a result of reaction with the Cys189 of the human erythrocyte AQP1 water channels. Further, treatment of erythrocytes with HgCl 2 is found to result in a cell volume decrease that can be related to activation of membrane K +-selective Gárdos channels and subsequent loss of intracellular K + and cell shrinkage. The variations in P d and volume of the erythrocyte were deduced from induced variations in the measured proton ( 1H) nuclear magnetic resonance (NMR) transverse ( T 2) relaxation functions of water exchanging between diamagnetic intracellular and paramagnetic extracellular compartments of the 20–25% hematocrit samples. The extracellular solvent contained 10 mM membrane-impermeable paramagnetic Mn 2+ ions. The 1H- T 2 NMR technique allows determination of the time constant τ exch (for exchange of the erythrocyte intracellular water) that is inversely proportional to the permeability coefficient P d when the intracellular water volume is left unmodified, as in the case of SNP-treated erythrocytes. However, for HgCl 2-treated erythrocytes, this technique showed simultaneous variation of both τ exch and the volume ratio V in/ V out of intracellular and extracellular water in proportions suggesting that P d was left unmodified. The HgCl 2 effect has been found to be partly reversible by the reducing activity of added mercaptoethanol.