Na-nitroprusside and HgCl2 modify the water permeability and volume of human erythrocytes

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 (Pd) 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 HgCl2 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 Pd and volume of the erythrocyte were deduced from induced variations in the measured proton (1H) nuclear magnetic resonance (NMR) transverse (T2) 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 Mn2+ ions. The 1H-T2 NMR technique allows determination of the time constant τexch (for exchange of the erythrocyte intracellular water) that is inversely proportional to the permeability coefficient Pd when the intracellular water volume is left unmodified, as in the case of SNP-treated erythrocytes. However, for HgCl2-treated erythrocytes, this technique showed simultaneous variation of both τexch and the volume ratio Vin/Vout of intracellular and extracellular water in proportions suggesting that Pd was left unmodified. The HgCl2 effect has been found to be partly reversible by the reducing activity of added mercaptoethanol.