Three main transport systems are involved in solute loss

Three main transport systems are involved in solute loss and red cell dehydration (summarised in Fig. 1: Lew and Bookchin, 2005): the deoxygenation-induced cation conductance (sometimes termed Psickle), the Ca-activated K+ channel (or Gardos channel) and the KCl cotransporter (KCC). Psickle is activated by deoxygenation and red cell shape change (Tosteson, 1955, Mohandas et al., 1986, Joiner, 1993). It allows entry of Ca (Rhoda et al., 1990) which may then activate the third transporter (Lew et al., 1997), the Gardos channel, with conductive K+ loss at high rates, and Cl− following separately through separate anion channels. KCC mediates coupled movements of K+ and Cl− (Ellory et al., 1982, Lauf et al., 1992, Gillen et al., 1996). Its activity is abnormally elevated in red Walrycin B sale from HbSS patients (Brugnara et al., 1986, Crable et al., 2005), and it also responds differently to modulatory stimuli such as O2 tension (Gibson et al., 1998), when compared to red cells from normal HbAA individuals. It may also be further stimulated by Mg depletion via Psickle (Ortiz et al., 1990, Delpire and Lauf, 1991). As noted above, with the exception of a few studies involving a handful of HbSC patients (Canessa et al., 1986), our understanding of these systems comes from work on red cells from SCA patients. The behaviour of red cells from HbSC patients and management of disease is largely extrapolated from these studies on HbSS - but this may not be justified. Our recent study comparing clinical parameters and K+ transport in red cells from HbSS and HbSC patients indicated significant differences between the two genotypes (Rees et al., 2015). In particular, KCC activity was higher in HbSC patients with more severe forms of SCD (Rees et al., 2015), whilst the same was not true for KCC activity in red cells from HbSS patients. These findings, along with differences in clinical pathology, support the hypothesis that HbSC disease is a distinct clinical entity. Since changes in red cell membrane permeability represent an early event in SCD pathogenesis, with a direct association with HbS polymerisation, further work on membrane transport in red cells from HbSC patients is an imperative. In this report, we characterise more fully the behaviour of the main K+ transport systems in red cells from HbSC patients and highlight important differences in comparison with red cells from patients with SCA.
Materials and Methods
Results
Discussion The present findings show significant differences in cation homeostasis comparing red cells from HbSC and HbSS patients. The sickling shape change occurred at higher levels of O2 tension in red cells from HbSS than HbSC patients, together with activation of the main conductive cation channels, Psickle and the Gardos channel. Both transport pathways showed a similar correlation with sickling. The level of activity of the two channels was significantly lower in HbSC cells compared to HbSS ones, and also required more profound hypoxia to become activated, consistent with a reduced participation of these systems in mediating solute loss and dehydration. By contrast, KCC activity was significantly higher in oxygenated red cells from HbSC patients than those from HbSS individuals. KCC activity varied considerably between HbSC individuals. It also showed a different relationship to O2 tension to that observed in red cells from HbSS patients, being inactivated at low O2 tension (as seen in normal HbAA red cells). There was a higher level of activity of KCC in denser HbSC red cells compared to that observed in lighter ones. Taken together, these findings are consistent with a greater role for KCC in dehydration of red cells from HbSC patients. They also present a characteristic of red cell membrane transport in HbSC patients which may be important in pathogenesis, and further substantiate the hypothesis that HbSC disease is a different entity to that of homozygous HbSS SCD.