Factors raising intracellular calcium increase red blood cell heterogeneity in density and critical osmolality

In normal human blood, RBC volume (V), surface area (A) and hemoglobin content (H) exhibit Gaussian distributions with coefficients of variation (CV) of 10-15%. Strikingly narrower distributions are observed for their cell density (CV approximately 0.5%) and filterability (CV approximately 5-7%). This implies that V is highly correlated with H and A. We hypothesize that the RBC is able to adjust its volume to parameters H and A. It is tempting to speculate that intracellular free calcium (Cai) is a mediator in this process, acting as an activator of the Gardos channel. We tested this hypothesis by experimentally varying Cai and measuring changes in RBC density and filterability distributions.

Three different approaches were used to raise Cai: (i) RBCs were loaded with Ca2+ in the presence of the calcium ionophore A23187; (ii) RBCs were incubated with Ca2+ in the presence of 1.0 mM ortho-vanadate; and (iii) the calcium pump was switched off by ATP depletion of RBCs. The density distribution of RBCs was determined by a phthalate technique. The distributions in filterability were obtained using a kinetic filtrometer.

Whatever the approach used, the density and filterability distributions of treated RBCs broadened significantly in comparison with those of control cells.

The results obtained suggest that (i) in vivo regulation of RBC volume is mediated by Cai and (ii) Cai probably depends on the A/V ratio, which determines the stress experienced by the RBC membrane in the circulation and, thereby, the calcium influx rate.