Comparison of the active calcium extrusion, calcium buffering capacity and ATPase activity in rabbit reticulocytes and mature red cells.

The purpose of the present work was to study the changes in the pattern of calcium homeostasis following the loss of intracellular organella during red cell maturation. Reticulocytes and mature red cells were prepared from anaemic rabbits blood after daily bleeding. Experimental protocols were designed to study the calcium buffering capacity in intact and digitonin-disrupted cells, the calcium pumping rate and, the Ca2+-translocating ATPase activity in the aforementioned red cells subpopulations. In digitonin-disrupted cells, a vesicular calcium pool, sharing the properties of mitochondria, could be detected in reticulocytes but no in mature red cells. Calcium content and calcium buffering capacity were significantly lower in reticulocytes than in mature red cells. The pattern of active calcium extrusion was quite similar in the two cell subpopulations, although reticulocytes had somewhat higher calcium affinity. Besides, an estimation of the calcium pumping rate gave higher values in reticulocytes than in mature erythrocytes. These values were 21 and 9 mmol/l cells per h, respectively. Maximal activities of the high-affinity (Ca2+ + Mg2+)-ATPase and basal Mg2+-ATPase were significantly higher in reticulocytes than in mature red cells, but no differences were observed regarding calcium affinity. The results show that changes in the properties of the Ca2+-translocating ATPase and intracellular calcium buffering systems are mechanisms involved in the process of red cell maturation.