Effect of ionophore-induced Ca²⁺ loads on the ATP content of AA RBCs in the absence and presence of 1 mM sodium orthovanadate in the RBC suspension. Results shown are from 1 experiment using the same conditions as those in the flow cytometry experiments (Figure 2, [K⁺]_o = 15 mM). Almost identical results were obtained in 2 additional experiments with K_o = 80 mM. The ATP content of the RBCs was related to the measured hemoglobin concentration in a parallel sample and is expressed per 340 g Hb, assumed equivalent of 1 L packed normal RBCs. The 1 mM vanadate inhibited more than 99% of PMCA-mediated Ca²⁺ transport and ATP consumption by the pump.⁵³  Note the steep fall in ATP content in the controls immediately after Ca²⁺ permeabilization, the period during which dehydration rates were measured, and the contrasting conservation of ATP levels in the presence of vanadate. Despite such differences, the conditions with and without vanadate, described in Table 1 and Figure 2, showed very similar results.