FIGURE 6.
Extracellular ATP hydrolysis in human, Xenopus, and dog erythrocytes. Time dependent changes in the concentration of extracellular ATP ([ATP]e) were determined by following the release of 32Pi from [γ-32P]ATP or from [α-32P]ATP, as described under “Experimental Procedures.” A, in human erythrocytes, release of [γ-32P]Pi was assayed at hematocrits 1 (●) or 20% (○), and adenosine production (i.e. release of [α-32P]Pi) was estimated at 20% hematocrit (◊). B, release of [γ-32P]Pi in X. laevis erythrocytes was assayed at hematocrits 1 (●) or 6.5% (○); C, release of [γ-32P]Pi in dog erythrocytes was assayed at hematocrits 1 (●) and 20% (○). In all cases, results are expressed as percentage of the total [γ-32P]ATP or [α-32P]ATP hydrolyzed and represent mean ± S.E. of quadruplicate assays from at least two independent preparations. The continuous lines represent the best fit function to experimental data. As explained under “Experimental Procedures,” the fitting procedure allowed calculating initial velocities for ecto-ATPase activities of erythrocytes from human, dog, and Xenopus and adenosine production from human erythrocytes. Accumulation of [γ-32P]Pi or [α-32P]Pi in the absence of cells was negligible.