Activation of Syk by deoxygenation is specific for SS cells, is reversed by reoxygenation, and is not mediated by increases in intracellular Mg²⁺ and Ca⁺⁺.

(A) AA or SS cells (n = 5) were incubated for 30 minutes under oxygenated (O2) or deoxygenated conditions (N2). (B) SS cells (n = 3) were incubated for 15 minutes under oxygenated or deoxygenated conditions, and samples from deoxygenated SS cells were reoxygenated for 60 minutes (N2/O2). (C) SS cells (n = 4) were incubated for 15 minutes under oxygenated or deoxygenated conditions in solution B (−) or in a buffered isotonic medium containing 0.15 or 1 mM MgCl₂, 0.1 mM EGTA, and 3 μM A23187 (+). In the latter conditions (A23187), the presence of EGTA and 0.15 mM MgCl₂, although preventing any increase in [Ca⁺⁺]_i and [Mg²⁺]_iassociated with deoxygenation of SS cells, did not abolish Syk activation. In the presence of A23187 and 1 mM MgCl₂, [Mg²⁺]_i increased to 1.7 mM33 in both oxygenated and deoxygenated samples, without affecting Syk activation. Syk activity was measured as indicated in the legend to Figure 2. Data shown are means ± SE. *Significantly different from oxygenated control; **significantly different from deoxygenated control. In C, the values in deoxygenated conditions without or with A23187 are not significantly different.