Lithium stimulation of in vitro granulopoiesis: evidence for mediation via sodium transport pathways.

Lithium (Li) stimulates granulopoiesis both in vitro and in vivo by increasing the number of committed granulocyte-macrophage colony forming stem cells (CFU-GM) either through direct or indirect mechanisms. In this report are described further studies designed to investigate if this Li stimulation could be altered by modulating Li transport using agents known to influence monovalent cation transport. Ouabain, a Na/K ATPase inhibitor, added to non-adherent bone marrow cells either before or immediately after the addition of ultra-pure Li (1 mEq) produced an irreversible reduction in CFU-GM indicating the importance of the Na/K ATPase in these processes. Further studies using the sodium and potassium ionophores gramicidin and valinomycin (5 micrograms/ml) followed by a delay in the addition of Li (0, 5, 15, 20, 30, 60 and 120 min) reduced CFU-GM; however, this reduction was less severe in the presence of gramicidin, indicating sodium transport pathways may be required for Li action. To further explore this hypothesis, studies were performed using the more specific sodium transport inhibitors, amiloride and phloretin. Both were effective in reducing the ability of Li to increase CFU-GM. Studies incorporating the calcium ionophore, A23187, demonstrated that in the presence of Li, a further reduction in CFU-GM was observed indicating that Li was unable to reverse this A23187 induced reduction in CFU-GM. These data suggest that in the presence of active calcium transport, the ability of Li to increase CFU-GM is restricted.