The specific binding of human granulocyte colony-stimulating factor (G- CSF) to its receptors on NFS-60 cells acts as a primer for cellular proliferation. There are approximately 400 binding sites per cell, with a binding constant of about 100 pmol/L. Before the proliferative response, the affinity constant of the membrane particulate fraction to 35S-labeled guanosine triphosphate-gamma-S (35S-GTP gamma S) and the intracellular cyclic adenylate monophosphate (cAMP) level increased in the presence of G-CSF to about 2.5-fold and about fivefold higher, respectively, than the levels seen in the absence of G-CSF. The increases were time-dependent, with a peak occurring 15 minutes after the addition of G-CSF at 37 degrees C. These findings suggest that, following the binding of the G-CSF to its surface receptors, the activation of the guanosine triphosphate (GTP)-binding protein/adenylate cyclase system may be involved in the proliferation of immature myeloid cells.

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