Ion channel expression in PMA-differentiated human THP-1 macrophages .3.

Ion channel expression was studied in THP-I human monocytic leukemia cells induced to differentiate into macrophage-like cells by exposure to the phorbol ester, phorbol 12-myristate 13-acetate (PMA). Inactivating delayed rectifier K+ currents, I-DR, present in almost all undifferentiated THP-I monocytes, were absent from PMA-differentiated macrophages. Two K+ channels were observed in THP-1 cells only after differentiation into macrophages, an inwardly rectifying K+ channel (I-IR) and a Ca2+-activated maxi-K channel (I-BK). I-IR was a classical inward rectifier, conducting large inward currents negative to E(K) and very small outward currents. I-IR was blocked in a voltage-dependent manner by Cs+, Na+, and Ba2+, block increasing with hyperpolarization. Block by Na+ and Ba2+ was time-dependent, whereas Cs+ block was too fast to resolve. Rb+ was sparingly permeant. In cell-attached patches with high [K+] in the pipette, the single I-IR channel conductance was similar to 30 pS and no outward current could be detected. I-BK channels were observed in cell-attached or inside-out patches and in whole-cell configuration. In cell-attached patches the conductance was similar to 200-250 pS and at potentials positive to similar to 100 mV a negative slope conductance of the unitary current was observed, suggesting block by intracellular Na+. I-BK was activated at large positive potentials in cell-attached patches; in inside-out patches the voltage-activation relationship was shifted to more negative potentials by increased [Ca2+]. Macroscopic I-BK was blocked by external TEA(+) with half block at 0.35 mM. THP-1 cells were found to contain mRNA for Kv1.3 and IRK1. Levels of mRNA coding for these K+ channels were studied by competitive PCR (polymerase chain reaction), and were found to change upon differentiation in the same direction as did channel expression: IRK1 mRNA increased at least 5-fold, and Kv1.3 mRNA decreased on average 7-fold. Possible functional correlates of the changes in ion channel expression during differentiation of THP-1 cells are discussed.