Development of a delayed outward-rectifying K+ conductance in cultured mouse peritoneal macrophages

Patch clamp techniques were used to study ionic currents in cultured mouse peritoneal macrophages. Whole-cell voltage clamp studies of cells 1-5 hr after isolation showed only a high-resistance linear membrane. After 1 day in culture, 82 of 85 cells studied had developed a voltage- and time-dependent potassium (K+) conductance similar to the delayed outward rectifier in nerve and muscle cells. The current activated when the membrane was depolarized above -50 mV. The sigmoidally rising current rose to a peak at a rate that increased with depolarization. Inactivation proceeded exponentially with a time constant of approximately equal to 450 ms. Recovery from inactivation was slow (tau = 12 s). The reversal potentials for varying extracellular K+ concentrations followed the Nernst predictions for a K+ -specific channel. The conductance was blocked by extracellular 4-aminopyridine and by intracellular tetraethylammonium chloride, barium, and cesium. Single-channel K+ currents comprising this net current had a conductance of 16 pS, exhibited bursting behavior, and inactivated with time. No inward currents were ever detected in macrophages cultivated for up to 4 days. Short-term exposure to chemoattractant and transmitter agents failed to activate an inward current. Macrophages may change their membrane electrophysiological properties depending on their state of functional activation. We postulate that the K+ conductance develops prior to depolarizing conductances involved in the macrophage's immunological functions.